I. Catarino

CUSTOMIZABLE GAS‐GAP HEAT SWITCH

The so‐called gas gap heat switch, in which the pressure is managed by a coupled small cryopump having no moving parts, is known to be a very reliable and simple heat switch.Mechanical design improvements can lead to optimized ON or OFF characteristics of a gas‐gap heat switch. Their ON conductance characteristics are mainly determined by the gas properties and the gap geometry. However, their operational temperature range is limited by the gas‐sorbent pair adsorption characteristics. Traditionally the gas chosen is helium, since it is the best conductive one below 100 K, and the sorbent used is activated charcoal. Such a switch is limited to be used at cold end temperatures below ∼15 K.In order to obtain a customizable device working at the whole range below 100 K, a gas gap heat switch was studied and extensively characterized. Hydrogen, Neon and Nitrogen were used as conducting gas, under different sorption conditions. A thermal model was built in order to determine the ON and OFF conductances over eac...

Magnetism of the series UFexAl12−x

Abstract Recent results from low-temperature specific heat and magnetization measurements performed on crystals of UFe x Al 12− x (3.8 x

Materials for damping the PTC-induced thermal fluctuations of the cold-head

The cold head on mechanical Pulse Tube Cryocoolers (PTCs) is subject to substantially less mechanical vibration and electromagnetic interference compared to that typically found in Gifford MacMahon coolers. However, thermal fluctuations at the PTC frequency are still present at the cold-head, typically at a level of 200 mK peak-to-peak at 1.4 Hz for a Cryomech Model PT405 cooler running at 4 K. It is highly desirable to damp out these fluctuations if PTCs are to be used successfully for running systems sensitive to such thermal fluctuations, for example, bolometeric detectors.We report here the characterization over the temperature range 2.5 K to 6 K of two materials, GOS (Gd2O2S) and GAP (GdAlO3), for use as low-pass thermal filters. These materials have antiferromagnetic transitions at around 4 K giving rise to an enhanced heat capacity and have a high thermal conductance. These are two highly desirable properties for thermal dampers in this application. Those materials were fired as ceramic discs to be tested as thermal dumpers. Thermal filter assemblies with discs of diameter 75 mm and thickness 2.5 mm and 1.6 mm (GOS and GAP, respectively) mounted in a PTC show thermal attenuation levels of x0.12 (GOS) and x0.11 (GAP) at 0.01Hz with a clean-side temperature of 4 K; the PTC induced fluctuations at 1.48 Hz are damped completely to within the noise limits (0.2 mK) of the thermometers. Experimentally determined thermal conductance and heat capacity data are reported. For this system, with a PTC cold-head (dirty-side) temperature of 3.3 K, a clean-side power dissipation of up to 30 mW is realized before its temperature rises above 4.2 K.

Gas gap heat switch for a cryogen-free magnet system

Cryogen-free superconducting magnet systems (CFMS) have become popular over the last two decades for the simple reason that the use of liquid helium is rather cumbersome and that helium is a scarce resource. Some available CFMS use a mechanical cryocooler as the magnets cold source. However, the variable temperature insert (VTI) for some existing CFMS are not strictly cryogen-free as they are still based on helium gas circulation through the sample space. We designed a prototype of a gas gap heat switch (GGHS) that allows a thermal management of a completely cryogen-free magnet system, with no helium losses. The idea relies on a parallel cooling path to a variable temperature insert (VTI) of a magnetic properties measurement system under development at Inter-University Accelerator Centre. A Gifford-McMahon cryocooler (1.5 W @ 4.2 K) would serve primarily as the cold source of the superconducting magnet, dedicating 1 W to this cooling, under quite conservative safety factors. The remaining cooling power (0.5 W) is to be diverted towards a VTI through a controlled GGHS that was designed and built with a 80 μm gap width. The built GGHS thermal performance was measured at 4 K, using helium as the exchange gas, and its conductance is compared both with a previously developed analytical model and a finite element method. Lessons learned lead to a new and more functional prototype yet to be reported.

Feasibility study of parallel conduction cooling of NbTi magnet and sample probe in a cryogen-free magnet system

The conduction cooling of both a 6 T superconducting magnet along with a sample probe in a parallel configuration is addressed in this work. A Gifford-McMahon (GM) cryocooler is directly cooling the NbTi magnet, which aims to be kept at 4 K, while a gas-gap heat switch (GGHS) manages the cooling power to be diverted to the sample probe, which may be swept from 4 K up to 300 K. A first prototype of a GGHS was customized and validated for this purpose. A sample probe assembly has been designed and assembled with the existing cryogen-free magnet system. The whole test setup and components are described and the preliminary experimental results on the integration are presented and discussed. The magnet was charged up to 3 T with a 4 K sample space and up to 1 T with a sweeping sample space temperature up to 300 K while acting on the GGHS. Despite some identified thermal insulation problems that occurred during this first test, the overall results demonstrated the feasibility of the cryogen-free parallel conduction cooling on study.

15 K liquid hydrogen thermal Energy Storage Unit for future ESA science missions

A thermal Energy Storage Unit (ESU) using liquid hydrogen has been developed as a solution for absorbing the heat peaks released by the recycling phase of a 300 mK cooler that is a part of the cryogenic chain of one of ESAs new satellites for science missions. This device is capable of storing 400 J of thermal energy between 15 and 16 K by taking advantage of the liquid-to-vapor latent heat of hydrogen in a closed system. This paper describes some results obtained with the development model of the ESU under different configurations and using two types of hydrogen storage: a large expansion volume for ground testing and a much more compact unit, suitable for space applications and that can comply with ESAs mass budget.