Nb/Cu, Nb3Sn/Cu Thin Film Cavity
Genfa Wu
Abstract
A thin film deposition method inside an elliptical cavity is proposed. The system uses the
substrate copper cavity as the vacuum chamber. The ECR plasma will be created to
produce direct niobium ion deposition, thus realize the energy controlled vacuum
deposition. An extended use of such deposition system to study the niobium-tin alloy
(Nb3Sn) is also proposed.
JLAB-TN-04-014
G. Wu
JLAB-TN-04-014
Nb/Cu, Nb3Sn/Cu Thin Film Cavity
Genfa Wu
Table of content:
1. Introduction
2. Current thin film effort
3. Copper based niobium thin film cavity by energy-controlled deposition
4. Copper based Nb3Sn thin film cavity
5. Cost and schedule
6. Conclusion
1. Introduction
Recent development in RF superconductivity (SRF) is both exciting and gloomy. The
single cell elliptical RF cavity based on solid niobium is reaching accelerating gradient of
40 MV/m [1]. Multicell cavity is not far behind, which is 35 MV/m for TESLA 9-cell
cavity [1]. While the solid niobium based SRF is reaching its theoretical limit. The cost
of the solid niobium remains much too high. The future development of SRF technology
much becomes reducing cost or finding alternative material to exceed the 40-50MV/m
limit due to the theoretical magnetic field limitation. The answer could well be the SRF
based on thin film technology: the copper based niobium thin film cavity for near term
cost reduction and Nb3Sn material based cavity for the accelerating gradient to reach well
beyond the 40-50 MV/m limitation.
The early successful LEP-II [2] proved the viability of thin film technology in particle
accelerator. Since copper material can be one tenth of the cost of niobium, plus the
potential for much lower manufacturing cost, the thin film technology has potential to
reduce the particle accelerator cost dramatically, which could have been beneficial for
TESLA proposal. The technology would also be a real benefit for muon storage ring,
which called for 200MHz large cavities [3]. The existing magnetron sputtered niobium
thin film cavity achieved 20 MV/m for single cell elliptical RF cavity at 1500MHz [4].
From the perspective of thin film deposition process, the magnetron sputtering technique
has some limitations to achieve certain film structure. The process has relatively low
impacting energy and has difficulty to form high quality thin film for some areas inside
an elliptical cavity due to the low deposition angle [5].
The Nb3Sn cavity effort was pioneered at Univ. of Wupertal. The new layer of Nb3Sn
was grown inside the high purity niobium cavity at high temperature up to 1200C. The
1.5GHz single cell cavity achieved 10MV/m accelerating gradient with Q0 of 109 at 4.2K
and 20MV/m with Q0 of 109 at 2K [6]. The result was promising at that time, since the
accelerating gradient was comparable to that of solid niobium and had much potential to
exceed.
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2. Current thin film effort
For the case of niobium thin film, different processes are being tried to improve niobium
thin films, including post-deposition laser annealing [7], DC post-magnetron sputtering
deposition [8], biased DC magnetron sputtering [9] and vacuum arc deposition [10-11].
For the aspect of thin film growth, the higher surface adatom mobility [12] and the
vacuum condition are believed helpful to get better film quality as required for
superconducting material like niobium. For a niobium thin film on a copper substrate,
increasing the substrate temperature is not an option to achieve greater surface adatom
mobility. Some processes such as ion-assisted deposition, biased magnetron sputtering,
ionized magnetron sputtering, vacuum arc deposition and energetic cluster deposition, are
expected to have higher impact energy during film growth, thus increasing the surface
adatom mobility. These techniques either require a working gas or entail risk of
microparticle contamination and lack good control of the deposition energy.
To take advantage of the high vacuum condition and the capability of controllable
deposition energy, the electron cyclotron resonance (ECR) plasma metal ion source
[13,14] is selected as an energetic deposition system to study the niobium thin film
deposition at different deposition energies [15].
