Cooling system for the soft X-ray spectrometer onboard Astro-H

The Soft X-ray Spectrometer (SXS) is a cryogenic high resolution X-ray spectrometer onboard the X-ray astronomy satellite Astro-H which will be launched in 2014. The detector array is cooled down to 50 mK using an adiabatic demagnetization refrigerator (ADR). The cooling chain from the room temperature to the ADR heat-sink is composed of superfluid liquid He, a Joule-Thomson cryocooler, and double-stage Stirling cryocoolers. It is designed to keep 30 l of liquid He for more than 5 years in the normal case, and longer than 3 years even if one of the cryocoolers fails. Cryogen-free operation is also possible in the normal case. It is fully redundant from the room temperature to the ADR heat-sink.     

Sub-Kelvin cooler configuration study for the Background Limited Infrared Submillimeter Spectrometer BLISS on SPICA

The Background Limited Infrared Submillimeter Spectrometer (BLISS) is an instrument proposed for the Japanese space borne telescope mission SPICA. The BLISS concept is a suite of grating spectrometers which combine to cover the 40-400 μm range at resolving power R∼700 with detector sensitivity approaching the natural photon background limits. To achieve the high sensitivity, the BLISS detectors require cooling to 50 mK, well below the 1.7 K cold stage provided on the SPICA spacecraft. We present a thermal architecture for BLISS that includes a thermal intercept stage actively cooled to a temperature in between the 1.7 K cold tip and the detector stage at 50 mK. This architecture requires, essentially, two coolers; one to cool the intercept stage from 1.7 K and one to cool the detectors from the intercept stage temperature to 50 mK. We compared several configurations of flight-heritage coolers to cool the intercept and detector stages. Of the various configurations studied, a continuous adiabatic demagnetization refrigerator (ADR) for each stage has the highest maturity, lowest heat dump at 1.7 K and total mass comparable to other approaches. Other options, such as a Herschel ³He sorption cooler-ADR hybrid and the recently demonstrated closed cycle version of the dilution cooler on Planck are also feasible for BLISS on SPICA.     

Development of double-stage ADR for future space missions

We report a development of a portable dewar with a double-stage ADR in it, and its cooling test results. The purpose of this system is to establish a cooling cycle of double-stage adiabatic demagnetization from 4.2 K to 50 mK, which is strongly desired for future space science missions. In our test dewar, two units of ADR are installed in parallel at the bottom of a liquid He tank. We used 600 g of GGG (Gadolinium Gallium Garnet) for the higher temperature stage (4 Tesla) and ∼90 g of CPA (Chromic Potassium Alum) for the lower temperature stage (3 Tesla). A passive gas-gap heat switch (PGGHS) is used between these two stages, while a mechanical heat switch between the He tank and the GGG stage. Using this system, 50 mK was achieved, and various kinds of cooling cycles with different operating temperatures and different sequences of magnetization were tested. We also evaluated the performance of the PGGHS, and interference of the magnetic field with each other during a stable temperature control.     

50 mK cooling solution with an ADR precooled by a sorption cooler

CEA/SBT is currently developing a 2.5 K-50 mK cooling solution composed of a small demagnetization refrigerator (ADR) precooled by a sorption cooler, equivalent to the high temperature stage of a two-stage ADR system. Thanks to the use of this dual technology, a low weight cooler able to reach 50 mK with a heat sink up to 2.5 K can be designed. Because the sorption cooler is probably the lightest solution to produce sub-Kelvin temperatures, these developments allow us to propose a solution to face the drastic reduction in the mass budget of space missions like SPICA or IXO. The European Space Agency (ESA) is funding the development of an engineering model able to produce 1 μW net heat lift at 50 mK. It is sized so that the sorption cooler provides an additional 10 μW at 300 mK. The ESA main requirements are an autonomy of more than 24 h and a recycling time smaller than 8 h. We present the design of the system able to meet these requirements as well as the expected performances and preliminary measurements.     

