A millikelvin cryocooler for space

Transcription

1 UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS MULLARD SPACE SCIENCE LABORATORY A millikelvin cryocooler for space Ian Hepburn Jo Bartlett, Graham Hardy, Alex Green, Matt Hills* Mullard Space Science Laboratory UCL * Now at RAL

2 UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS MULLARD SPACE SCIENCE LABORATORY MSSL millikelvin Cooler Technology Development Outline Single shot system Continuous cooling Space Engineering Model (ESA/STFC funded) ( ) Baselined Edge Origin XEUS Mission proposals Flight qualification of new parts (heat switch) Current activity Design study of space version ESA / UCL Completed Space based Milli-Kelvin Cryo-Cooler Development mkcc (2010-) Ground based MKID optical detector development (MSSL + OU) -STFC KIDSpec (OU+ UCSB + MSSL, Camb.) -STFC System for space (Phase 2 flight system) 2017 start (TDC) System for space (phase 1 preliminary design) March 2016 start, 1 yr Quantum Microwave Sensor (Sussex + MSSL) -EPSRC

3 Millikelvin cryocooler Continuous cooling based on rapid recycling tandem ADRs interface to 4 K or lower. Cooling power is set by how quickly we can recycle the ADR Higher recycling frequency higher cooling power Higher recycling frequency Lower mass for a fixed cooling power D Tf, B=0 B=0 T A Ti, B=0 B=1 T 1 Entropy/R C Tf, Bf B T i, B= Temperature (K) 3

4 Millikelvin cryocooler Tandem Magnetic Refrigerator (single-stage millikelvin cryocooler - smkcc Solid state No moving parts Just electrical input Continuous cold surface Heat switch Refrigerant Heat switch Hot surface Single stage Currently being assembled for test 4

5 Multi-stage millikelvin cryocooler (mkcc) ~50 mk 0.2 K By using multiple stages lower temperatures will be achievable Each stage provides a continuous selectable temperature interface 0.2 K - 4K 2 stage system will be constructed for the microwave sensor project next year for instrument integration in K 2 stage 2 continuous temperatures. 5

6 Talk outline: Phased tested programme, flight study and future plans Single millikelvin cryocooler (smkcc) smkcc components CPA pill Tungsten heat switch Support structure Superconducting magnets Performance of a single ADR using a tungsten heat switch Test set up Measured recycling and hold times Predicted smkcc performance Current smkcc status Millikelvin cryocooler for space requirements Modifications to ground based components for flight Tungsten heat switch Support structure Results of millikelvin cryocooler for space study

7 Single millikelvin cryocooler (smkcc) smkcc has been developed for ground based applications Ground based astronomy Kinetic Inductor Detectors (KIDs) Quantum technology (single photon microwave detectors) University of Sussex, UK Tandem continuous ADR utilising two ADR chains One chain provides cooling whilst the other recycles Each ADR chain is comprised of: a CPA salt pill two tungsten magnetoresistive heat switches three 2 T superconducting magnets two support structures Operates continuously at any temperature in the range 0.25 K to 4 K Operates from a 4 K bath Dimensions: 56 x 120 x 228 mm Mass of 4.67 kg 3.96 kg due to mass of magnets and magnet shields The smkcc ADR insert

8 smkcc components (1) Fast thermal response Chromium Potassium Alum salt pill Chromium Potassium Alum salt pill 30 mm long x 24 mm outer diameter (13.57 cm 3 volume) Contains 22g of CPA Gold plated copper thermal bus mm wires inside Contact area of cm 2 Grown from saturated CPA solution directly in pill casing Measured sub-second thermal response CPA pill thermal bus Thermal response step test: CPA temperature profile as magnet current is stepped (after demagnetisation) Growing the CPA pill Bartlett et. al Design and performance of a fast thermal response miniature CPA salt pill for use in a millikelvin cryocooler Cryogenics 65 (Jan 2015)

9 smkcc components (2) Tungsten magnetoresistive (MR) heat switch Made from single crystal of tungsten Wire EDM cut into 7 layer slotted design % purity Measured Residual Resistivity Ratio (RRR) of 32,000 Free path length of 31 cm Effective 1.5 mm square cross section 12.5 mm in diameter, 32.3 mm high (12 mm excluding mounting flanges) Calculated MR heat switch thermal conductivity for 0, 1, 2, and 3 T magnetic fields 7 layer single crystal tungsten MR heat switch for the mkcc For more information on tungsten theory see: Hills et. al. Thermal Magnetoconductivity of Tungsten below 6 K: Combining the zero-, low- and highfield cases. J. Low Temp Phys 178 (Jan 2015) Switching ratio when using 2 T field: 9,000 at 3.6 K 19,800 at 200 mk Switching ratio when using 3 T field: 15,200 at 3.6 K 38,800 at 200 mk

