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PULSE-TUBE PRECOOLED AND HYPERFINE-FIELD-ENHANCED NUCLEAR REFRIGERATION WITH NOISE THERMOMETRY
Aya Shibahara, Microkelvin Workshop 2013
JRA1 Task 2JRA4 Task 3a
Nanoscience community for fast turnaround µK measurements.
Oxford Instruments for industrial/commercial reasons
Us for testing noise thermometry
Staying below 1mK on a cryogen-free fridge
InK: Implementing the new Kelvin To resolve the long-standing discrepancy
between the PLTS 2000 measurements through low uncertainty primary thermometry.
Praseodymium Nickel-5: PrNi5 Hyperfine enhanced paramagnet Contains rare earth ion Pr3+, spin 5/2 At low temperatures, the 4f electrons have an
electronic singlet non-magnetic ground state. External magnetic field mixes higher non-singlet states
into the ground state, inducing an electronic magnetic moment.
This generates a hyperfine field Bint at the Pr nucleus, which is enhanced compared to the externally applied field B.
K = Bint/B = 11.2
B enhanced by (1+K), ln enhanced by (1+K)2
Entropy curves
Comparison of PrNi5 with Copper
Large entropy reduction possible
Less eddy-current heating
Less than 1 mol required
Only a maximum of 20% entropy reduction possible
High conductivity 10-100 mol used
PrNi5 Copper
Spontaneous ferromagnetic nuclear ordering T = 400 µK
Internal field 65 mT
Spontaneous antiferromagnetic nuclear ordering T = 50 nK
Internal field 0.36 mT
The Oxford Instruments Triton 200
Commercial cryogen free dilution fridge
Decoupled pulse tube cooler first and second stages from the refrigerator plates
Brass MC shield and Still shield
Dry 8 T magnet mounted at the second pulse tube stage 3K plate
Schematic of shields and magnet on Triton 200
The Nuclear stage
128 g of PrNi5 (0.3 mol) Nine rods, 6 mm Ø × 50 mm long 99.99% Cadmium solder to 1 mm Ø Cu
wires One wire per rod to upper plate Eight wires per rod to lower plate From Jeevak Parpia, Cornell Aluminium heat switch supplied by
Richard Haley, Lancaster. Noise thermometer heat sunk to lower
plate with 37 × 0.7 mm Ø annealed Cu wires
Current sensing Noise thermometer
Noise sensor:
0.24 mΩ copper foil resistor
Heat sinking
ground
Annealed copper
holder
Heat sinking copper washer
Heat sinking
Nb foil
Single calibration at 4.2 K Simple to install Compact
Nb screw
terminals
Current sensing Noise thermometer
C636 G24 XXL SQUID from PTB Input coil Li = 1.8 µH
Noise spectra at various temperatures
Here we present the performance of the PrNi5 nuclear stage on the Triton 200
Results
Typical demag procedure
Pump on vacuum can overnight With magnet, base temperature of DU reached in
48 hours Precool field limited to 6.2 T (90 A)
Due to stray field at aluminium heat switch But high enough field for significant entropy reduction
Precool duration 24 h for 20 mK Typically 40-45 h weekend precool for 19 mK Corresponds to 80% entropy reduction
Demagnetisation from 6.2 T to 0 T in 6 hours In steps from 90 A to 0 A with rates from 0.5 A/min to 0.03
A/min
How long does it stay cold?
Base temp of 600 µK reached in zero field
20 nW heat leak: 16 hours below 1 mK
5 nW heat leak: over 24 hours below 1 mK
Increased hold time for 210 mT
Heat capacity of PrNi5
Kubota et al. Phys. Rev. Lett. 45, 22 (1980)
Thermometer performance on a wet nuclear demag fridge
Traditional copper nuclear stage on a wet system Noise thermometer of the same design, base T = 190 µK
Fast PtW noise thermometer
TN = 130 µK
Precision of Fast noise thermometer
R = 1.29 Ω Dilution fridge 1% precision in
100 ms
Conclusions
A bolt-on PrNi5 nuclear demag stage for a pulse-tube pre-cooled system was shown to cool to 600 µK and remain below 1 mK for over 24 hours, with a heat leak of 5 nW.
The use of a current sensing dc SQUID noise thermometer allowed the direct measurement of these sub-mK temperatures.
A precool in a field of ~6.2 T and a starting temperature of ~20 mK with a 6 hour demag is feasible.
Compact and easy to use Cryogen-free sub-mK platforms seem to be a
realistic prospect, dramatically improving the accessibility of ultra-low temperatures
Thank you to all our collaborators and funding bodies!
And thank you for listening!arXiv:1307.7049
Heat leak measurements
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