An Experimental Device for Critical Surface
Characterization of YBCO Tape Superconductors
by
Franco Julio Mangiarotti
Submitted to the Department of Nuclear Science and Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Nuclear Science and Engineering
ARCHVES
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2013
@ Massachusetts Institute of Technology 2013. All rights reserved.
Author............ . . . . . . . . . . . . . . . . . . . . . ... . . .
. . . . .
ranco Julio Mangiarotti
Department of Nuclear Science and Engineering
January 18, 2013
Certified by ..........
..........
.................................
Joseph V. Minervini
Senior Scientist, Division Head, Fusion Technology & Engineering
,Thesis Supervisor
Certified by.......
.......
Anne White
Assistant Professor of Nuclear Science and Engineering
A ./
Thesis Reader
Accepted by .........................
...........
S
u-'d S. Kazimi
TEPCO Professor of Nuclear Engineering
Chair, Department Committee on Graduate Students
An Experimental Device for Critical Surface
Characterization of YBCO Tape Superconductors
by
Franco Julio Mangiarotti
Submitted to the Department of Nuclear Science and Engineering
on January 18, 2013, in partial fulfillment of the
requirements for the degree of
Master of Science in Nuclear Science and Engineering
Abstract
The twisting stacked tape cabling (TSTC) method for YBCO superconductors is very
attractive for high current density, high magnetic field applications, such as nuclear
fusion reactors and high energy physics experiments. Industrial scale assembling
methods have been proposed, and cable samples have been tested at 77 K and 4.2 K.
A new experimental device has been designed and built to measure critical current
of YBCO tapes and TSTC as a function of magnetic field and temperature. The probe
allows controlling the temperature between 4.2 K and 80 K within ±1 K in liquid
and gaseous helium ambient, and can be used in a 2 T magnet facility at MIT-PSFC
and a 14 T magnet facility at NHMFL-FSU. Its current leads are designed to carry
up to 5 kA.
The device consists in a 0.9 m long, 25 x 38 mm rectangular vacuum-insulated
canister. The superconducting sample and a superconducting current return lead
fit inside the canister, in such a way that the Lorentz force and torque produced
by the external magnetic field is cancelled. The sample temperature is controlled
in a 200 mm long area inside the canister where critical current measurements are
performed.
Critical current measurements were performed on a single YBCO tape at self-field
at temperatures between 20 K and 70 K. The results are similar to data provided
by the superconductor's manufacturer. The temperature reached the set point in
approximately 10 minutes, and was controlled within ±1 K. Results of heating power
required and difference between set point temperature and measured temperature as
functions of set point temperature are presented for two temperature control methods.
Thesis Supervisor: Joseph V. Minervini
Title: Senior Scientist, Division Head, Fusion Technology & Engineering
Thesis Reader: Anne White
Title: Assistant Professor of Nuclear Science and Engineering
3
4
Acknowledgments
I would like to express my deep gratitude to my advisor, Dr. Joseph Minervini, for the
great opportunity he gave me. Dr. Minervini has been very supportive and helpful,
always giving great advice both academic and "for real life".
I am very grateful to Dr. Makoto Takayasu, who had infinite patience with me,
showing me how to perform good experiments and how to get useful data, even when
everything fails. His help was also invaluable during the several 30-minutes meetings
we had practically every day, helping me solve every little problem I had.
Thanks to Prof. Anne White for reading this thesis, and for her patience in making
corrections until the very last moment.
I would also like to thank Don Strahan, for his help designing and building the
probe. He also taught me and helped me doing some basic machining on my own.
Thanks to my fellow coworkers Dr. Andre Berger and Dr. Michael Cheadle, for
being great friends and for the great talks during lunch. Their ability of thinking
outside the box would always help me solve many issues in just a few minutes.
Thanks to everyone in the group, especially Dr. Leslie Bromberg for help with
Comsol, and Darlene Marble for her assistance.
I am grateful for all the support and help I received from my family. I am very
very thankful to Peter and Ruth, without their help I would not be here now. Thanks
to them, and Andreas, Omar and Betty for receiving me in their house every now
and then, for a much needed family break. Thanks to my parents, Celia and Daniel,
and to my siblings, Dante, Irene and Soffa, for their support at six thousand miles
away. And special thanks to Leticia, she is always there for me.
Thanks to my friends in Boston. Mikhail, the best roommate ever; James, Emilio
and lain, tied in the second place (hahaha); Joaquin, who keeps following me from
Argentina; Hans, Noemie, and the rest... And to my friends around the world: Fer,
Julia, Pablo, Diego, Pablito, Ricar, Marie, Fede, Leo, Manna, Cris, Tito...
This work was supported by the U.S. Department of Energy, Office of Fusion
Energy Science under Grant DE-FC02-93ER54186.
5
U
Contents
1
2
3
13
Introduction
. . . .
. . . . . . . .
13
. . . . .
. . . . . . . .
17
1.3
Status of Development of YBCO TSTC .
. . . . . . . .
20
1.4
Motivation and Scope of Thesis . . . . .
. . . . . . . .
21
1.1
Background of Superconductivity
1.2
Superconductivity Applications
Design of the Experimental Device
23
2.1
Cross Section Design . . . . . . . . . . . . . . . . . .
24
2.1.1
Size Constraints . . . . . . . . . . . . . . . . .
24
2.1.2
Heat Loss Considerations . . . . . . . . . . . .
25
2.1.3
Mechanical Considerations . . . . . . . . . . .
27
2.2
Operational Design . . . . . . . . . . . . . . . . . . .
30
2.3
Thermal Simulations . . . . . . . . . . . . . . . . . .
34
2.3.1
Properties of Materials at Low Temperatures .
35
2.3.2
Model Description
. . . . . . . . . . . . . . .
37
2.3.3
Simulation Results . . . . . . . . . . . . . . .
41
Device and Experiment Construction
47
3.1
Device Construction
. . . . . . . . . . . . . . . . . .
47
3.2
Sample Construction . . . . . . . . . . . . . . . . . .
55
3.3
Instrumentation . . . . . . . . . . . . . . . . . . . . .
57
3.4
Experimental Setup . . . . . . . . . . . . . . . . . . .
59
7
4
5
Experimental Results and Discussion
63
4.1
Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
4.2
Critical Current Measurements
. . . . . . . . . . . . . . . . . . . . .
68
4.3
Current-Temperature Interaction . . . . . . . . . . . . . . . . . . . .
71
Conclusions
77
5.1
Sum m ary
5.2
Future Work.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . .
8
77
78
List of Figures
1-1
Resistance of a wire of mercury as function of temperature . . . . . .
14
1-2
Critical surface of a commercial alloy of NbTi . . . . . . . . . . . . .
15
1-3
Critical surfaces of NbTi and BSCCO-2223 . . . . . . . . . . . . . . .
15
1-4
Critical current measurement of a commercial YBCO superconductor,
at 77 K and self field . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
. . . . . . . . . . . .
19
1-5
Commercial SuperPower 2G HTS YBCO Wires
1-6
Cross section of SuperPower 2G HTS YBCO Wire, not to scale
. . .
19
1-7
Illustration of four different YBCO cabling methods . . . . . . . . . .
20
2-1
Schematic of the experimental device and how it is to be used
. . . .
24
2-2
Cross section of the magnets and the maximum probe cross section
.
25
2-3
Geometry used to calculate maximum heat loss due to conduction in
the cross section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
2-4
Canister configuration for a vacuum insulation . . . . . . . . . . . . .
27
2-5
Forces in the sample and current lead, depending on the orientation of
the m agnetic field . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
. .
31
2-6
Schematic cross section of the 14 T split pair magnet at NHMFL
2-7
Schematic cross section of the 2 T dipole magnet at MIT-PSFC
. . .
31
2-8
Schematic geometry of the probe, not to scale . . . . . . . . . . . . .
32
2-9
Solid model of the probe, and detail of the vacuum canister . . . . . .
33
2-10 Cross section of the sample holder . . . . . . . . . . . . . . . . . . . .
34
2-11 Longitudinal section of the sample holder . . . . . . . . . . . . . . . .
34
. . .
35
2-12 Cross section of SuperPower 2G HTS YBCO Wire, not to scale
9
2-13 Specific heat of YBCO tape and its materials as function of temperature 36
2-14 Thermal conductivity of YBCO tape and its materials as function of
temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-15 Reduced geometry for thermal simulations: exploded view
. . . . . .
2-16 Reduced geometry for thermal simulation: bottom and top views
. .
37
37
38
2-17 Cross section of the bottom copper joint, as modeled for the thermal
simulations
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
2-18 Cross section of the stainless steel structure, as modeled for the thermal
sim ulations
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
2-19 Solid model used for thermal simulations . . . . . . . . . . . . . . . .
41
2-20 Example of thermal simulation result . . . . . . . . . . . . . . . . . .
42
2-21 Results of a simulation of temperature as function of time (top) and
input heating power as function of time (bottom), with parameters
fi= 1.0, f2=0.1, f3= 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . .
42
2-22 Results of a simulation of temperature as function of time with parameters f1=1.0, f2=0.0, f3=1.0 . . . . . . . . . . . . . . . . . . . . . . .
43
2-23 Results of a simulation of temperature as function of time with parameters f1=1.1, f2=0.0, f3=0.9 . . . . . . . . . . . . . . . . . . . . . . .
44
2-24 Results of a simulation of temperature as function of time with parameters fi=1.13, f2=0.03, f3=0.87 . . . . . . . . . . . . . . . . . . . . .
44
2-25 Simulated steady state axial temperature distribution in the sample at
16 W total heating power, for two sets of shape factors . . . . . . . .
45
3-1
Picture of the top of the vacuum canister . . . . . . . . . . . . . . . .
48
3-2
Picture of the bottom of the vacuum canister
. . . . . . . . . . . . .
48
3-3
Picture of the vacuum canister . . . . . . . . . . . . . . . . . . . . . .
49
3-4
Picture of the probe without the vacuum canister . . . . . . . . . . .
50
3-5
Picture of the top flange of the probe . . . . . . . . . . . . . . . . . .
51
3-6
Detail of the joint between the current leads and the sample . . . . .
52
10
3-7
Detail of the joint between the current leads and the sample, with the
sam ple m ounted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3-8
Instrumentation wires connectors and top flange of the vacuum canister 53
3-9
Picture of the bottom joint . . . . . . . . . . . . . . . . . . . . . . . .