Thermal conductivity measurements of pitch-bonded graphites at millikelvin temperatures: Finding a replacement for AGOT graphite

Pitch-bonded graphites are among the best known thermal insulators at sub-kelvin temperatures, but are very good conductors at higher temperatures. This makes them ideal for mechanical supports which must provide good thermal isolation at an operating temperature below 1 K, but must have good conductance at higher temperatures to aid in initially cooling down an instrument (a “passive heat switch”). One type of graphite, AGOT, has been known as having the lowest thermal conductivity below 1 K not only among graphites, but also compared with any other material. It is, however, no longer available. We have carried out thermal conductivity measurements at temperatures between 60 mK and 4 K on a proposed replacement, POCO AXM-5Q graphite, as well as a sample of AGOT graphite. Our measurements show that both graphites have a difference of about six orders of magnitude in conductivity between room temperature and 100 mK, but that AGOT graphite is not as good an insulator as previously believed. We conclude that AXM-5Q graphite is not only a suitable replacement for AGOT, but in fact is somewhat superior.     

A closed cycle He-He dilution refrigerator insensitive to gravity

The cooling power and the lifetime of an open cycle dilution refrigerator as developed for the Planck mission (100 nW at 100 mK during 30 months) are limited by the quantity of the helium isotopes carried on the satellite at launch, because the helium mixture obtained after the dilution process is rejected into space. Future space missions require to increase the cooling power and lifetime significantly (1 μW at 50 mK during 5 years).Therefore we are extending the open cycle dilution refrigerator with a helium isotope separator operating at 1 K to close the cycle. A first prototype to demonstrate the principle of the closed cycle dilution refrigerator has been tested and a cooling power of 1 μW at temperatures below 60 mK has been obtained. We present the apparatus and the experimental results and give some elements for its integration in a complete cooling chain. The advantages (continuous operation, absence of magnetic field, less weight) of a closed cycle dilution refrigerator with respect to an adiabatic demagnetization refrigerator are also discussed.     

A vibration free cryostat using pulse tube cryocooler

This paper introduces a new vibration free cryostat cooled by liquid helium and a 4 K pulse tube cryocooler. The cryogenic device mounts on the sample cooling station which is cooled by liquid helium. The boil off helium is recondensed by the pulse tube cryocooler, thus the cryostat maintains zero boil off. There is no mechanical contact between the cryogenic part of the cryocooler and the sample cooling station. A bellows is used to isolate the vibration which could transfer from the cryocooler flange to the cryostat flange at the room temperature. Any vibrations generated by the operation of the cryocooler are almost entirely isolated from the cryogenic device. The cryostat provides a cooling capacity of 0.65 W at 4.21 K on the sample cooling station while maintaining a vapor pressure of 102 kPa. The sample cooling station has a very stable temperature with oscillations of less than ±3 mK during all the operations. A cryogenic microwave oscillator has been successfully cooled and operated with the cryostat.     

Development of mechanical cryocoolers for Astro-H/SXS

The Soft X-ray Spectrometer (SXS) is a high-resolution spectrometer with an X-ray micro-calorimeter array onboard the Japanese X-ray astronomy satellite Astro-H, planned for launch in 2013. The micro-calorimeter is operated at cryogenic temperature of 50 mK provided by the Adiabatic Demagnetization Refrigerator (ADR) with a heat sink of 1.3 K liquid helium stored in the SXS Dewar. To extend the liquid helium lifetime to over 3 years in orbit, two types of mechanical cryocoolers are installed: 20 K-class double-staged Stirling (2ST) coolers and a 1 K-class Joule-Thomson (JT) cooler. Improvement of mechanical cryocoolers has been investigated and verified for higher reliability and cooling performance. The engineering model (EM) of upgraded mechanical cryocoolers was fabricated for a long lifetime test. The required cooling power of 200 mW at 20 K for the 2ST cooler and 10 mW at 1.7 K for the JT cooler are achieved by EM test.     

Improved performance of an engineering model cryogen free double adiabatic demagnetization refrigerator

This paper describes the design, development and performance of the engineering model double adiabatic demagnetization refrigerator (dADR) built and tested under contract to the European Space Agency for its former mission XEUS (now IXO). The dADR operates from a 4 K bath and has a measured recycle and hold time (with a parasitic load of 2.34 μW) at 50 mK of 15 h and 10 h, respectively. It is shown that the performance can be significantly improved by operating from a lower bath temperature and replacing the current heat switches with tungsten magnetoresistive (MR) heat switches, which significantly reduce the parasitic heat load. Performing the latter gives an anticipated recycle and hold time of 2 and 29 h (with a 1 μW applied heat load in addition to the parasitic load), respectively. Such improved performance allows for a reduction in mass of the dADR from 32 kg to 10 kg by operating from a 2.5 K bath (which could be reduced further by optimising the magnet design). Ultimately, continuous operation could be achieved by linking two dADRs to a common detector stage and operating them alternately. Based on this design the mass of the continuous ADR is estimated to be about 4.5 kg.     

