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Control of Multi-stage ADRs

Control of Multi-stage ADRs. Peter Shirron Cryogenics & Fluids Group. ADR Operation. Adiabatic Demagnetization Refrigeration Magnetic cooler based on the magnetocaloric effect:

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Control of Multi-stage ADRs

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

  2. 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 point switch Tsalt~Tsink • Slowly demagnetize • Turn off heat to maintain stable T switch as salt absorbs heat

  3. 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

  4. Prototype 20 min

  5. 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

  6. 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 OFF Feedforward ON

  7. 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

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