53
3-10 Picture of the probe, assembled and prepared for an experiment . . .
54
3-11 Pictures of a sample and the current return lead, with sample holder
55
3-12 Schematic drawing of the three different samples . . . . . . . . . . . .
56
3-13 Picture of the sample with instrumentation, and schematic drawing of
the sam ple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
3-14 Picture of the dewar and 2 T magnet . . . . . . . . . . . . . . . . . .
60
3-15 Setup to leak-check the inside of the vacuum canister . . . . . . . . .
61
3-16 Current supply response to a "dump" signal . . . . . . . . . . . . . .
62
4-1
Evolution of the temperature measured by sensor C as a function of
time for different set points
4-2
. . . . . . . . . . . . . . . . . . . . . . .
Difference between the measured and set point temperature as a function of set point temperature for the temperature control method #1
4-3
65
Difference between the measured and set point temperature as a function of set point temperature for the temperature control method #2
4-4
65
Heating power required as function of temperature for the temperature
control method # 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-5
..
67
..................................
V-I curves of a single 4 mm wide Super Power YBCO tape, measured
at self-field and temperatures from 20 K to 70 K.
4-8
66
Comparison of axial temperature distribution simulated and measured
at 70K .......
4-7
66
Heating power required as function of temperature for the temperature
control method # 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-6
64
. . . . . . . . . . .
69
V-I curves of a single 4 mm wide Super Power YBCO tape, measured
at self-field and temperatures from 20 K to 70 K in log-log scale. .
11
.
.
69
4-9
The critical current at 100 pV/m and n-value of a single 4 mm wide
SuperPower YBCO tape as a function of temperature . . . . . . . . .
70
4-10 Critical current at self-field normalized to the value at 77 K, self-field,
as a function of temperature measurements for a single SuperPower
YBCO tape. SuperPower data of normalized critical current at selffield as a function of temperature is shown for comparison
. . . . . .
70
4-11 Temperature variation while performing critical current measurements
in liquid nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
4-12 Temperature variation while performing critical current measurements
in gaseous helium . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
4-13 Temperature runaway and quench of sample #1 . . . . . . . . . . . .
73
4-14 Temperature runaway and quench of sample #2 . . . . . . . . . . . .
73
4-15 Picture of sample #1 after quench . . . . . . . . . . . . . . . . . . . .
74
4-16 Picture of sample #2 after quench . . . . . . . . . . . . . . . . . . . .
75
12
Chapter 1
Introduction
Superconducting materials brought a breakthrough in the design and construction
of large-scale electric and magnetic devices. The non-resistive characteristic of these
materials allows achieving performances that would be either too demanding or economically impossible with conventional materials.
In this chapter, the main characteristics of superconductors and its applications
are discussed. A general background of superconductivity is given in the first section.
The following section summarizes the most important applications for superconductors, and the engineering challenges in order to build them. In the third section the
motivations for the work of this thesis are presented.
1.1
Background of Superconductivity
Superconducting materials have virtually zero electrical resistivity under certain conditions. Most commonly, the superconductive state is reached by sufficiently lowering
the temperature of the material. In Figure 1-1 the transition from normal to superconductor for a mercury wire is shown, as measured for the first time by H. K. Onnes
in 1911.
The other two major factors that influence in the superconducting property of
these materials are the electrical current density through the superconductor, and
the external magnetic field applied to it. These two factors and the temperature of
13
T1
4 10
I~
_____________________________V
OOf
to
0,00 .
ivo0
4''10
470
Figure 1-1: Resistance of a wire of mercury, in , as a function of temperature, in K.
At 4.2 K the mercury wire becomes superconductive. This is the first superconductive
material discovered, in 1911 [11].
the material define what is known as the critical surface of the superconductor. As
an example, the critical surface of a commercial alloy of NbTi is shown in Figure 1-2.
For a superconductor to remain in the zero resistivity zone, its current density
has to be lower than the critical current density: the current density of the point in
the critical surface determined by the material's temperature and the magnetic field
applied to it.
From Figure 1-2 the parameter critical temperature of the superconductor can be
defined. It is the temperature on the critical surface such that both the magnetic field
and the current density are zero: at temperatures higher than the critical temperature
the material is not superconductive.
Often, commercially available superconductors are divided in two groups: low
14
JkA/m2]
T [K]
F r 12B [T
Figure 1-2: Critical surface of a commercial alloy of NbTi [261.
J[A/cm 2 ]
-10'
- Z.2
I%.
K
P4I[T]
100
T[K]
Figure 1-3: Critical surfaces of NbTi and BSCCO-2223 [10].
15
temperature superconductors (LTS) such as Nb3 Sn and NbTi, and the high temperature superconductors (HTS) such as YBCO 1 and BSCCO2 . In Figure 1-3 the critical
surfaces of a LTS (NbTi) and a HTS (BSCCO-2223) are compared.
There are several differences between LTS and HTS; the most important are the
critical temperature (for LTS close to liquid helium, for HTS higher than liquid nitrogen), and the material characteristics (LTS are metallic alloys or compounds, while
HTS are ceramics). Also, LTS were discovered decades earlier than HTS, and thus
the development of fabrication and cabling methods for LTS conductors are much
more advanced.
200
d 150
a 100
o 50
0
0
20
40
60
80
Current [A]
100
120
Figure 1-4: Critical current measurement of a commercial YBCO superconductor, at
77 K and self field.
In order to experimentally obtain the critical current density of a superconductor,
measurements of voltage drop along its length are performed as a function of current
through it. The voltage drop varies smoothly with current near the critical current,
as is shown in Figure 1-4 for a commercial YBCO superconductor. In this work, the
critical current will be obtained as the value of current, at a fixed magnetic field and
temperature, when a voltage drop of 100 pV/m is developed through the superconductor. From Figure 1-4, the critical current is 108 A. The critical current density is
1Yttrium
2
Barium Copper Oxide (YBa 2 Cu 3 Or).
Bismuth Strontium Calcium Copper Oxide. There are two main compounds: BSCCO-2212
(Bi 2 Sr 2 CaCu208+x) and BSCCO-2223 (Bi 2 Sr 2 Ca 2 Cu 3 010+x).
16
obtained dividing the critical current by the cross section of the superconductor, in
this case 0.4 mm, and the critical current density is 270 A/mm 2. This convention is
usually referred to as "critical current density at 100 pV/m".
The steepness of the transition is an indicator of the quality of the superconductor. An ideal superconductor would have a step-like transition, while a degraded
superconductor would present a flatter transition. The parameter that quantifies the
steepness of the transition is the n-value, and is obtained from fitting a power law to
the transition data:
VcJ"
(1.1)
where V is the voltage drop in the superconductor, J the current density, and n the
n-value. In the case shown in Figure 1-4, the n-value is 26.
1.2
Superconductivity Applications
Superconductors are very attractive for high current density or high magnetic field
applications, where the overall energy consumption would be prohibitive if a normal
conductor (such as copper) was used.
There are six major fields of application of superconductivity:
" Fusion Energy: magnetic confinement nuclear fusion reactors require high magnetic fields and high current densities to operate. The use of normal conductor
coils would be prohibitive as they would consume 30% - 50% of the electric
power generated. Superconducting coils are ideal for this application, and the
next milestone in fusion energy investigation and development, the International
Thermonuclear Experimental Reactor (ITER) will be built using superconductors (NbTi and Nb 3Sn) in all its magnet systems.
" High Energy Physics: in high energy physics experiments, superconducting
magnets are used to accelerate, focus and analyze beams of energetic particles.
17
The largest particle accelerator in the world, the Large Hadron Collider (LHC),
has more than 1600 NbTi magnets.
* Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI):
superconducting magnets are widely used in NMR and MRI applications, since
they produce a very stable and intense DC magnetic field over large volumes.
The use of conventional magnets would consume much more power, and would
not achieve the imaging quality of superconducting magnets.
" Power Transmission Cables: the zero resistance property of superconductors
allows these materials to carry DC high currents with practically no power
dissipation. In certain cases, the installation and operation cost of the superconductor cables (including the cryogenic systems to keep it below the critical
temperature) are lower than the costs of normal conducting transmission cables.
A superconducting power transmission cable has been built in the Long Island
Power Grid, and it is operating since 2008.
" Superconducting Magnetic Energy Storage (SMES): a superconducting magnet
does not experience DC power dissipation, and as such it can be used to store
large amounts of magnetic energy with virtually no losses. Several SMES systems have been demonstrated since 1985, and are now commercially available.
" Magnetic Levitation (Maglev): several projects of magnetic levitation trains
using superconductors are being developed, including the JR-Maglev in Japan
which holds the world speed record for all trains. Magnetic levitation allows
reducing maintenance of the trains, because they do not require the traditional
moving parts (such as gears, wheels, axles...).
These applications require different superconducting cables technologies. For instance, the main challenge of power transmission cables is cooling the superconductors, and the external magnetic field in the superconductor is zero, thus high temperature superconductors operated with liquid nitrogen or gaseous helium are favored. For
nuclear fusion reactors, the fusion power density increases dramatically with larger
18
magnetic fields, and subcooling the superconducting coils for increased performance
is an economically attractive option.
Figure 1-5: Commercial SuperPower 2G HTS YBCO Wires [9].
Copper
Stabilizer
Silver Ovrlayer
20 pm
-
--
(RE)BCO - HTS (epitaxial)
Buffer Stack
2 pm
-0.2 pm Substrate
'not to scale: sSI4050
l8y
20 pm
Figure 1-6: Cross section of SuperPower 2G HTS YBCO Wire, model SCS4050, not
to scale [9]. The superconductor is 1 pm thick.
Low temperature superconductors NbTi and Nb 3 Sn, and high temperature superconductor BSCCO-2212 are commercially available in fine round wires, and cabling methods for conventional conductors can be used to bundle them. YBCO and
BSCCO-2223 conductors are tapes, usually 0.1-0.3 mm thick and 4-12 mm wide, and
new cabling methods must be developed in order to use them for engineering applications. In Figure 1-5 a picture of SuperPower YBCO tapes is shown, and in Figure
1-6 a detail of the cross section of a standard commercial configuration. Depending
on the manufacturer and model, different tape configurations are available; however,
in all cases layers with high electrical resistivity are present (in the case of the figure,
the substrate -hastelloy- and the buffer layers), allowing current into the YBCO layer
only from one side of the tape.