Experimental measurement of vapor-liquid equilibrium for (trans-1,3,3,3-tetrafluoropropene (R1234ze(E)) + propane (R290))

The VLE data for the binary system of R1234ze(E) + R290 were measured with a recirculation method at four temperatures (258.150, 263.150, 273.150 and 283.150 K). The measured uncertainties of the temperature, pressure, and compositions are ±5 mK, ±0.0005 MPa, and ±0.005, respectively. All the experimental data were correlated by the Peng-Robinson (PR) EoS with the Huron-Vidal (HV) mixing rule involving the non-random two-liquid (NRTL) activity coefficient (PR-HV-NRTL) model. Azeotropic behavior can be found at the measured temperature range.     

Advances in silicon carbide X-ray detectors

The latest advances in SiC X-ray detectors are presented: a pixel detector coupled to a custom ultra low noise CMOS preamplifier has been characterized at room and high temperature. An equivalent noise energy (ENE) of 113 eV FWHM, corresponding to 6.1 electrons r.m.s., has been achieved with the detector/front-end system operating at +30 °C. A Fano factor of F=0.10 has been estimated from the ⁵⁵Fe spectrum. When the system is heated up to +100 °C, the measured ENE is 163 eV FWHM (8.9 electrons r.m.s.). It is determined that both at room and at high temperature the performance are fully limited by the noise of the front-end electronics. It is also presented the capability of SiC detectors to operate in environments under unstable temperature conditions without any apparatus for temperature stabilization; it has been proved that a SiC detector can acquire high resolution X-ray spectra without spectral line degradation while the system temperature changes between +30 and +75 °C.     

Thermal conductivity of ME771 glass-epoxy laminate from millikelvin temperatures to 4 K

Permaglas ME771 is a glass-epoxy laminate which is suitable for use at cryogenic temperatures. We have measured the thermal conductivity of a sample of this material between 64 mK and 4.2 K in the direction parallel to the reinforcing fibres, enabling us to make a comparison with the better known material G-10CR. The thermal conductivity follows the form that would be expected for such a material, and is similar to that of G-10CR, which has a similar (room temperature) tensile strength. We comment on some confusion that has arisen over the difference between G-10CR, a material specifically produced for cryogenic use, and G-10, the more common equivalent.     

Thermal conductivity of Tecamax® SRP from millikelvin temperatures to room temperature

Tecamax® SRP (self-reinforced polyphenylene) is a new commercially available amorphous polymer which is suitable for use at cryogenic temperatures. It has a high tensile strength (210 MPa at room temperature), resulting from the molecular structure of the polymer rather than by the addition of reinforcing materials. We have measured the thermal conductivity between 60 mK and 280 K. We find that the conductivity below 10 K is similar to, but lower than, most amorphous materials, and the material offers a good combination of low conductivity at low temperatures and high tensile strength. Our results suggest that the material may in fact have a small crystalline component, which may be a partial explanation for the low conductivity. Above 10 K, the temperature dependence of the conductivity is different from most amorphous materials. We are unaware of previous measurements of the thermal conductivity of this material, even at room temperature.     

Ir TES Electron-Phonon Thermal Conductance and Single Photon Detection

We have operated an Ir TES as detector for single photon at 450 nm in a temperature range of 100–120 mK. The decoupling of the electron gas from the phonons in the film, caused by the fifth power dependence of the temperature, is measured from the pulse decay time. The detection of single photon generated by a laser diode with a pulse length of 500 ns in a 25×25 μm² detector area at a base temperature of 90 mK is shown.      

