Cryostats and Vacuum
Inevitably, infrared detectors and optics must be held
stably at cryogenic temperatures
Detector temperatures must be kept low to minimize dark
current (photon detectors) or thermal noise (bolometers).
HgCdTe (0.8-2.5 um) 80K
InSb (0.8 – 5.5um) 40K
Si:As (3 – 30um) 10K
Bolometers (100 – 3000 um) 0.1K
Optics, support structures and internal components must be
cold enough that the blackbody Wein peak is well below the
longest detectable wavelength.
T 
1 2900um
5
longest
Cryostats – a.k.a Dewars (with a Capital D)
Achieving and maintaining low
temperature requires significant
isolation from the ambient warm
environment.
the one exception being in deep space
vacuum comes for free and radiative loads
come from limited directions (Sun, Earth)
On Earth, isolation from the hot world
is generally achieved by using a double
walled flask.
“silvered” low-emissivity walls minimize
radiative load
vacuum minimizes thermal gas
conduction.
Sir James Dewar
(1842 - 1923)
Thermal Isolation of the Cryogenic
Environment
The ideal cryostat would
maintain its internal volume at a
temperature lower than that of
the ambient environment without
cryogens or refrigeration –
complete isolation
Sources of thermal loss
radiation
imperfect insulation
windows
conduction
necks, actuators, wires
residual vacuum
power dissipation
Radiative Loads
Environmental radiation loads are surprisingly significant.
Each square meter receives 460 watts at 300K
simply
T4
Surfaces radiate (reflect) this load proportional to their
emissivity, 
Polished metals have emissivities of a few percent.
More insulating layers are better....but one must account for the
“trapped” radiation bath between reflective layers.
Consider parallel reflectors of equal emissivity
Hot
T1
Cold
T2
2
Heat Load W / m  =
 T 14 −T 42 
2
−1

Radiative Loads
Consider 0.1 square meters of surface separating 300K
from 77K with emissivity=0.05
Heat transfer is 1/39th emissivity=1 case
but still 1.1 Watts
Is 1.1 Watts a lot? See capacities of cryogenic liquids and
refrigerators which follow
Hot
T1
Cold
T2
2
Heat Load W / m  =
 T 14 −T 42 
2
−1

Radiative Window Loads
An instrument must “see” the
outside world, usually this
implies a significant solid angle
open to outside radiation.
worst case 460W/m^2 of hole area
ideally the hole is baffled so that <
solid angle is seen by the interior of
the instrument
an internal cold blocker can reflect
unwanted wavelengths back to the
outside world.
2
460 W / m ∗
view
2
Thermal Isolation of the Cryogenic
Environment
The ideal cryostat would
maintain its internal volume at a
temperature lower than that of
the ambient environment without
cryogens or refrigeration –
complete isolation
Sources of thermal loss
radiation
imperfect insulation
windows
conduction
necks, actuators, wires
residual vacuum
power dissipation
Thermal Conduction
Thermal conduction thorough a solid part is
proportional to
cross sectional area (A)
thermal conductivity ((T)) – note temperature dependence
temperature gradient per unit length
Heat Flow Watts= A  T 
dT
dl
Example thermal conductivities ( W/m/K )
Copper
Aluminum (1100)
Aluminum (6061)
Stainless Steel
G-10 Fiberglass
300K
400
200
160
10
0.5
77K
600
300
150
5
0.2
Temperature Dependence of 
Heat Flow Watts= A  T 
dT
dl
Heat Flow Example
Heat load from dewar “neck”
Liquid cryostats are filled through rigid
tubes which connect the outside world
to the cryogenic storage volume.
These tubes have to be of small cross
section and low thermal conductivity to
avoid a significant heat load, but high
strenth to support significant weight –
stainless steel
–
–
length – several cm; radius 1 cm
typical wall thickness - ½ mm
3x10−5 m 2  ∗ 7 Wm−1 K −1 ∗
223K
0.1m
= 470 mW
Heat Flow Watts= A  T 
dT
dl
Heat Flow - Wires
Copper is an ideal electrical
conductor
The same free electrons (and
their mobility) which give rise
to electrical conductivity, also
account for thermal
conductivity.