A few cabling methods for making high current YBCO cables have been proposed,
19
Figure 1-7: Illustration of four different YBCO cabling methods. From left to right:
helical winding on a round former [13], ROEBEL cable [7], CORC cable [24], TSTC
cable [18].
such as helical windings on a round former [13], ROEBEL cabling of tapes cut in a
zigzag pattern [7], conductors on round core (CORC) [24] and twisting stacked tape
cables (TSTC) [20, 18]. These cabling methods are illustrated in Figure 1-7. TSTC
provides high current density, efficient tape usage and allows constructing cables for
high current density and high magnetic field applications, such as nuclear fusion
reactors.
1.3
Status of Development of YBCO TSTC
The excellent high current capabilities at high magnetic fields of HTS, especially
YBCO, make them very attractive for magnet applications. In particular, the use of
YBCO conductors instead of LTS for coils in nuclear fusion would greatly improve
the performance, as has been proposed in several studies [8, 2, 1]. Though ITER is
being built with LTS magnets, the next step in the development of magnetic confinement nuclear fusion energy, the DEMO reactor, could greatly benefit by using YBCO
magnets.
Of the four cabling methods mentioned in the previous section, TSTC is the only
20
one suitable for making long lengths of high current, high current density magnetgrade cables. The helical winding on a round former design has low current density,
and is ideal for transmission cables. The ROEBEL and CORC designs, due to manufacturing difficulties, are hard to scale-up to the long lengths required for magnet
construction, and the proposed electrical joint mechanisms between cables are very
complex.
TSTC, on the other hand, can meet the performance requirements on current and
current density, and a simple industrial scale assembling method has been proposed
[19]. As demonstrated in [21], a 2 m TSTC cable has been built and tested at self
field and 77 K, and a 2.5 turn pentagon shaped coil of less than 165 mm diameter
has been constructed and tested at a background field of 19.7 T at 4.2 K. A relatively
simple method for building low-resistance electrical joints has been developed and
tested [18].
For engineering applications of YBCO TSTC, a complete critical surface characterization is required. Some measurements have been performed at 77 K and 4.2 K,
but intermediate temperatures (around 20-50 K) can be of practical interest due to
lower cooling costs than 4.2 K, while keeping a much better performance than at 77 K.
Single tape data is readily available, but some degree of degradation is expected due
to increased self magnetic fields, and full cable experiments are required.
1.4
Motivation and Scope of Thesis
Conventional experimental apparatus for testing superconducting samples can not
be used for testing YBCO TSTC cables, due to their large characteristic length.
An experimental device for testing TSTC, CORC and other YBCO cables has been
developed by Barth et al. at the Karlsruhe Institute of Technology [3], but the
temperature uncertainties of his measurements may be as high as ±5-10 K.
In order to perform a complete and precise characterization of YBCO TSTC, a
new experimental device for YBCO TSTC critical surface characterization has been
21
designed, built and tested. This probe3 allows to characterize TSTC cables at temperatures between 4.2 K and 80 K within ±1 K, and in magnetic fields up to 2 T
or 14 T. The cables will be subject of currents up to 5 kA. Characterization experiments can be performed in two different magnetic facilities, with magnets submerged
in liquid helium, and the experimental device fits in both of them while providing
adequate temperature control.
In this thesis, the design and construction of this experimental probe are detailed,
and results of temperature control and critical current measurements performed at
self field in one of the magnet facilities for a single YBCO tape are presented. The
critical current results are similar to data from the superconductor manufacturer,
validating the design of the probe for TSTC characterization.
The device consists of a helium gas filled canister, with a surrounding vacuum
layer. The superconducting sample is in contact with liquid helium far away from the
measuring area, and the temperature of the sample is controlled with three electric
heaters. The design of the experimental probe is detailed in the second chapter of
this thesis. The construction of the probe and the experimental setup is described
in the third chapter. In the fourth chapter the results are presented and discussed.
Finally, the fifth chapter concludes this thesis with a summary of the results, feedback
to improve the experiment and planned future work.
3
Throughout this thesis the terms "device" and "probe" will be used indifferently, referring to
the same experimental device.
22
Chapter 2
Design of the Experimental Device
The experimental probe is designed to measure the critical current of YBCO TSTC
samples as a function of magnetic field and temperature. The samples will be tested
with currents up to 5 kA, and their temperature will vary between 4.2 and 80 K.
The twist pitch of the superconducting cable is approximately 130-200 mm. To
reduce end effects and current redistribution in the measurement area, the electrical
joints will be placed at least a twist pitch away from it (making the total length of
the sample at least 600 mm).
The sample will be kept straight. In order to get a perpendicular magnetic field
for such a long sample length, a dipole or split-pair magnet will be used.
Avail-
able magnetic facilities are a 2 T superconducting dipole magnet, located in building
NW22 at MIT-PSFC, and a 14 T superconducting split-pair magnet, located in the
National High Magnetic Field Lab (NHMFL) at Florida State University, in Tallahassee, Florida.
The selection of magnetic facilities, sample geometry and current and temperature
ranges of operation give the conditions the probe must fulfill:
1. Operate at 4-80 K, with low temperature gradients in the measurement area.
2. Fit in both magnet facilities.
3. Operate with DC currents up to 5 kA, and be able to mechanically support the
associated Lorentz forces.
23
4. Allow long YBCO TSTC samples to be mounted, tested and removed easily.
5. Consume as little power as possible and allow quick measurements, for liquid
helium economy.
Cross Section Design
2.1
2.1.1
Size Constraints
The 2 T dipole magnet has a 50 mm diameter circular bore, and a homogeneous field
region approximately 220 mm long. The 14 T split-pair magnet has a 30 x 70 mm
rectangular bore, and a homogeneous field region approximately 150 mm long. The
magnetic field is perpendicular to the 70 mm side of the bore. Both magnets have to
be immersed in liquid helium during operation. The probe, then, will have to operate
in a liquid helium environment. A schematic of the device and how it is to be used is
shown in Figure 2-1.
Top of Dewar
Structural
support
Probe
Magnet bore
Measurement
area
Magnet
Transverse
magnetic field
Bottom of Dewar
Figure 2-1: Schematic of the experimental device and how it is to be used
In order to fit in both magnets, the probe has to have a small enough cross section.
In Figure 2-2 the maximum possible probe cross section is shown.
24
Florida magnet cross section
Local magnet cross section
Maximum canister cross section
Figure 2-2: Cross section of the 2 T dipole magnet at MIT-PSFC, and the 14 T
magnet at NHMFL. The maximum cross section of the probe is such that fits in both
bores.
Commercially available stainless steel components are preferred for the probe.
The tube with the largest cross section that fits in both magnet bores is a rectangular
25.4 x 38.1 mm (1" x 1.5") tube, with wall thickness 1.65 mm (0.065").
2.1.2
Heat Loss Considerations
Two thermal insulation methods were considered for the sample: a vacuum layer, or
a thermal insulator.
An estimate of the maximum heat loss due to conduction through a thermal
insulator has been done, considering the geometry shown in Figure 2-3: the inside of
the rectangular steel tube filled with Styrofoam, with a small rectangular section of
7 x 21 mm where the sample would be placed.
The thermal conductivity of Styrofoam is approximately constant in the 4-80 K
range, 0.01 W/mK [14]. Temperature gradients in the sample and steel tube have
been neglected since the thermal conductivity of these materials is at least 100 times
larger than that of Styrofoam. Heat loss per unit length can be approximated as:
25
1.37"= 34.9 mm
0.87" 22.1 mm
liquid He 4 K
Figure 2-3: Geometry used to calculate maximum heat loss due to conduction in the
cross section.
q' =:ktydT
T2
- (T 1 - T2 ) -
[kJty
i
(2.1)
-[-
where P(x) is the cross sectional perimeter at position x along the path of heat flow,
T1 is the inner temperature (80 K), T2 is the outer temperature (4.2 K), and kstv is
the thermal conductivity of Styrofoam (0.01 W/mK). As a conservative estimation
of the heat path integral, P(x) was taken to be the outer perimeter of Styrofoam,
114 mm; with x the horizontal distance between the sample area and the border of
the Styrofoam, 7 mm. In this situation, the heat loss per unit length is 12 W/m.
An estimate of the heat loss due to radiation through a vacuum insulation has
been done, considering the geometry shown in Figure 2-4: the system is composed
of two circular steel tubes, with outer diameter 15.9 mm (5/8") and wall thickness
0.89 mm (0.035"), inside the rectangular tube.
Heat loss per unit length can be estimated as:
q'= -cPT4
(2.2)
where o is the Stefan-Boltzmann constant, c is the emissivity of stainless steel, approximately 0.1 [5]; P the outer perimeter of the two tubes, and T the temperature
of the tubes (80 K). In this situation, the heat loss per unit length is 0.023 W/m.
26
liquid He
vacuum
Figure 2-4: Canister configuration for a vacuum insulation
The vacuum layer insulation method was chosen in order to minimize the heat
loss on the cross section.
2.1.3
Mechanical Considerations
The cross section shown in Figure 2-4 can balance the Lorentz forces in the conductors,
if the magnetic field is aligned in such way that the forces in the sample and in the
current return lead are inward against each other (Figure 2-5). In this situation, a
small misalignment of the probe with respect of the magnetic field would not produce
a net force, but a net torque.
An estimate of the maximum torque expected is the maximum misalignment in
the 14 T split pair magnet. In that case, the probe can be misaligned at a maximum
angle such that the opposite corners of the cross section are in contact with the
magnet bore walls. That angle 0 can be estimated as:
- a
0 ~,w a
b
(2.3)
where w is the width of the magnet bore (30 mm), and a and b are the probe dimensions (25.4 mm and 38.1 mm respectively). The net torque T on the canister will
be:
27
current
lead,
sample
F
B
F
Configuration with net force = 0 and torque = 0
current
lead
sarnple
8
FF
Configuration with net force = 0 but torque # 0
Figure 2-5: Forces in the sample and current lead, depending on the orientation of
the magnetic field. "+" and "-" indicate current direction.
T = 0 -d - l - I - B
(2.4)
where d is the distance between the sample and current return lead (15.9 mm), 1 the
length of the magnetic field zone (approximately 600 mm), I is the current intensity
(maximum 5 kA), and B is the magnetic field intensity (maximum 14 T). The value
of the torque is 81 Nm.
