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=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

Prototype

0.1

1.0

0 1000 2000 3000 4000 5000 6000

Time (s)

Stage 1

Stage 2

Stage 3

Stage 4

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

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