UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS
MULLARD SPACE SCIENCE LABORATORY
Milli-Kelvin Cryo-Cooler Development at the Mullard Space
Science laboratory – I. Hepburn, J. Bartlett, G. Hardy
Engineering Model (2002-2008)
44 kg
IXO Development (2008 -)
<10kg
Milli-Kelvin Cryo-Cooler
Development (2010 -)
<2 kg
1
Further information
• Bartlett et. al. Cryogenics 50 (2010) 582-590 “Improved
performance of an engineering model cryogen free double
adiabatic demagnetisation refrigerator”
• Bartlett et. al. Cryogenics 50 (2010) 647-652 “Thermal
characterisation of a tungsten magnetoresistive heat
switch”
2
Engineering model - 50 mK ADR cooler
• ESA Contract to Astrium (Stevenage)
in association with MSSL in 2002.
– Develop and qualify 50 mK ADR cooler.
• Base temperature < 30 mK
– To be able to be pre-cooled by 4 K J-T
(Planck cooler)
– To have high level magnetic shielding to
protect spacecraft and X-ray detectors
on cold finger
Formal transfer of system to ESA June 2008.
(System has stayed at MSSL on loan for continued component development)
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50 mK EM ADR
DGG stage
CPA stage
4 K or lower
interface
Cold finger/detector
stage
DGG
magnet
DGG magnet
booster coils
CPA magnet
booster coils
CPA magnet
cancellation coils
DGG magnet
cancellation coils
4
Construction and qualification
Modular construction to enable sub-systems to be
used in future systems.
key sub-systems qualified
• Kevlar suspension system
• Low current superconducting magnets
• Heat switches
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Kevlar suspension
Kevlar suspends cooling parts
of the ADR
• 6 kg suspended mass
• Kevlar tensioned to 720 N
• Passed vibration qualification
• Test system has remained
strung since March 2004 (6.5
years) with no change.
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Superconducting magnets
Low current to magnetic field superconducting
magnets (0.79 Amps per Tesla)
– Low current to minimize Joule heating in leads
– Minimize electrical power required
Magnetic field lines
Magnet coil layout
Magnet system on vibration
table
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Heat switches
• Required to extract heat during
magnetisation phase of ADR
cycle
• Provide thermal isolation during
cold phase.
• Superconducting heat switch
qualified for use at the milliKelvin end of ADR
Superconducting switch connected
to cold finger
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Modelling
Mathematical thermal model developed during
project.
– Model agrees very well with real system
– Model incorporates low temperature heat flow (i.e.
Thermal boundary resistance)
– Model includes
•
•
•
•
All heat loads.
Base temperature.
Hold time at required temperature.
Recycle times
– Model used to investigate performance of future
systems e.g IXO and milli-Kelvin cryo-cooler
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System mass breakdown
Sub-system
Assembly component
Paramagnetic including
Complete CPA and DGG assembly including
cold finger
cold finger
Magnet
CPA mass
0.25 kg
DGG mass
0.9 kg
Magnet assembly (of which 11.196 kg is
Mass (kg)
5.526
24.080
superconducting wire)
60% magnet mass due to cancellation
coils and associated former structure
Magnetic shielding
Focal plane magnetic shield (passive shield)
12.9
Heat switches
Gas switch
0.127
Superconducting lead switch
0.097
Miscellaneous ADR parts
1.942
Miscellaneous
Sub total
44.672
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As delivered performance (2008)
•
System kept at 4 K for just over 1 year (cooled by a 4 K pulse tube
cooler).