To estimate the stress this torque generates in the tubes, one tube will be considered as a free tube, subject to half the torque. In this situation, the maximum shear
28
stress T can be estimated as [22]:
r -
T
Tr
r25
2.27r 3 t
2J
(2.5)
where r is the radius of the tube (7.9 mm), t its thickness (0.89 mm), and J the polar
moment of inertia (equal to 27r 3 t in the case of a thin cylindrical tube). The result
is 115 MPa.
Additional stresses will come from the pressure difference between the liquid helium at atmospheric pressure and the vacuum layer. The maximum stress of the
system will be in the wider face of the rectangular tube. Since the length of the tube
is much larger than the width, the stress on the tube depends on a parameter a [23],
determined as the smaller positive root of:
a(1 - a) =
354 D)2
(2.6)
where D is the flexural rigidity of the face of the tube, p the pressure difference
(approximately 1 bar), t is the wall thickness of the tube (1.65 mm) and a the width
(38.1 mm). D is calculated as:
D
=
Et 3
12(1 - V2)
(2.7)
where E is the material's Young modulus (approximately 250 GPa [16]), and v its
Poisson's ratio (0.3).
The maximum tensile stress u
and bending stress o7" are
calculated as:
a~r 2 D
ta=2
,,
X
3 pa2 2 [cosh2 (
4 t2
(2.8)
a) - 1]
) cosh(
)
And the maximum stress is the sum of them. Using the values previously given,
the maximum stress in the rectangular tube is 41 MPa.
29
The yield strength of stainless steel at cryogenic temperatures is about 400 MPa
[5]. Since the maximum combined stress in both circular and rectangular tubes is
much lower than their yield strength, the design is strong enough for this application.
2.2
Operational Design
The 14 T split pair magnet is fixed inside a 1.9 m deep Dewar. In Figure 2-6 a
schematic is shown of the 14 T magnet in its Dewar. The 2 T dipole magnet will be
located in a 2.4 m deep Dewar. In Figure 2-7 a schematic is shown of the 2 T magnet
in its Dewar. The final position of the magnet inside the Dewar can be adjusted, in
such way that the same probe fits both magnets with the sample measurement area
in the uniform magnetic field zone of each of them.
The proposed geometry of the probe is shown in Figure 2-8, and a solid model of
the probe is shown in Figure 2-9, with a detail of the vacuum canister. The bottom
part ("A" in Figure 2-8) houses the electrical joint between the sample and the current
return lead. Most of the space in that area is occupied by a copper joint between the
superconductors, in order to reduce joule heat generation. The middle part ("B") is
the vacuum canister, where the measurements will be performed. The top part ("C")
acts as mechanical support and features the current leads from room temperature to
the cryogenic area.
The longitudinal dimensions of the probe are determined by the 14 T magnet.
The bottom of the probe is about 10 mm higher than the bottom of the magnet's
Dewar. The top of the vacuum canister is about 100 mm higher than the top of the
magnet.
In order to keep a gaseous helium ambient inside the vacuum canister, it needs
to be tightly sealed on the top. To reduce complexity of that joint, the sample
and current return lead will be mounted from the bottom of the canister, with the
instrumentation wires coming out of the measurement area also from the bottom.
This way, the only openings required in the top of the canister are for the sample and
current lead, and the structural components can be welded in place.
30
top of Dewar
1138 mm
magnet bore
split
384 mm
magnet
uniform
field
394 mm
zone
I
I
I
I
bottom of Dewar
Figure 2-6: Schematic cross section of the 14 T split pair magnet at NHMFL
top of Dewar
1300 mm
magnet bore
dipole
magnet
210 mm
-4
uniform
field
zone
876 mm
bottom of Dewar
Figure 2-7: Schematic cross section of the 2 T dipole magnet at MIT-PSFC
31
Current Lead
Ambient temperature
Support
Gaseous He
Zone C
Uquid He
Rectangular tube
Circular tube
Vacuum
Zone B
Sample
Gaseous He
Zone A
Copper Joint
Liquid He
Figure 2-8: Schematic geometry of the probe, not to scale
A steel tube section separates liquid and gaseous helium in zone "A". For easy
access to the electrical joint, the tube is not welded to the vacuum canister, and
32
A
1.9 m
9?
1
0.9 m
t
Figure 2-9: Solid model of the probe, and detail of the vacuum canister. References:
(1) outer rectangular tube; (2) inner circular tubes; (3) sample.
instead just held mechanically. Hermetically sealing this area is desirable but not
critical, since the heat generation in the joint would evaporate any small amount of
liquid helium that comes into contact with it.
Inside the vacuum canister, the sample and current lead must be supported against
the Lorentz force, and electrically isolated from the metallic structure. The support
design is shown in Figures 2-10 and 2-11, made from half a thin stainless steel tube
with inner diameter 7 mm, supported by "C"-shaped G10 pieces. The G1O pieces fit
tightly inside the circular tubes of the vacuum canister. The distance between the
33
R3.500
R4.763
R7.049-
Figure 2-10: Cross section of the sample holder. The white component is made of
G10, the grey component is half a stainless steel tube. Dimensions in mm.
Figure 2-11: Longitudinal section of the sample holder. The white components are
made of G10, the grey component is half a stainless steel tube. Dimensions in mm.
G1O pieces is such that the tube will support the maximum Lorentz load expected.
At a magnetic field of 14 T with 5 kA of transport current, this force is 70 kN/m.
2.3
Thermal Simulations
In the real experiment, the temperature of the sample has to be increased in steps
of approximately 10 K, and the time between measurements needs to be short (10
to 15 minutes), to avoid consuming too much liquid helium. The temperature in the
measurement area has to be very uniform, otherwise the "hot spot" will be a weak
point in the sample and damage may occur.
To check if appropriate temperature gradients can be theoretically achieved, and
34
in a short time period, a thermal simulation has been performed with the commercial
software Comsol Multiphysics. A simplified geometry has been analyzed, with similar
boundary conditions to those of the real experiment, as shown in Sections 2.3.2 and
2.3.3.
2.3.1
Properties of Materials at Low Temperatures
As mentioned in the Introduction, commercial YBCO tapes are composed of several
layers of different materials, depending on the manufacturer. The experimental probe
was designed to measure SuperPower SCS4050 YBCO tapes. These tapes are 4 mm
wide, and less than 0.1 mm thick; the cross section is illustrated in Figure 2-12.
From a thermal perspective, the most important materials in the YBCO tapes are
copper, silver and the substrate (Hastelloy), because they fill the largest area of the
cross-section.
Copper Stabilizer
Silver Overlayer20 pm
(RE)BCO - H TS (epitaxial)
Buffer Stack
2 pm
~M
0.2 pm Substrate
50 pm
* oscale
o
sc451.8
pm
20pm
Figure 2-12: Cross section of SuperPower 2G HTS YBCO Wire, model SCS4050, not
to scale [9]. The superconductor is 1 pm thick. From a thermal point of view, the
most significant layers are the substrate (Hastelloy, 50 pm thick), the silver overlayer
(approximately 3.8 tm) and the copper stabilizer (40 pm).
For modeling simplicity, this complicated cross section can be reduced to a homogeneous cross section, with an equivalent mass density (p), specific heat (6) and
thermal conductivity (I). The expression to obtain each equivalent property are:
p = E(2.10)
35
.- Itipici
c i(2.11)=
(tipi
Et ki
k= E t
(2.12)
zti
where t%is the thickness of the material i, pi its mass density, ci its specific heat
and ki its thermal conductivity. The mass density of these materials varies much less
than the other properties in the temperature range of interest (4-80 K), and will be
assumed constant; these values, along with the equivalent mass density, are shown in
Table 2.1.
Mass density [kg/m 3 ]
10490
8940
8890
8979
Thickness [pm]
3.8
40
50
93.8
Material
Silver
Copper
Hastelloy
Equivalent
Table 2.1: Thickness and mass density of the materials in a SuperPower 2G YBCO
tape [4].
3
---10
-V -Copper
102 -Hastelloy
-Silver
010 -Equivalent
u
!10
0
1
10
0
1
10
Temperature [K]
2
10
Figure 2-13: Specific heat of Copper [15], Silver [6] and Hastelloy [12, 25] as function
of temperature, and equivalent specific heat for a homogeneous material with the
same thermal characteristics. Matlab functions by Dr. A. Berger [4].
The specific heat and thermal conductivity of these materials as function of temperature, with the equivalent specific heat and thermal conductivity, are shown in
Figures 2-13 and 2-14 respectively.
36
13
10F
-Co6pper
-Hastelloy
10 -Silver
1-0
S10
-Equivalent
10
...
..
0
10
10
Temperature [K]
2
10
Figure 2-14: Thermal conductivity of Copper [15], Silver [17] and Hastelloy [12, 25]
as function of temperature, and equivalent thermal conductivity for a homogeneous
material with the same thermal characteristics. Matlab functions by Dr. A. Berger
[4].
2.3.2
Model Description
It was inconvenient to perform a thermal simulation of the entire system, as shown in
Figure 2-8, because the only part of interest is the sample, which represents a small
fraction of the volume of the probe and is thermally isolated from those components.
A reduced geometry was proposed, to obtain some insight of the thermal dynamics of
the system. The reduced geometry is composed of six components: sample, current
return lead, bottom electrical joint, sample holder, current return lead holder, and
stainless steel structure. An exploded view of the reduced geometry is shown in Figure
2-15, and its bottom and top views are shown in Figure 2-16.
11m
T
2
Figure 2-15: Reduced geometry for thermal simulations: exploded view. References:
(1) Sample; (2) Current lead; (3) joint between them; (4) Sample holder; (5) Current
lead holder; (6) Stainless steel tubes.
37
Bottom view
/
8
Top view
\3
6
Figure 2-16: Reduced geometry for thermal simulation: bottom and top views. References: (1) Sample; (2) Current lead; (3) joint between them; (4) Sample holder; (5)
Current lead holder; (6) Stainless steel tubes.
The sample and current return lead were simulated as cylinders of 7 mm and
8.4 mm in diameter respectively, and 1055 mm in length. The material has the
equivalent properties calculated in the previous section.