10 smkcc components (3) Superconducting magnets Fast ramping 0 to 2 T in 30 seconds 30 mm long, 43.6 mm outer diameter Each magnet has its own mild steel magnetic shield Stray field at the cold stage is 69 mt with magnet at 2 T Maximum stray field 150 mm from centre of cooler is 5.79 mt smkcc magnet with magnetic shield Stray magnetic field at the continuous stage Support structure Designed for a lab system Made from G10 fibre glass and Vespel SP1 to minimise heat load More practical for a lab system than Kevlar Stray magnetic field within sample space volume G10/Vespel SP1 support structure Bartlett et. al. Millikelvin cryocooler for space and ground based detector systems Proc. of SPIE 8452 (2012)

11 Mini ADR with MR heat switch performance (1) Miniature ADR tested 1 x fast response CPA pill 1 x tungsten MR heat switch 2 x superconducting magnets 1 x support structure Size: 56 x 56 x 158 mm Bath of 3.6 K provided by PTR Mass 1.48 kg 1.32 kg magnets/magnet shields Measured recycle time of 82 s Measured 17 min hold time at 300 mk Miniature ADR setup 82 second recycling profile µw parasitic heat load Recycling and operation at 300 mk Measured hold times of the miniature ADR Bartlett et. al. Performance of a fast response miniature ADR using a single crystal tungsten heat switch Cryogenics 72 (December 2015)

12 Mini ADR with MR heat switch performance (2) Maximum cooling power measured to give a 3 min hold time 3 minutes chosen as this is ~ twice the recycle time Power applied onto the cold finger via a10 kω heater Power adjusted until a 3 minute hold time was reached For the smkcc, this is expected to be the maximum hold time needed for each CPA pill. Cooling power ascertained from 250 mk to 3 K 45 µw cooling power at 300 mk 317 µw cooling power at 1 K 1.09 mw cooling power at 3 K Measured cooling power to give 3 min hold time Relationship between hold time, total heat load and operating temperature Recycling and hold at 300 mk with 45 µw applied heat load Bartlett et. al. Performance of a fast response miniature ADR using a single crystal tungsten heat switch Cryogenics 72 (December 2015)

13 Predicted smkcc performance Predicted smkcc performance is based on the single miniature ADR results and the thermal model The thermal model is used to estimate the maximum heat load from the recycling ADR based on a 4 K peak recycling temperature Requirement for smkcc is for the hold time of each CPA pill to exceed the recycle time Maximum hold time needed estimated at 3 min (~ double the recycle time) Minimum hold time estimated to be 90 seconds (few seconds longer than recycle time) Based on the max. and min. hold times, can estimate the upper and lower limit of the smkcc cooling power. smkcc ADR insert Estimated maximum cooling power of: µw at 250 mk µw at 300 mk 2.1 mw at 3 K Predicted cooling power of the smkcc Bartlett et. al. Performance of a fast response miniature ADR using a single crystal tungsten heat switch Cryogenics 72 (December 2015)

14 Current smkcc status and mkcc construction The smkcc is assembled on the bench and testing will start Jan 2016 We have funding to build a two-stage smkcc for the University of Sussex (UK) for their detectors For use in the lab Extra magnetic shielding will be required for the detectors (target of nt currently stray field is mt) Operation from 4 K (pulse tube cooled) Two continuous stages to be provided by stacking one smkcc on top of the other Target of at least 80 mk for lowest temperature stage Building starts summer 2016, integration in quantum microwave system in 2017.

