A Continuous Adiabatic Demagnetization Refrigerator for Cooling to

A Continuous Adiabatic Demagnetization
Refrigerator for Cooling to 10 mK and below
Dr. Peter Shirron
Cryogenics and Fluids Group
NASA/Goddard Space Flight Center
August 3, 2005
Bolometer Arrays
! Low temperature detectors for far-IR missions
– Large format detector arrays (32x32)
– Growing number of sensor types
• Semiconducting, Superconducting, Magnetic
– Background-limited detection: NEP <10–19 W/! Hz
• Achievable only with cooling to ~20 mK
• Cooling power of ~1 "W at 10 mK
Continuous ADR
Stage
Detector
interface
1
2
3
4
.05 K
.045-.275 K
.25-1 K
.9-6+ K
Superconducting
magnet
"Salt pill"
refrigerant
Cryocooler
Cold Tip
Heat switch
Recycling Sequence
!
Load is cooled by a “continuous” stage
!
Upper stages cascade heat to the heat sink
– Number of stages and temperature range
depends on heat switch properties
!
Cycle time can be short, ~1 hour
– 1-2 orders of magnitude higher cooling
power per unit mass
– ~8 kg mass for 5-10 "W at 50 mK
!"Stage CADR
!
Uses 4.2 K helium bath
!
Total mass of 7.7 kg
!
Magnets are fully shielded
!
Fully automated operation
50
40
Efficiency (% Carnot)
30
20
Cooling Power (µW)
10
0
40
50
60
70
80
T (mK)
90
100
110
!"Stage Cycling
! Control algorithm minimizes cycle period to maximize cooling power
! Peak heat rejection rate is 10 mW
Temperature (K)
Stage 4
1.0
Stage 3
Stage 2
0.1
Stage 1
0
1000
2000
3000
Time (s)
4000
5000
6000
Heat Switches
! Superconducting Switches for
less than about 0.3 K
– Simple design and construction
! Passive Gas-Gap Switches for
0.2 K to over 4 K
– Instantaneous turn on/off
3
10
He3
2
10
He3/H2
He3
1
K (mW/K)
10
0
10
-1
10
-2
10
-3
10
-4
10
0.1
1
T (K)
10
Laboratory#Detector Test Dewar
! Continuous ADR
Cryocooler
! Low power cryocooler
! Efficient dewar
G-10
Supports
Flexible
Thermal
Links
Coldplate
CADR Con$guration
CADR
Experiment
Space
Cryogen"Free System
! SRDK-101D
cooler with small
efficient dewar
! Large experiment
space
! Cooling power
comparable to
small dilution
refrigerators
! Cooled to 2.75 K without
ADR wiring
! Expect < 4 K in final
configuration
! Need to shorten cooldown
time (2 days)
Development E%orts
! Extend low temperature operation to 10 mK
– Efficient refrigerants
! Extend heat sink capability to >10 K
– High temperature magnets
– High temperature materials
Challenges for &' mK Operation
! 5-stage ADR
Load
Continuous
stage
Second
stage
Third
stage
Fourth
stage
Fifth
stage
.01 K
.009.05 K
.045-.3 K
.25-1.1 K
1-6+ K
Heat
Sink
Superconducting
heat switches
! Materials with ordering temperature < about 5 mK
– Ordering T scales with electron spin and ion density
– Low ordering T means low refrigeration capacity
• Extremely important to minimize parasitic heat loads
! Thermal boundary resistance grows as T-3
– Non-metallic compounds will have poor heat transfer efficiency and
potentially long thermal time constants
Material Properties
!
!
Material
Formula
Ordering
Temperature (TC)
Typical
Magnetic Field
Max Entropy
Density
(J/K/cm3)
CMN
Ce2Mg3(NO3)1224H2O
1.5 mK
0.1 T
0.012
peroxychromates
M3CrO8
(M=Li, Na, K, Rb, Cs)
1 mK to >4 K
0.1 T
0.030
Van Vleck nuclear
paramagnets
PrNi5, PrCu6
0.4 mK, 2.5 mK
0.5-1 T
0.31
CMN has a history of use in early ADRs
–
Hydrated salt - must be hermetically sealed
–
Low entropy capacity, very high boundary resistance
Peroxychromates are a better choice than CMN
–
Tailorable TC via stoichiometry
• KCrO8 has TC~0.85 K, while a 50% mixture KCrO8/KNbO8 has TC~1 mK
–
!
Not hydrated - do not require hermetic packaging - higher entropy density
Nuclear paramagnets
–
Relatively high entropy capacity per volume
–
Higher field requirements, but still lower overall mass and size
–
Metals!
Thermal Buses for Non"Metals
! Non-metallic refrigerants require a thermal bus with large surface
area for heat exchange
– Copper cylinder machined by wire-EDM
– Thermal bus occupies ~25% of volume
7.5 cm
– Very high thermal conductance
! Have fabricated an 87 g CMN salt pill, and integrated with 3 upper
ADR stages
Single"Shot Cooling Tests (CMN)
! CMN demagnetized from
various starting
conditions
0.250
– Parasitic heat load of
0.6 "W (small)
0.200
! Deviation from nominal
temperature is being
investigated
– Appears to be eddy
current or other
heating in the
thermometers
– Temperature saturates
at ~30 mK regardless
of starting conditions
Temperature
– 1.2 T, 0.22 K is shown
Demagnetization Data
0.150
0.100
0.050
Expected Temperature
0.000
0
0.5
1
1.5
Current
2
2.5
3
Development Plan
! Contracts in place to fabricate materials
– Peroxychromate fabrication by Prof. Naresh Dalal at Florida State U.
– PrNi5, PrCu6 fabrication (arc melting) by Materials Preparation Center
at Ames Laboratory, Ames, IA
! CMN work allows some fundamental issues of operating at ultralow T to be addressed
– Reduction of parasitic heat loads by shielding salt pills
– Thermometry
! New materials will be delivered in next 1-2 months
! Goal: operational system by end of 2006
High Temperature ADR Stages
! Magnets for operating at >10 K
– Nb3Sn developments
• Work is being done on both “wind and react” and “react and
wind” techniques (Shahin Pourrahimi at Superconducting
Systems Inc.)
• Successfully demonstrated 3 T/8 A at 10 K
! High Temperature Refrigerants
– GdLiF4 and other Gd/Dy compounds
• Funded by Numazawa
Summary