The bottom joint is a 70 mm long copper piece. Its cross section is shown in
Figure 2-17. The two near-horizontal faces are tangent to the cylindrical faces, and
these match perfectly the sample and current lead.
The sample holder is as shown in Figures 2-10 and 2-11. The length is 985 mm.
It is composed of two materials, G1O and stainless steel 316. The current return lead
holder was modeled as a solid 985 mm long half-tube of G10, with inner diameter
8.4 mm and outer diameter 15.9 mm.
The stainless steel structure represents the half of each of the circular steel tubes in
the canister, with a steel filling between them, simulating a welded joint, to improve
thermal contact. The length of the structure is 985 mm. The outer parts of the
circular steel tubes have not been simulated because they have lower effect in thermal
conduction, due to small cross section and not being in contact with other materials.
A cross section of the simulated steel structure is shown in Figure 2-18.
The cryogenic thermal properties of copper, stainless steel 316 and G10 were
included in the simulation software. The system was modeled as completely isolated,
except in the bottom where the bottom face of the sample, current lead and copper
joint are forced to be 4.2 K (simulating those faces being in contact with liquid
helium), and in the top where the top face of the sample and current lead are forced
38
----- 15.520
-
-
R3.500
R,0
8.367
15.875
Figure 2-17: Cross section of the bottom copper joint, as modeled for the thermal
simulations. dimensions in mm.
to be at 4.2 K.
For this simulation, gaseous helium was completely neglected. This simplification
removes a cooling mechanism from the simulation. Since the design of the probe does
not provide a path for gaseous helium to circulate through the sample area, convective
cooling of the sample would be minimal. The thermal conductivity of YBCO is very
high, and conduction cooling axially through the sample will dominate the thermal
dynamics. As a result, the heating power required to maintain a certain temperature
will only be slightly underestimated.
Simulating the control heaters, a distributed superficial heat source q"(z, t) was
placed around the sample, according to:
(2.13)
q"(z, t) = q'w(t) -f (z)
with q'/'(t) a factor related to the total heating power q(t), and
39
f (z)
a shape factor,
R7.049
R7.938
15.7 5
13.106
2
6.917
-
15.875
Figure 2-18: Cross section of the stainless steel structure, as modeled for the thermal
simulations. dimensions in mm.
function of the axial position. The shape factor was defined as follows:
z < 360
0
fi 360 < z < 370
f(z)=
f2
370 < z < 500
f3
500 < z < 510
0
z > 510
(2.14)
with z in mm, and z = 0 at the bottom of the sample. The values of fi,
f2
and
f3
are
input parameters, kept constant during each simulation run. The relation between
q'(t) and q(t) is:
q(t) =
q"(z, t)dA = q'1'(t
7)
f (z)dz
q(t) = q'i(t) (fi + 13f2 + f3) - 2.2 - 10-4M2
40
(2.15)
(2.16)
To simulate the temperature PID control that will be used for the experiment, the
heating power will be set with steps every 10 minutes; each step has an initial constant
overshooting to improve the thermal dynamics, during the first 150 seconds, and then
a lower constant power value during 450 seconds. The overshooting factor, the shape
factors, and the power as function of time were changed between simulations, and a
selection of results are presented in the next section.
2.3.3
Simulation Results
In Figure 2-19 the solid model used for thermal simulations is shown.
The area
marked as "measurement area" corresponds to the area where the critical current
measurement voltage taps are located, and is where the distributed heating source
is located. Four point-temperature measurements were set up, three in the sample
(Ti-T 3 ) and one in the current return lead (T4 ). The axial position of the temperature
measurements is: Ti: 510 mm, T2 : 420 mm, T3 : 360 mm, T4 : 420 mm.
Boundary condition: 4.2K
T1
T2
measurement
Boundary
condition: 4.2K
0
_001
Figure 2-19: Solid model used for thermal simulations. The area marked as "measurement area" corresponds to the area where critical current measurement will be
performed. The position of four point-temperature measurements is shown.
The results of the simulation are the temporal evolution of the temperature of the
system. A qualitative example is shown in Figure 2-20, for a maximum temperature
41
A 94.726
06
0,5
0
0
#'1
V 42
Figure 2-20: Example of thermal simulation result. The color code is not enough to
measure the temperature gradients, and point temperature data is required.
60
--- T
T2
50
T3
--
40
3020
S
0
Time [min)
15
E10
-
0
10
20
30
Time [min]
40
50
60
Figure 2-21: Results of a simulation of temperature as function of time (top) and
input heating power as function of time (bottom), with parameters fi=1.0, f2=0.1,
f3=1.0.
42
on the sample of 95 K. For quantitative analysis, the evolution of the four pointtemperatures mentioned before (T 1-T4) and the input heating power as a function of
time are better figures to analyze.
The first simulation was intended to relate heating power with steady state temperature on the sample. A heating power overshooting of 20% was used, and the shape
parameters were: fi=1.0, f2=0.1, f3=1.0. In Figure 2-21 the results of temperature
as function of time are shown, along with the input heating power as function of time.
Additional simulations were performed, changing the overshooting and the shape
parameters. The goal of changing these parameters is to obtain a flat temperature
profile in the measurement area, and reach a steady state temperature distribution in
ten minutes. To better show the improvements, the results of temperatures T1, T2, T3
versus time for three sets of shape parameters and overshooting factors (obtained by
trial and error) are shown in Figures 2-22, 2-23 and 2-24. For clarity only the last two
temperature steps are shown. As can be seen in the figures, the progressive changes
in the shape parameter helped obtain a flatter temperature distribution through the
sample area.
.100 -T
--
'T
90
x
3
.80
70
65
70
80
75
Time [min]
85
90
Figure 2-22: Results of a simulation of temperature as function of time with parameters
fi=1.0,
f2=0.0, f3=1.0.
For the experiment, the heater can be set up to mimic the simulated heating power
distribution (for example, different winding density in case of a wire heater); this
43
0)
65
70
80
75
Time [min]
85
90
Figure 2-23: Results of a simulation of temperature as function of time with parameters f1=1.1, f2=0-0, f3=0.9.
4-2
E-I
65
70
80
75
Time [min]
85
90
Figure 2-24: Results of a simulation of temperature as function of time with parameters f1-1.13, f2=0.03, f3=0.87.
44
1001
75
50
25
-f = [1.10, 0.00, 0.90]1
0
-f = [1.13, 0.03, 0.87]
0.2
0.4
0.6
0.8
Axial position [m]
0
1
Figure 2-25: Simulated steady state axial temperature distribution in the sample at
16 W total heating power, for two sets of shape factors. The shape factor is expressed
as f = [fi f2 f3], and the corresponding areas are shown. The profile only changes
in the measurement area, between the two temperature peaks: the distributed heat
in the measurement area helps reduce the temperature gradients. The goal for the
experiment is to have a flat temperature profile in the measurement area.
solution however does not provide flexibility adjusting the shape factor if required.
Dividing the heater in three separate heaters: one for the sample area, and one
additional in either end of the sample area, and using separate power sources for
these three heaters will help achieve a good temperature distribution while allowing
to adjust the shape parameters during the experiment.
The effect of the shape parameters can be seen in Figure 2-25. In that figure
are shown the simulation results of steady state axial temperature distribution in the
sample, at 16 W total heating power, for a simulation with shape factors fi=1.1,
f2=0.0, f3=0.9 (corresponding to the simulation shown in Figure 2-23), and a simu-
lation with shape factors fi=1.13, f2=0.03, f3=0.87 (corresponding to the simulation
shown in Figure 2-24).
Distributed heating along the measurement area helps to
achieve a more uniform temperature profile in the area of interest. The temperature
achieved in the measurement area is approximately 90 K.
Several conclusions can be drawn from the simulations.
First, that the tem-
perature evolves quickly, and if given the correct overshooting value a steady state
45
temperature distribution can be reached in ten minutes. For a real experiment, the
heating power can be feedback-controlled, for example with a PID controller. Effects
not taken into account, such as cooling by helium gas, are not expected to influence
greatly in the time evolution of the system. For the experiment, the settling time is
expected to be close to ten minutes.
The real power requirements are more difficult to estimate. To reach 80 K in
the simulations, a total heating power of approximately 15 W is required. However,
heaters such as the simulated may be difficult to construct.
46
Chapter 3
Device and Experiment
Construction
The probe was built according to the design described in the previous chapter, and
shown in Figure 2-9. This chapter describes the construction of the probe and samples, and the experimental setup.
In the first section critical points of the design and construction of the probe are
shown and explained. The following section deals with sample design and construction. The third section summarizes the instrumentation and measuring devices used
in the experiment. Finally, the last section describes the experiment setup, and the
critical steps performed in order to obtain high quality measurements.
3.1
Device Construction
The probe was built according to the design shown in Figure 2-9. In this section the
critical points of the design are shown and explained.
The top of the vacuum canister is shown in Figure 3-1. The rectangular tube and
the circular tubes described in Figure 2-4 are welded to the top flange, and to a bottom
cap (shown in Figure 3-2).
A vacuum connection is provided through a 6.35 mm
(1/4") stainless steel tube connected and welded to the side of the rectangular tube.
The top of the canister flange is covered by a rectangular stainless steel plate. In its
47
Figure 3-1: Picture of the top of the vacuum canister. Left: during its construction.
Right: after finishing the canister construction. References: (1) vacuum port; (2)
outer rectangular tube; (3) top flange; (4) inner circular tubes; (5) top flange cover;
(6) canister vent; (7) sample and current return lead openings; (8) holes for structural
attachment to the probe.
Figure 3-2: Picture of the bottom of the vacuum canister. References: (1) bottom
sample termination; (2) instrumentation wires; (3) bolt holes for holding bottom joint
area cover; (4) inner circular tubes of canister; (5) bottom cap of canister; (6) current
return lead in sample holder; (7) instrumentation wires connectors; (8) threaded rods
for canister attachment to the probe.
48
final form, the top of the canister has only two openings for the sample and current
return lead, and a 6.35 mm stainless steel tube for venting. The tube is connected
to a valve that vents to the outside. The four circular holes in the top flange are for
structural attachment to the rest of the probe.
Figure 3-3: Picture of the vacuum canister. References: (1) outer rectangular tube;
(2) vacuum port; (3) bottom cap; (4) sample termination and instrumentation wires;
(5) bottom joint area cover; (6) current return lead and sample holder; (7) rest of the
experimental device (see Figure 3-4).