15 A space system ESA+UCL project has looked at making the mkcc robust enough to survive an Ariane 5 rocket launch. Finite Element Analysis shows in principle no show stoppers. 15

16 Study cooler requirements Based on the requirements for the cooling chain for the XMS instrument on IXO (Now ATENA) 45 mk level: Cooling TES and electronics Intermediate level: K 4 K /2 K bath temperature: Provided by external cooler Cooling power requirement depends on temperature of higher stages 1.6 μw at 45 mk from 0.3 K 2.0 μw from 0.6 K 3.0 μw from 1.0 K Continuous cooling

17 Millikelvin Cryocooler for Space Configuration Based on two smkcc s stacked together to provide two constant temperature stages: Cold stage Intermediate stage Retain magnet design Retain CPA pill design Increase contact area in thermal bus to account for thermal boundary resistance Modify heat switch design Change support structure design

18 Vibration Specification Frequency (Hz) Sine test 5-18 ± 11 mm g g Frequency (Hz) Random test db / octave g2 / Hz db / octave Target: First normal mode > 140 Hz

19 Magnetoresistive Heat Switch Cut from a single crystal of tungsten Current 7-layer design not intended for spaceflight

20 Finite Element Analysis of smkcc Heat Switch Mesh Deformation stress Normal modes

21 Ground Based smkcc Design Example of random vibration along Z-axis Stress concentration due to sharp corners Also makes modelling less reliable Random stress: 110 MPa 3-sigma stress: 330 MPa C.f. yield strength: 550 MPa Very little margin! New design

22 Heat Switch Design for Millikelvin Cryocooler for Space Coil shape minimises sharp angles 4 layers Mounting arms Dimensions: Path cross-section: 1 mm x 1 mm Free path length: 19 cm Gaps: 0.25 mm between arms Off thermal isolation: 0.3 μw parasitic between 45 mk and 0.6 K 3.6 μw parasitic between 0.6 K and 2 K

23 Random Vibration Modelling Same test: Random vibration, Z-axis 3-sigma stress: 77 MPa Reduced by factor 4 No point concentrations Maximum stress is on inside of arm

24 Random Vibration Modelling X-axis Y-axis

25 Sine Vibration Modelling First normal modes: Sine vibration test (Z-axis): Lower stresses

26 Displacement Vibration Modelling Sine vibration test, Z-axis Gap displacements: Maximum: mm Separation: 0.25 mm

27 Flight heat switch FE analysis predicts new design can be flight qualified Preparation for manufacture underway First vibration in early 2016

28 Support Structure for Millikelvin Cryocooler for Space Kevlar suspension: Kevlar mm and 0.25 mm diameter 5 cm length Groups of 6 and 12 supports Heritage: ESA ADR designed at MSSL Strung for 13 years

29 New Support Structure Can be modelled as 4 independent systems One pill and two heat switches in each unit Units connected by flexible thermal straps

30 Vibration modelling First normal mode: 182 Hz Simplified model used but has good agreement with solid model ~ 50 % additional stress in the sine test still more than a factor of 20 below yield strength

31 Results of Millikelvin Cryocooler for Space study Cold stage: Overall: Continuous operation at 45 mk 4 paramagnetic pills 2.0 μw cooling power at 45 mk 8 heat switches Operating from 0.6 K intermediate stage 12 magnets Chromium Potassium Alum 1 T magnetic field Dimensions: 16 cm x 16 cm x 32 cm Intermediate stage: Continuous operation at 0.6 K Mass: Up to 100 μw cooling power at 0.6 K 11 kg Operating from 2 or 4 K Chromium Potassium Alum 2 T magnetic field Rejected heat: ~ 20 mw to warm stage (peak, during recycling) Expected to meet requirements of the study

32 Conclusion Millikelvin cryo cooler is being developed for space to provide two continuous temperature interfaces of: 45 mk (50 mk with 2 µw predicted) 0.3 K (0.6 K with 100μW predicted) Based on the MSSL ground based millikelvin cryo-cooler. o This cooler is currently progressing through a phased test programme. So far testing has agreed very well with the model predictions. o A 2 stage cooler is due to be delivered to Sussex University for integration in a quantum microwave sensor system in Preliminary mechanical analysis for space shows no show stoppers. o First vibration test of new component (magnetoresistive heat switch) will be performed early All other components are based on qualified heritage. Preliminary design of a flight system due to start March yr duration (Phase 1) Construction of a flight system anticipated to follow completion of Phase 1 starting in 2017.

33 Acknowledgements This work is supported by the following Engineering and Physical Research Council (EPSRC), Millikelvin cryocooler, Quantum microwave sensor Science and Technology Facilities Council (STFC), KIDs European Space Agency & UCL, preliminary space adaptation work 33

34 Thank-you