ADR design for the Soft X-ray Spectrometer instrument on the Astro-H mission

In its instrument suite, the Japanese Astro-H mission will include the Soft X-ray Spectrometer (SXS), whose 36-pixel detector array of ultra-sensitive X-ray microcalorimeters will be cooled to 50 mK. This will be accomplished using a two-stage adiabatic demagnetization refrigerator (ADR). A complicating factor for its design is that the ADR will be integrated into a superfluid helium dewar at 1.3 K that will be coupled to a 1.8 K Joule-Thomson (JT) stage through a heat switch. When liquid helium is present, the coupling will be weak, and the JT stage will act primarily as a shield to reduce parasitic heat loads. When the liquid is depleted, the heat switch will couple more strongly so that the ADR can continue to operate using the JT stage as its heat sink. A two-stage ADR is the most mass efficient option and it has the operational flexibility to work well with a stored cryogen and a cryocooler. The ADR’s design and operating modes are discussed, with emphasis on how they reflect the capabilities and limitations of the hybrid cryogenic system.     

A method for accurate thermal conductivity measurements of small samples at ultra-low temperatures

A specific experimental arrangement has been developed for low temperature measurements of thermal conductivity of small samples such as single crystals of magnetic insulators with a typical length of a few millimeters. A frame of low conductance, serving as a mechanical support for ruthenium thermometers recording the temperature gradient on a sample, has been tested in the temperature range from 150 mK to 5 K by using commercial 99.95% purity polycrystalline non-annealed molybdenum. The applicability of the setup is discussed for the samples with the thermal conductance in the range 10⁻⁵-10⁻³ W/K.     

Effect of carbon nanofibers (CNFs) content on thermal and mechanical properties of CNFs/SiC nanocomposites

Carbon nanofibers dispersed β-SiC (CNFs/SiC) nanocomposites were prepared by hot-pressing via a transient eutectic phase route at 1900 °C for 1 h under 20 MPa in Ar. The effects of additional CNFs content between 1 and 10 wt.% were investigated, based on densification, microstructure, thermal and mechanical properties. The CNFs/SiC nanocomposites by the CNFs contents below 5 wt.% exhibited excellent relative densities over 98% with well dispersed CNFs. However, the CNFs/SiC nanocomposites containing the CNFs of 10 wt.% possessed a relative density of 92%, accompanying CNFs agglomerates and many pores located inside the agglomerates. The three point bending strength gradually decreased with the increase of CNFs content, but the indentation fracture toughness increased to 5.7 MPa m^1/2 by the CNFs content of 5 wt.%. The thermal conductivity was enchanced with the increase of CNFs content and represented a maximum value of 80 W/mK at the CNFs content of 5 wt.%.     

Fractal-to-compact transition in step decorations obtained by KMC simulations of MBE

With increasing temperature the fractals grown on a surface during submonolayer epitaxy lose their fractal character and eventually become compact islands. Experimental determination of the transition temperature is difficult and it is not known if the fractal-to-compact transition is sharp or arises gradually. In the present work, the transition is examined for the Cu/W(1 1 0) system based on kinetic Monte Carlo simulations. The simulations refer to the fractal growth at an ascending step oriented in the [1 1 0] direction on bcc(1 1 0). The fractal character of step decorations is measured by their fractal dimension calculated by the mass-radius relation. The fractal dimension changes from 1.71±0.02 in the fractal growth mode to 2.00 in the compact growth mode. It is shown that the fractal-to-compact transition takes place gradually at temperatures ranging from 220 to 270 K.     

Processing map for hot working characteristics of a wrought 2205 duplex stainless steel

Processing map on a wrought 2205 duplex stainless steel under hot compression conditions has been developed based on the dynamic material model theories in the range 1223–1473 K and 0.01–10 s⁻¹. The various domains in the map corresponding to different deformation characteristics have been discussed in combination of microstructural observations. The results show that the power dissipation efficiency (η) depends strongly on the dynamic recrystallization (DRX) of austenite which plays a dominant role in microstructural evolution, while the ferrite phase mainly continues to exhibit relatively well-developed dynamic recovery (DRV) at large strain. The optimum hot working domain of wrought 2205 duplex stainless steel is obtained to be in the temperature range 1373–1473 K and at strain rate of 0.01 s⁻¹, with peak efficiency 50% occurring at about 1423 K, in which more uniform microstructure is developed due to the occurrence of complete DRX of austenite. The unstable hot working regimes are predicted by Prasad instability criterion, in good agreement with the macro-and microstructural observations. As predicted, flow instability, which are manifested as twinning, bands of flow localization and the absence of DRX in austenite are observed at lower temperatures and higher strain rates (1223–1273 K and 1–10 s⁻¹); in other cases, wedge cracking is responsible for instability phenomena observed at the temperature range 1373–1423 K and strain rate of 10 s⁻¹.