In Figures 3-2 and 3-3 other views of the vacuum canister are shown. A sample
with instrumentation is mounted inside the canister, the instrumentation wires come
out through the bottom. The current return lead is shown, lying next to the canister,
49
mounted in a sample holder. The rectangular steel tube for covering the joint area,
shown in Figure 3-3, during operation is held in place with long bolts screwed into the
tapped holes shown in Figure 3-2. The instrumentation wires that come out of the
canister are terminated in pin connectors, which are plugged during the experiments
to the socket connectors shown in Figure 3-2.
Figure 3-4: Picture of the probe without the vacuum canister. References: (1) top
flange; (2) vapor cooled current leads; (3) instrumentation wires; (4) cryogenic valve;
(5) current leads and joint; (6) instrumentation socket connector; (7) canister vent
connection; (8) canister vacuum port connection; (9) structural rods for canister
attachment. The structure of the probe is made with 9.5 mm stainless steel threaded
rods and 12.7 mm thick G10 plates.
The probe without the vacuum canister is shown in Figure 3-4. The top flange is a
50
25 mm thick G1O plate; it has several penetrations to accommodate instrumentation
wires, vacuum and vent tubes, valve operation and current leads. The structure of
the probe, made with 9.5 mm stainless steel threaded rods and 12.7 mm thick G1O
plates, is attached to the inside side of the flange. A detail of the top flange is shown
in Figure 3-5.
Figure 3-5: Picture of the top flange of the probe. References: (1) sample current
connector; (2) vapor cooled current leads; (3) instrumentation connector; (4) canister
vent; (5) cryogenic valve handle; (6) canister vacuum tube; (7) instrumentation wires
port.
The current leads are composed of two parts: a vapor cooled current lead on the
top, which carries current from outside the cryogenic area to the inside, above the
liquid helium level; and a flexible current lead made with copper "rope" style cables
that connects the bottom of the vapor cooled current leads to the superconducting
sample immersed in the liquid helium space. The current leads are rated for 5 kA.
For the joint between the current lead and the superconducting sample a copper
piece with a slot and hole is used; a detail of this piece, with the "rope" current
leads, is shown in Figure 3-6. During operation, the joint mechanically clamps the
superconducting sample as shown in Figure 3-7; the stainless steel bolts used for
clamping are electrically insulated to avoid a short circuit between the copper pieces.
51
Figure 3-6: Detail of the joint between the current leads and the sample. The sample
is terminated in a copper tube that fits in the circular hole, and it can be clamped to
reduce the electrical resistance of the joint. In the background, "copper rope" style
current leads.
Figure 3-7: Detail of the joint between the current leads and the sample, with the
sample mounted. References: (1) sample termination; (2) copper joint; (3) G10 plate
for clamping; (4) electrically insulated stainless steel bolts for clamping.
52
Figure 3-8: Instrumentation wires connectors and top flange of the vacuum canister.
The sample and current return lead are mounted in place, and the epoxy (in blue)
has already cured.
Figure 3-9: Picture of the bottom joint. References:(1) instrumentation wires; (2)
copper terminator; (3) vacuum canister; (4) bolts for holding bottom joint area cover;
(5) sample; (6) current return lead.
53
The canister vent and vacuum tubes shown in Figure 3-1 are connected with
fittings to tubes, shown in Figure 3-4 as number 7 and 8 respectively. This design
allows the vacuum canister to be entirely separated from the rest of the probe if
necessary. The vent tube is connected as well to a cryogenic valve (number 4), that
can be operated from outside of the cryogenic area (Figure 3-5, number 6).
Figure 3-10: Picture of the probe, assembled and prepared for an experiment. The
instrumentation wires are tied to the canister.
In Figure 3-8 the final view of the top flange of the vacuum canister is shown.
The clearance between the top flange holes and the copper tubes of the sample and
current return lead is filled with a G1O ring, and sealed with an epoxy (Stycast).
The bottom joint consists in two copper pieces with cylindrical grooves that clamp
54
the bottom termination of the sample and the current return lead together. A picture
of the bottom joint, prepared for operation, is shown in Figure 3-9.
The final view of the probe, once it is assembled and prepared for an experiment,
is shown in Figure 3-10.
3.2
Sample Construction
The samples tested are composed of a 1090 mm long, 4 mm wide SuperPower SCS4050AP YBCO tape, two 7.9 mm outer diameter copper tube terminations (330 mm long
for the top termination, 114 mm long for the bottom termination), and a metallic
structural support. The YBCO tape and the metallic support are mounted inside the
copper tubes. The tubes are filled with copper strips and 60-40 SnPb solder. The
soldered length is 114 mm in both ends. The current return lead is constructed in
the same way, with two YBCO tapes to increase its current capacity, and with two
0.3 mm thick, 1090 mm long, 4 mm wide copper strips in each side of the tape as
structural support. In Figure 3-11 pictures of a sample and the current return lead
are shown.
Figure 3-11: Pictures of a sample and the current return lead, with sample holder.
Left: detail of the top terminations. The soldered area is marked. Right: entire
sample and current lead. The bottom copper tube is completely filled with solder
and copper strips. The wooden ruler is 305 mm (12") long.
Three samples were built, with different structural support mechanism, as shown
in Figure 3-12. Sample #1 has 0.3 mm thick, 4 mm wide copper strips on either side,
55
1.
Insulated Cu Strip
YBCO Tape
Cu Strip
2.
YBCO Tape
SS strip
3.
YBCO Tape
Cu Strip
Joint are Joinarsasample
Joint area
Figure 3-12: Schematic drawing of the three different samples. The first one was
supported by two copper strips, with insulated copper in the sample area to avoid
current sharing. The second one was supported by one stainless steel strip, attached
to the hastelloy side of the sample. The third one was supported by two copper strips,
not insulated to improve current sharing.
cut and insulated in the middle section to avoid current sharing. Tape MIT ID#
12-01C.
Sample #2 had a 0.3 mm thick, 1090 mm long, 4 mm wide stainless steel strip for
protection on the back side of the tape (i.e. the hastelloy side of the tape). Current
sharing between the superconductor and the stainless steel strip is nominally possible,
but is negligible due to the low electrical conductivity of steel and the high electrical
conductivity of the terminations. Tape MIT ID# 12-01F.
Sample #3 had a 0.3 mm thick, 1090 mm long, 4 mm wide copper strip in each
side of the tape, without any insulation to reinforce current sharing between the
superconductor and the copper. This design improves the thermal stability of the
superconductor. For critical current measurements, the current sharing is negligible.
At the criteria of 100 pV/m, at 30 K, the copper strips are expected to transport
approximately 2 A, while the superconductor would transport around 800 A. Tape
MIT ID# 12-03F.
56
3.3
Instrumentation
During the experiment, the temperature of the sample must be measured and controlled, the level of liquid helium must be monitored and kept over a minimum height
above the canister, and the voltage, current and magnetic field on the sample must
be measured. Temperature sensors, heaters and voltage taps are mounted in the
sample, as shown in Figure 3-13; a helium level sensor and a magnetic field sensor
are mounted in other areas of the probe, as described below.
(Bottom)
E
H3 D
C
v2l
11"W
H2 B
A
HI
M
Figure 3-13: (Top) Picture of the sample with instrumentation mounted. (Bottom)
Schematic drawing of the sample, showing voltage taps (in black: V1, V2), heaters
(red: H1, H2, H3) and temperature sensors (grey squares: A, B, C, D, E).
For temperature measurement, five LakeShore Cernox temperature sensors were
mounted along the sample for measurements of its temperature distribution. These
temperature sensors are well suited for cryogenic temperature measurements at high
magnetic fields (2-14 T). Sensors A, B, C, D (model number CX-1050-SD) are calibrated between 4 K and 100 K; sensor E (model number CX-1070-SD-HT) is calibrated between 4 K and 325 K. Sensors A, C and E were operated with a LakeShore
336 Temperature Controller (LS336-TC); sensors B and D were monitored with a
LakeShore 218 Temperature Monitor (LS218-TM).
The temperature of the sample was controlled with three 25 Q heaters. Heater
HI and H3 are 150 mm long, heater H2 is 200 mm long. Heaters H1, H2 and H3
correspond loosely to the areas fl, f2 and
f3
in the thermal simulation; however, the
length of the simulated heaters could not be reproduced due to mechanical limitations
57
of the materials. Temperature sensor A is used to control the power in heater HI;
sensor C is used for heater H2, and sensor E is used for heater H3. The power sources
are the LS336-TC (it has a 100 W output, and a 50 W output; each can be controlled
with a PID loop using any of the temperature measurements as input) and a Keithley
2440 5A SourceMeter (K2440-SM, manually operated and limited to 1 A and 50 V).
Two control mechanisms were considered: using the LS336-TC for heaters H1 and
H3 and the K2440-SM for heater H2, and using the LS336-TC for heaters H2 and H3
and the K2440-SM for heater H1.
Liquid helium level measurement in the canister was performed with a 914 mm
long NbTi liquid helium level sensor, and a LakeShore silicon diode temperature
sensor (model number DT-470-SD-12A). The NbTi level sensor is mounted along
the current return lead, and measures the level of liquid inside the vacuum canister.
The diode sensor is located just below the bottom electrical joint, and measures
whether that area is in contact with liquid helium (if the temperature is 4.2 K) or
not (if the temperature is higher). Additionally, liquid helium level measurements are
implemented in the magnetic facilities, and during operation the helium level must be
kept such that it completely covers the part of the sample that is outside the vacuum
canister.
For magnetic field measurements, a F. W. Bell bulk Indium transverse Hall sensor
(model number BHT-021) was prepared. It was mounted outside the vacuum canister,
on the wide rectangular face, 355 mm from the bottom of the vacuum canister.
However, the magnet was not used in the experiments reported in this thesis, and
there was no need for a magnetic field measurement.
Voltage taps were soldered to the sample and current lead. The distance between
the sample voltage taps (V) is 203 mm within ±3 mm; the distance between the
sample control voltage taps (V2 ) and between the current return lead control voltage
taps (V3 ) is in both cases 560 mm within ±5 mm. The voltage taps wires are twisted
to avoid electromagnetic interference. Voltage was measured with Keithley 2182A
Nanovoltmeters. The current in the sample was measured with a calibrated shunt,
at room temperature, with a Keithley 2010 Multimeter.