Table 1. Comparison of niobium films on sapphire by several coating processes
RRR
***
Crystallization measured
by (X-ray diffraction)
Range from oriented to
less oriented, depends on
deposition angle.
Film structure by
XTEM analysis
Columnar growth
Some voids present at
high deposition angle
Columnar growth
10-80
N/A
Preferred orientation,
other orientations exist
50
Perfectly oriented
Coating process
Tc(K)
Tc(K)
Magnetron Sputtering
9.5
0.3
5-10
9.6
>1K
7-15
9.25
<0.02
9.1
0.07
Biased Magnetron
Sputtering
Vacuum Arc
deposition *
Energetic Vacuum
deposition **
N/A
Epitaxial in some
films.
* Tc measured by different process
** Sample made at deposition energy around 123 eV on sapphire.
*** RRR as measured on sapphire substrate
From the several parameters for different processes listed in Table 1, one can see vacuum
arc deposition and energetic vacuum deposition have clear advantages. While macro
particle contamination remains the major challenge for vacuum arc deposition, the
energetic vacuum deposition has better chance to achieve the high performance thin film
cavity.
According to Matricon and Saint-James [16], the superheating critical field for Nb3Sn can
be as high as 4000 Oe, which suggests up to 100MV/m for a typical elliptical cavity. But
the best superheating critical field obtained for Nb3Sn was around 1033 Oe [17], which is
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JLAB-TN-04-014
much lower than previous predicted. It could simply be the reason of different quality of
Nb3Sn, or more seriously the wrongly predicated superheating critical field [18]. Never
the less, the Nb3Sn’s ability to achieve high Q above 4.2K makes it quite attractive. And
the possibility to use copper as substrate cavity can further reduce the cost of particle
accelerator just likes that of the copper based niobium thin film cavity. Due to the
University of Wupertal’s quitting the SRF business, the Nb3Sn thin film effort was
virtually stopped. The vapor diffusion process to make the Nb3Sn cavity was based on
solid niobium technology. Even though the high accelerating gradient at 4.2K is
rewarding, the cost issue always comes into minds. Early Nb3Sn was prepared through
carefully monitored two-source evaporation [19]. Other than the co-evaporation and
vapor diffusion process [6,20], the Nb3Sn thin film actually can be made though RF
magnetron sputtering process in which a Tin embedded Nb [21] or reacted Nb3Sn [22,23]
is the target.
3. Copper based niobium thin film cavity by energy-controlled deposition
Based on successful ECR plasma operation result [15], it is natural to extend this
energetic vacuum coating technique to the elliptical cavity as a substrate. In this case, the
cavity itself would serve as a vacuum chamber. To generate niobium plasma inside the
cavity vacuum, at least three components are needed: neutral niobium vapor, RF power at
certain frequency, proper static magnetic field which is perpendicular to the electric field
of RF power and satisfies the ECR condition. To create energy controllable depositing
niobium ions, a cylinder grid will be inserted into the cavity vacuum. The grid confines
the RF field, floats at certain potential above the cavity to provide the controllable
accelerating voltage for niobium ions. The concept is illustrated in figure 1.
Figure 1. Illustration of ECR plasma creation inside an elliptical cavity.
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(a)
(b)
(c)
Figure 2. The permanent magnet configuration (a), axial field distribution (b), and midplane field distribution (c)
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A 1.3 GHz single cell elliptical cavity is envisioned for the cavity deposition. The RF
power would be commercial grade 2.45 GHz power source, which needs to be able to
provide 500 W at least. The cylinder grid would be made out of niobium, both to reduce
contamination and provide high heat tolerance. To meet ECR condition, the magnetic
field needs to be around 875 Gauss. The actual field was designed as shown in figure 2.
The field is configured to have relatively large area uniform field of 875 Gauss. The RF
power is fed into the cylinder grid in TE11 mode. The cavity, RF field and ECR magnetic
field is superimposed in figure 3.
Figure 3. A 1.3GHz cavity with permanent magnet and RF power.