Wiedemann-Franz law
For “low-current” applications –
more resistive, but less
thermally conductive wires are
preferred.
Cryogenic Liquids
Commonly used cryogenic liquids
Liquid
Boiling Mass/liter Heat of Vapor. Hours/liter
point (K) kg
KJoules/liter
1 Watt Load
Helium
Hydrogen
Neon
Nitrogen
Argon
Oxygen
Water
4.2
20.4
27.1
77.3
83.8
90.2
373
0.13
0.07
1.20
0.81
1.40
1.14
1.00
2.7
31.2
103.2
161.2
225.4
242.8
2257.0
0.7
8.7
28.7
44.8
62.6
67.5
626.9
Heat of Vapor. Cost/liter Cost/MJoule
KJoules/Kg
Dollars
Dollars
20.4
446
86
199
161
213
2257
The boiling points above are measured for
ambient atmospheric pressure.
Often, cryogens are “pumped” to produce a
lower vapor pressure over the liquid/solid and
thus a lower temperature.
Nitrogen ~50K
Hydrogen ~7K
Helium ~2K
Helium³ ~0.3K
10
3770.74
0.5
3.1
Cryogenic Safety
Cryogenic liquids present several hazards
Frostbite
Liquid cryogens or vent gas can instantly freeze skin
–
evolved gas cushions initially, but thermal contact is quick (and
painful). Especially avoid spilling on clothes.
Volume expansion
Gas volume is approximately 1000 times liquid volume
at room temperature contained cryogens inevitably become gas
– steel containers can pop like balloons with devastating results
asphyxiation – evolved gas quickly displaces oxygen
–
Combustibility (oxygen, hydrogen, methane)
note that oxygen can be condensed by liquid nitrogen
Fracture of embrittled materials
transfer cryogens in appropriate containers to avoid flying shards of
fractured glass.
http://www2.umdnj.edu/eohssweb/aiha/accidents/cryogens.htm
Cryogenic Refrigerators
A variety of thermodynamic cycles/processes can
deliver cryogenic temperatures without expendable
cryogenic liquids.
Typical cycle involves
isothermal compression of a working gas – compression in contact
with a heat sink
gas could be liquified or simply compressed
expansion/boiling at cold load
–
–
heat exchange warms expanded gas for return to compression
stage.
Cryogenic Refrigerators
Closed cycle refrigerators use circulating Helium gas in
a “Gifford-McMahon” cycle (similar to a Stirling cycle)
Cryogenic Refrigerators
The “CryoTiger” in use at Fan Mountain is a simple
Joule-expansion cooler capable of removing a few watts
at 80K.
Cryostat Construction
As discussed above with thermal conductivity, material
properties change with temperature.
Other factors to consider are
Mechanical strength and ductility
Length/volume contraction
can lead to significant stress due to thermal gradients in
materials and use of dis-similar materials in construction
Electrical conductivity
–
Metallurgy 102 – Cryogenic Behavior
Almost without exception the elastic moduli of metals
becomes greater at lower temperatures
Metallurgy 102 – Cryogenic Behavior
Deformation behavior has some interesting systematics.
BCC (carbon steel) and FCC (copper, aluminum, “austenitic”
stainless steels) crystalline metals have different stress/strain
behaviors.
As is the general rule strength improves with decreasing temperature
BCC alloys, however, tend to become brittle
Given cryogenic stresses, BCC materials are to be avoided.
Thermal Contraction
Materials decrease
proportionally in length with
decreasing temperature.
From room temperature to 80K
the thermal contraction exceeds
the yield strain for most
materials!
Dis-similar metal junctions will
experience significant stress.