58
Due to the low voltages that develop along the sample during critical current
testing (tens to hundreds of pV), the voltage taps must be mounted in such a way
that thermal voltages are avoided as much as possible.
For that reason, there is
only one connection for the voltage taps wires, inside the cryogenic area, and always
covered in liquid helium to serve as a static reference temperature (see Figure 3-4,
number 6); from that connection, the wires come out of the cryogenic area (through
the port shown as number 3 in Figure 3-5) and are connected to the nanovoltmeters.
The rest of the instrumentation handles much higher voltages: tens of mV to several
V. Thermal voltages are not a problem in this case, and for convenience the wires of
the rest of the instrumentation have two connectors: one in the cryogenic area just
like the voltage taps, and another on the top flange (number 4 in Figure 3-5).
3.4
Experimental Setup
The experiments were performed in the 2 T MIT-PSFC magnetic facility. However,
due to a failure in the power supply of the magnet, the tests were performed without
an external magnetic field. The facility has a 2 T magnet inside a double-layer dewar.
In operation, the inner volume is filled with liquid helium, and the outer jacket is filled
with liquid nitrogen to reduce radiation heat loss. The experiments are performed in
the inner volume. In Figure 3-14 are shown the 2 T magnet and the dewar.
Before mounting the probe inside the magnet dewar, the inside of the canister
(that in operation must be filled with gaseous helium) was checked for leaks. To do
this, the instrumentation wires were disconnected and placed in a plastic bag, which
was in turn tightly taped to the bottom of the canister (Figure 3-15). The canister
vent was used to input gaseous nitrogen inside the canister, and all the joints were
checked for leaking nitrogen gas with Snoop Liquid.
The instrumentation was connected and the probe mounted in the magnet dewar.
The day before the experiment, liquid nitrogen was poured in both volumes of the
dewar, and left overnight to cool all components down to 77 K. The day of the
experiment, the liquid nitrogen left inside the inner layer of the dewar was pushed to
59
Figure 3-14: Picture of the dewar and 2 T magnet. References: (1) magnet; (2) liquid
helium feed line; (3) dewar; (4) inner volume of the dewar; (5) outer volume of the
dewar.
the outer jacket by pressuring with nitrogen gas; after all liquid nitrogen was removed
from the inner volume, liquid helium was transferred into the dewar.
Two separate computers were used during the experiment. One collected data
from the temperature monitors, and the other was used for critical current measurements.
A 1 kA current supply was used to provide electrical current to the sample. The
current supply was controlled with a remote controller that allows varying the current smoothly. During the first use, an unexpected interaction between the remote
controller and the current supply provoked damage to samples #1 and #2.
This
interaction occurred when the remote controller sent a "dump" signal. Instead of
immediately reducing the current to zero, the current supply responded with a sharp
increase in current before going to almost zero amps. When operating at 640 A and
dumping, the maximum current reached with this off-normal event is 760 A. This
issue was resolved by adding an electronic adapter between the remote controller and
the current supply. The adapter consists in a switch that either connects the remote
60
Figure 3-15: Setup to leak-check the inside of the vacuum canister
controller to the current supply input, or connects the two current supply input terminals together, effectively dumping the supply's current to zero without an increase
of current. In Figure 3-16 a detail of the current supply response to a "dump" signal
is shown, before and after solving the issue.
For each critical current measurement, a new temperature set point was chosen.
Once the temperature was stable, and the difference between the temperatures measured by sensors B, C and D was as small as possible, typically ±I K, the critical
current measurement was performed. The current was increased slowly, at a rate of
approximately 10 A/s, and voltage was measured. The current was dumped once any
61
.700
600-
500
140
150
160
Time [s]
170
180
Figure 3-16: Current supply response to a "dump" signal. The original problem was
a current spike when a "dump" signal is sent. In this case, dumping at 640 A elevated
the current to a maximum of 760 A (black curve). The overshoot lasts 0.4 s. After
solving the problem, the "dump" signal does not provoke a current peak.
one of the three voltages that were being monitored (sample voltage V1 , sample control voltage V2 , or current return lead control voltage V3) reached a value equivalent
to 1000 pV/m, or when the temperature started to increase due to joule heating in
the sample.
62
Chapter 4
Experimental Results and
Discussion
Three liquid helium experiments were performed, one per sample. Samples #1 and #2
were damaged early in their experiments, as was mentioned in the previous chapter.
Sample #3 was tested successfully, and the results for temperature control and critical
current of this sample are presented below.
In the first section details of the temperature control operation are given, with
measurements of the time evolution of temperature, temperature difference between
different points in the sample, and required heating power.
The second section
presents the results of critical current measurements at self-field results for sample
#3. Finally, in the last section the interaction between temperature control and critical current measurement are shown, including the case of quenching samples #1 and
#2.
4.1
Temperature Control
As mentioned in section 3.3, two heater control methods were considered:
9 Method #1:
heater HI and H3 controlled with a PI loop, using as input tem-
perature sensors A and E respectively; and heater H2 controlled manually.
63
* Method #2:
heater H2 and H3 controlled with a PI loop, with sensors C and
E as inputs (respectively); and heater Hi controlled manually.
Using the control method #1, and starting with the vacuum canister filled with liquid helium, the temperature was first raised to 75 K and then gradually stepped down.
The temperature evolved quickly when changing the set point, typically reaching the
set point temperature within t1 K in 10-15 minutes. In Figure 4-1 the temporal
evolution of the temperature measured by sensor C is shown, for different set points.
The quality of the temperature control is indicated by the difference between the set
point and the measured temperature in the sample area (sensors B, C, and D) at the
start of the critical current measurement, and is shown in Figure 4-2. The heating
power required at each set point is shown in Figure 4-4. At 75 K, the total heating
power required was 65 W, and 30 W at 20 K.
80
Setpoint:60K
50K
40K
30K
S40.
li40
20OK
20
0
5
10
Time [min]
15
20
Figure 4-1: Evolution of the temperature measured by sensor C as a function of time
for different set points. The system reaches the set point temperature, within ±1 K,
in 10-15 minutes. For each evolution the temperature set point is selected at time=0.
The experiments with control method #2 were performed increasing the temperature from 4.2 K. The temporal evolution of the temperature after changing the set
point was similar to the previous results, reaching the set point temperature in approximately 10 minutes. Since the sensor C was used as input for a PI loop, the
temperature it measured at the start of the critical current measurement was almost
64
1.
-0.
-I
+-1.
20
60
40
Set point temperature [K]
80
Figure 4-2: Difference between the measured and set point temperature as a function
of set point temperature for the temperature control method #1. The curves are
marked B, C, D for the three temperature sensors located on the sample as shown in
Figure 3-13.
1.2
D
S0.6
e
0
B
-UA3
H
0
20
40
60
Set point temperature [K]
80
Figure 4-3: Difference between the measured and set point temperature as a function
of set point temperature for the temperature control method #2. The curves are
marked B, C, D for the three temperature sensors located on the sample as shown in
Figure 3-13.
65
H1
S20-7
0
0.~ 10
H2
0
60
20
40
Set point temperature [K]
80
Figure 4-4: Heating power required as function of temperature for the temperature
control method #1. The curves are marked H1, H2, H3 for the three heaters mounted
on the sample as shown in Figure 3-13.
30
H1
20[
0
b.O
10
H2:
H3
0
0
20
40
60
Set point temperature [K]
80
Figure 4-5: Heating power required as function of temperature for the temperature
control method #2. The curves are marked HI, H2, H3 for the three heaters mounted
on the sample as shown in Figure 3-13.
66
exactly the set point temperature after 10 minutes, as shown in Figure 4-3; at the
same time, the temperature measured by sensors B and D was within ±1 K of the set
point. The required heating power as function of set point temperature is presented
in Figure 4-5. The total heating power required at 70 K was 60 W, and 25 W at
20 K.
Control method #2 provides a very accurate temperature measurement (sensor C),
while control method #1 does not. The maximum difference between temperatures in
the sample measurement area is lower for control method #2 at lower temperatures;
however, at 70 K the difference between temperatures B and D with control method
#2 is almost 2 K, while with control method #1 the maximum temperature difference
at 75 K as approximately 1.2 K.
D
C
B
A
-Experiment
60
z
-Simulation
E
40-
W20-
0
0.2
0.4
0.6
0.8
Axial position [m]
1
Figure 4-6: Comparison of axial temperature distribution simulated and measured.
The simulated data was obtained for shape factors fi=1.13, f2=0.03, f3=0.87 and
total power 13 W. The experimental data was obtained with the temperature control
method #2, and set point 70 K. The white squares in the experimental data series
represent the boundaries of the vacuum canister. Those points were not measured
but are assumed to be at liquid helium temperature.
The heating power required during the experiment was much higher than that
was calculated in section 2.3. This is due to the difference in the heater distribution
assumed for the thermal simulation, and the real heater distribution as constructed.
The total length of the constructed heaters is 500 mm, while the simulated heaters
67
span 150 mm. This gives rise to a different temperature profile, with increased heat
losses, as shown in Figure 4-6. In that example, the required total heating power to
achieve 70 K in the sample area was 13 W, while during the experiment the heating
power required was 60 W.
In order to obtain a smaller power requirement, the heaters must be modified
to reduce their total length. This can be achieved by using a different wire, with
higher resistance per unit length. To obtain a better temperature control quality, one
additional P1 power controller is necessary, as well as also changing the control inputs
for heaters H1 and H3 to sensors B and D, respectively.
4.2
Critical Current Measurements
For each set point temperature, the critical current measurements of sample #3 were
performed once the temperature was stable (about 10-15 minutes after the set point
change, as mentioned in the previous section). For the measurements between 20 K
and 60 K temperature control method #1 was used, and method
#2
was used for
the measurement at 70 K. The current ramping up rate was kept between 2 A/s and
4 A/s.
Voltage-current results for sample #3 at self-field and temperatures between 20 K
and 70 K are shown in Figure 4-7 in linear scale, and in Figure 4-8 in logarithmic
scale. In those figures, the 20 K curve showed a slight change of slope at 40 pV/m
due to a temperature control problem.