– No change during the year long operation
•
Base temperature: 25 mK
– Limited by thermal boundary resistance inside pill
•
Recycle time: 15 hours (Required 4 hours)
– Limited by superconducting heat switch
– Limited by maximum heat load to
envisaged 4 K J-T cooler (peak load < 5 mW)
•
Hold time at 50 mK with 1 µW: 10 hours (Required 24 hours)
– Limited by heat load from superconducting lead heat switch
•
Hold time at 30 mK: 2.2 hours
•
Thermal stability at 50 mK: ± 8 µK
– Limited by thermometer readout electronics
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Improvements & new systems
•
•
•
EM system on loan to MSSL for continued technology development
(e.g. heat switches)
Mathematical thermal model identified main area affecting
performance - Superconducting heat switch.
New heat switch developed – magnetoresistive (MR) heat switch –
solid state switch
– Qualification project just
started.
– Improvement in performance
demonstrated with lab MR switch.
• Purity of material not high
enough to obtain full performance.
• With required purity, performance
will exceed requirement.
New switch in EM ADR
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IXO development (X-ray micro-calorimeter
spectrometer.)
• IXO reduction in stray magnetic field enables reduced
magnet coils
significant mass saving (2 magnet coils rather then 10).
• No cold finger enables splitting of cooler stages and thus
optimization.
significant mass saving.
• Can use existing Kevlar suspension system as EM
system.
• Same cooling stages as EM system.
• Magnet wire/manufacture same as EM system.
• Same heat switches as EM system (2 K operation) or
with MR heat switch for 4 K operation.
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IXO Magnet design
•Same wire as used in ESA system (can run
at 4 K or below)
•FEA magnet design performed
•FEA thermal investigation performed
•FEA mechanical investigation performed with
Ariane 5 vibration loads.
Only 2 coils required
rather then 10 for the
EM system
Magnet design robust
Magnet field
lines
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IXO system model performance and masses
Interface Temperature
2K
2K
4K
S/C Switch +
Gas Switch
MR Switch +
Gas Switch
MR Switch +
Gas Switch
CPA mass
750 grams
500 grams
750 grams
CPA 50 mK hold time
31 hours with 2
µW load
(+parasitic)
32 hours with 2 31 hours with 2
µW load
µW load
(+parasitic)
(+parasitic)
DGG mass
1000 grams
900 grams
1000 grams
DGG stabilised
temperature (ADR 1st
stage temperature)
0.6 K with 7µW
load (+parasitic)
0.6 K with 7
µW load
(+parasitic)
0.6 K with 7µW
load
(+parasitic)
Re-cycle time
9 hours
<6 hours
<6 hours
Pill stage mass
(including structure)
2 350 grams
2000 grams
2350 grams
Magnet mass
7094 grams
7094 grams
7094 grams
Heat switch mass
~250 grams
~500 grams
~500 grams
Total mass
9694 grams
9594 grams
9950 grams
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Milli-Kelvin cryocooler
• Development started on a ground based milli-Kelvin
cryo-cooler.
– Provide continuous cooling from 4 K down to 50100 mK from a 4 K cryo-cooler.
– Based on multi-stage ADR units
– 2 year project started in July 2010 with £0.5M
funding from EPSRC (Engineering and Physical
Sciences Research Council, UK).
– Cooler using technology developed as part of the
EM ADR cooler and IXO development (e.g.
Magnets and MR switch)
 Easier to transfer to flight system
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Milli-Kelvin cryo-cooler
Project time scales
• PDR – January 2011
• CDR – May 2011
• Testing – end of 2011
Projected mass ≤ 2 kg for mK cryocooler module.
mK cryo-cooler
module
Cryo Mech pulse
tube cooler
4 K cryo-cooler
interface
Cold surface
50-100 mK base
temperature.
Module made from multiple ADR stages. Therefore
temperature settable over range 50-100mK ≤ T< 4K.
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Summary
• On-going 50 mK cooler programme builds on
existing technology development
– ESA EM ADR
– Heat switch development
• Application to missions like IXO and the cryogenic
spectrometer instrument with significant mass
saving from original system.
• New generation of ground based cooler providing
continuous mK temperatures in a very low mass
system in development.
– Adaptable for use in space since largely using
technology/components from EM / IXO development
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