From the above discussion, one can see the clear advantages for this deposition process:
1. No working gas like argon
2. High vacuum means reduced impurities
3. Controllable deposition energy, and 90-degree deposition flux
4. Excellent bonding
5. No macro particles
4. Copper based Nb3Sn thin film cavity
Based on the survey discussion in section 2, several coating processes are imagined to
form the Nb3Sn inside the copper cavity. One is certainly the magnetron sputtering
process; the target can either be reacted Nb3Sn, or the geometrically separated Nb and
Tin target. Second process would be the Nb and Tin co-evaporation. These two processes
use little of the proposed niobium cavity deposition system. The following two processes
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are based on the energetic vacuum deposition principle, i.e. the direct ion deposition
through ECR plasma process.
The sequential process creates the niobium film on copper first, and then second
energetic tin ions are delivered to the newly formed niobium surface to allow the
diffusion process to occur. Since Tin only starts to diffuse into niobium around 1100 C,
it is not acceptable to heat the copper substrate to that high temperature. The energetic
vacuum deposition has shown that the energetic impinging ions do enhance the surface
adtom mobility [24], which is equivalent to surface layer temperature. It is possible to use
the same energetic deposition system to create high enough surface layer temperature to
form niobium-tin alloy on cold copper surface. The nature of ionization also provides the
novel method to deliver the vapor to inner surface of an elliptical cavity.
The parallel process actually tries to create plasma of both niobium and tin. The twosource e-beam gun is commercially available to provide accurate amount of niobium and
tin vapor. It is convenient to extend this cavity deposition system to study the feasibility
of the plasma of co-existing niobium and tin, then subsequent Nb3Sn coating on copper
substrate.
5. Cost and schedule
The deposition system is designed to use many off the shelf commercial product to
reduce the equipment cost. The elliptical copper cavity can be obtained through lab
collaboration. The only part that needs some attention is the niobium cylinder grid.
Table 2 lists the estimated cost for several subsystems. The author assumes most of the
labor. Total one man/year’s technician support should help to expedite the project.
Table 2. Estimated cost to build the energetic vacuum deposition
Equipment/activity
Estimated cost
Microwave power source
$3,000
Microwave accessory
$2,000
Vacuum pump
$5,000
Vacuum system accessory
$5,000
Vacuum Tees, supporter
$3,000
Permanent magnet
$5,000
Magnetic coil
$2,000
E-beam gun system
$15,000
E-beam gun second source
$7,000
Bias voltage control
$1,000
Cylinder grid assembly
$3,000
Total
$51,000
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Note:
Includes waveguide
Includes power meter, adapters, etc.
Vacuum getter pump
Includes RGA, vacuum gauge, etc.
Includes Nd-Fe-B and supporting structure
Coils and power supply
Includes power controller
Includes power supply, insulator assembly
G. Wu
JLAB-TN-04-014
The goals and milestones for thin film cavity deposition
Activity
1st Month
2nd Month
3rd Month
4th Month
5th.Month
6th Month
7th Month
18-19th
Month
20th month
and beyond
Design review
and finalize
Order PM, grid
RFpowerSource
Making flanges,
Tees, vacuum
Assembly
Vacuum test,
Grid test with
RF power
Magnet, coil
assembly
E-gun, niobium
feeder
Test the plasma
Test bias voltage
Deposition in
cavity
The goals and milestones for thin film cavity deposition (continued)
Activity
8-9th
Month
10-11th
Month
12-13th
Month
14-15th
Month
16-17th
Month
Deposition in
cavity & testing
Nb3Sn sample
deposition &
characterization
Nb3Sn Cavity
deposition &
testing
6. Conclusion
Following the ever-increased size and cost of modern particle accelerator, both the cost
reduction and future technique to achieve even higher energy is critical for the future of
particle accelerators. The proposed research topic can be both feasible and rewarding.
Acknowledgement
The author would thank W. Funk, R. Rimmer and C. Reece for their constant support, L.
Phillips, P. Kneisel and A-M. Valente for very constructive discussions.
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