Critical current at 100 pV/m and n-value are shown as function of temperature in
Figure 4-9. The uncertainties of the critical current measurement are negligible. The
uncertainties of the temperature measurement are due to the temperature distribution
along the sample area, as shown in Figures 4-2 and 4-3, and to the variation of
temperature during the critical current measurement, as described in section 4.3. In
all cases, the temperature uncertainty is approximately ±1 K. The n-value does not
have an uncertainty associated, and was calculated with two methods:
e Two point method: from the value of critical current at 100 pV/m (I100) and
68
0L
400
600
Current [A]
1000
Figure 4-7: V-I curves of a single 4 mm wide Super Power YBCO tape, measured at
self-field and temperatures from 20 K to 70 K.
3
101
2
10
3
Current [A]
10
Figure 4-8: V-I curves of a single 4 mm wide Super Power YBCO tape, measured at
self-field and temperatures from 20 K to 70 K in log-log scale.
69
1000
--.
*Critical Current
800
+n
- two points
50
45
60n - curve fitting
S600400
0x
35
2003
0
20
30
60
50
40
Temperature [K]
70
5
Figure 4-9: The critical current at 100 pV/m and n-value of a single 4 mm wide
SuperPower YBCO tape as a function of temperature. Uncertainties in critical current
are negligible. Uncertainties in temperature are represented by the size of the plot
markers.
o12
-Measurements
-SuperPower data
8
."
0 0
0
Z
60
40
20
Temperature [K]
80
Figure 4-10: Critical current at self-field normalized to the value at 77 K, self-field,
as a function of temperature measurements for a single SuperPower YBCO tape. SuperPower data of normalized critical current at self-field as a function of temperature
is shown for comparison. Uncertainties in the critical current measurement are negligible. Uncertainties in the measured temperature are represented by the size of the
plot markers.
70
at 10 pV/m (Iio) the n-value was calculated as:
n = log
[9
'100
10
(4.1)
Curve fitting method: a power law was fitted to the data using a Matlab routine,
and the n-value is the exponent of the current in the regression.
In Figure 4-9 the dependence of the n-value with temperature showed a minimum
at 40 K. Further investigation is required to understand this phenomenon better.
The critical current can be compared to the superconductor's manufacturer's data.
In Figure 4-10 normalized critical current as a function of temperature at self-field
for sample #3 is compared to the manufacturer's data of a similar tape. The critical
current was normalized to the value in liquid nitrogen, self-field. The critical current
for sample #3 at 77 K was not measured, but extrapolated from the data in Figure
4-9. The value obtained is 102 A, which is consistent with manufacturer's data for
the YBCO spool used. As shown in that figure, the normalized critical current of
sample #3 is very similar to the manufacturer's data.
4.3
Current-Temperature Interaction
The temperature of the sample slightly varies when approaching the critical current.
For sample #1, at self-field in liquid nitrogen, reaching critical current increases the
sample temperature about 3 mK, as shown in Figure 4-11. The temperature measurement resolution was 0.8 mK. The peak current achieved was 113 A at approximately
260 pV/m, and this gives rise to a joule heating in the sample of 30 mW/m.
In gaseous helium, for a set point of 40 K and using temperature control method
#1, the temperature of sample #3 increases approximately 1 K, as shown in Figure
4-12. The peak current achieved was 611 A at 1360 pV/m, and this gives rise to a
joule heating in the sample of 800 mW/m. The temperature sensors B, C and D show
that the sample temperature gradually decreases as the sample current is increased,
and then the temperature increases at the critical current similarly to that shown in
71
30,
0-*Temp- C
200
77.403
100
100
477.401
ii-
01
0
20
40
60
80
Time [s]
100
77.399
120
Figure 4-11: Temperature and voltage measurements as a function of time, while
performing critical current measurements for a single SuperPower YBCO tape, in
liquid nitrogen (77 K) and self-field. The temperature measurement resolution is
0.8 mK.
1600
42
1200
41.5
C
800.4
0400
B
40.5
VoltageD
011-0
' 140
4
2
Time [min]
6
Figure 4-12: Temperature and voltage measurements as a function of time, while
performing critical current measurements for a single SuperPower YBCO tape, in
gaseous helium at 40 K and self-field.
72
12,
H
0
20
Time [s]
40
60
Figure 4-13: Temperature runaway and quench of sample #1. The temperature rises
very quickly, in a few seconds, and surpasses 100 K. The calibration limit of sensor
D was 100 K.
101
10)
[Time [s]
40
60
Figure 4-14: Temperature runaway and quench of sample #2. The temperature rises
very quickly, in less than a second, and it reaches about 240 K. Sensor C was not as
precisely calibrated above 100 K as it was between 4 K and 100 K; the calibration
error for temperatures higher than 100 K might be a few degrees kelvin.
73
Figure 4-11. The initial reduction of temperature shown in Figure 4-12 (at 2-4 min)
is due to joule heating of the bottom joint proportional to the square of the sample
current (I). This forces a reduction of the bottom heater power by the bottom heater
controller. As current increases, a voltage is developed along the superconducting
sample (at 4-5 min), and a distributed heating power (proportional to In+)
gives
rise to an increase of temperature along the sample.
The joule heating in the gaseous helium case was about 25 times higher than in
the liquid nitrogen case, but the temperature difference was 300 times higher due to
the worse cooling properties of gaseous helium, compared to liquid nitrogen.
Figure 4-15: Picture of sample #1 after quench. A kink is observed at 260 mm
(10.25") from the bottom of the sample, close to the location of temperature sensor
D.
The temperature increase for samples #1 and #2 while testing in gaseous helium
was uncontrolled, due to the current supply interaction with the remote controller,
as mentioned in section 3.4. In Figures 4-13 and 4-14 the temperature runaway for
samples #1 and #2 (respectively) is shown. The temperature rose very quickly, in
2-5 seconds; however, the cool down of the sample back to the initial temperature
took much longer, from 25 seconds to about a minute, due to the poor cooling characteristics of gaseous helium.
74
Figure 4-16: Picture of sample #2 after quench. Two damaged zones are observed:
one at 400 mm (16") from the bottom of the sample, between temperature sensors B
and C, and the other at 265 mm (10.5"), close to the location of sensor D.
During the temperature runaway the voltage measurement devices were off-line,
probably because the measurement range was not as larger as the large voltage developed in the sample. After the thermal runaway incidents, samples #1 and #2 were
found to be completely resistive upon ramping up the current from zero.
An inspection of the samples after the quench showed physical damage to them,
as shown in Figures 4-15 and 4-16 for samples #1 and #2 respectively. Sample #1
had a kink, close to the location of sensor D, and the insulating teflon tape that
covered the structural copper strip in that area was blackened. Sample #2 had two
completely burned zones, one at the position of sensor D and another between sensors
C and B; in this case, the temperature reached during the quench was enough to melt
the cross section of the superconductor.
While performing critical current measurements in a controlled temperature environment, the additional heating from the sample itself must be taken into account,
and current must be controlled very precisely. The additional heating may increase
the temperature only a few milikelvin, as shown in Figure 4-11, or several hundreds of
kelvin, as in Figure 4-14. In order to prevent this in future test, the sample should be
stabilized by adding extra copper in thermal and electrical contact with the sample.
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76
Chapter 5
Conclusions
5.1
Summary
A new experimental probe for critical current measurements of YBCO tapes and
TSTC cables has been designed and built. The probe allows measurement of critical
current in gaseous helium at various temperatures between 4.2 K and 80 K, in two
high magnetic field facilities with magnetic field up to 14 T, and sample current up to
5 kA. A vacuum insulation around the test area provides enhanced thermal insulation,
reaching low temperature gradients in the measurement area.
Critical current and n-value of a single YBCO tape in self field were tested between 20 K and 70 K. The values of critical current as a function of temperature
obtained for a single YBCO tape are similar to data provided by the superconductor's manufacturer, validating the probe design.
The temperature of the sample reached the set point temperature in about 10
minutes. The total heating power required at 70 K was 60 W, and 30 W at 20 K.
The temperature distribution along the 200 mm sample area was between t0.2 K at
20 K, and ±0.9 K at 70 K.
Critical current measurements challenged the temperature control system. As the
current (I) increases, the self-heating of the sample and joule heating of the bottom
joint increase in different proportions (self-heating as In+1, joint heating as 12), and
the external heating power required to keep the temperature distribution decreases.
77
In consequence, the sample temperature varies, with a variation of about 0.7 K at
40 K. For better control of temperature, the current ramp up rate has been kept
between 2 A/s and 4 A/s.
Taking into account the temperature variation during the critical current measurements, and the temperature distribution along the sample, the experimental probe
allows to perform critical current measurements with a temperature uncertainty of
approximately +1 K. This design is good compared to others because of the low
temperature uncertainty. For example, the device at KIT [3] has a temperature uncertainty of about ±5-10 K.
This experimental device can be used to obtain precise critical current data of
YBCO tapes and cables, with temperature between 4.2 K and 80 K, and magnetic
field up to 14 T. Characterization of YBCO TSTC using this probe will be performed,
as is discussed in the next section.
5.2
Future Work
There are some improvements that can be done to the experimental probe in order to
reduce the liquid helium consumption and to obtain a better temperature distribution
along the sample:
" Redesign of the heaters, using a different wire, to make them shorter. This
will allow locating the heaters closer to the measurement area and further away
from the ends of the sample, reducing conduction heat leak.
" Replace the manual heater controller with a PI closed loop temperature controller; this will prevent large variations of temperature during critical current
measurements.
" Modify the setting of the instrumentation to be able to control heater power with
temperature sensors B, C and D as input. Keep sensors A and E as monitors,
to ensure that the temperature is the highest in the sample area. These last two
78
modifications will allow to get a better temperature distribution, and probably
to reduce the transient duration.
Follow-up experiments are planned. After fixing the magnet's power supply at
MIT-PSFC, critical current measurements of single and multi-tape TSTC will be
performed, for temperatures between 4.2 K and 80 K and magnetic fields up to 2 T.
These measurements will be completed with measurements at higher magnetic field,
in the 14 T magnetic facility in NHMFL, Tallahassee, Florida.
The characterization of the TSTC cables will be continued with the measurement
of AC losses. The design of the sample holder allows the addition of a small pick-up
coil around the sample, and a racetrack-style excitation coil would be constructed
outside the probe. These modifications would allow to measure AC losses at various
temperatures and with a background field of up to 14 T.
79
80
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