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ADR Operation• Adiabatic Demagnetization Refrigeration
– Magnetic cooler based on the magnetocaloric effect:• Increasing the magnetic field in a paramagnetic material raises its
temperature; decreasing the field lowers the temperature
• B=0 • Magnetize salt • Magnetize to • Demagnetize• Tsalt≤Tsink until Tsalt≥Tsink full field until Tsalt reaches
• Turn on heat • Wait until operating pointswitch Tsalt~Tsink • Slowly demagnetize
• Turn off heat to maintain stable Tswitch as salt absorbs heat
Operational ModeRecycling
Detectors
Heat sink
SaltPill
T≤Tsink
Heatswitch
Magnet
detectordissipation
heat switchwires, suspension
system, etc.
Continuous ADR
• Stages connected in series pump heat from a load (detectors) up to a warm heat sink
• Stage 1 is always at low T• Upper stages cascade heat repeatedly using same basic mechanism
as for single-stage ADR• Some benefits are 1) continuous operation, 2) higher cooling power,
3) higher operating temperature range
Detector
interface
.05 K .045-.275 KCryocooler
Cold Tip
.25-1 K .9-6+ K
Superconducting
magnet
"Salt pill"
refrigerant
Heat switch
Stage 1 2 3 4
T = 6+ K
Needed Improvement
• Temperature Stability– Magnetic field is control variable for regulating temperature– Periodic reversals of heat flow at stage 1 cause temperature
fluctuations due to finite thermal conductances of components
– Slowing the cycle improves stability, but reduces cooling power
0.04980
0.04990
0.05000
0.05010
0.05020
0.05030
0.05040
0.05050
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 1000 2000 3000 4000 5000
T (K)
Magnetic Field (T)
Time (s)
Feedforward Technique
• Can use knowledge of ADR’s thermodynamic state and components to calculate changes in magnetic field needed to compensate for changes in heat flow
• Techniques shows promise, but needs improvement
-0.0010
-0.0005
0.0000
0.0005
0.0010
0.0015
0.0020
22000 22500 23000 23500 24000
V actualV calc
Time (s)
0.0998
0.0999
0.1
0.1001
0.1002
0.1003
25000 30000 35000 40000 45000 50000
Time (s)
Feedforward ON OFF
Possible Project Elements
• System control– Autonomous control system should be capable of
(re-)establishing low temperature operation from arbitrary starting condition
– Needed for initial startup, and recovery from disturbances or off nominal behavior
– But… control system should be as simple as possible
• Temperature stability– Develop control system that achieves required temperature
stability (~1 microkelvin rms a factor of 10x improvement!)• Evaluate ADR design and thermal characteristics to improve/optimize
control system• Evaluate utility of feedforward technique(s)
• Prototype CADR is available for testing/evaluation