Control of Multi-stage ADRs Peter Shirron Cryogenics & Fluids Group

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=0Magnetize saltMagnetize toDemagnetize T salt ≤T sink until T salt ≥T sink full fielduntil T salt reaches Turn on heatWait untiloperating point switchT salt ~T sinkSlowly demagnetize Turn off heatto maintain stable T switchas salt absorbs heat

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 T = 6+ K

Prototype 20 min

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

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 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