Neon gas-gap heat switch

A self-contained neon gas-gap heat switch featuring an internal charcoal adsorption pump has been developed and tested. This heat switch can be used with cold base temperature ranging from 17 K to 40 K offering an extension to sorption based helium gas-gap heat switch limited to below 20 K. For this prototype, an ON conductance about 74 mW/K and an OFF resistance about 3000 K/W were obtained, giving an ON/OFF conductance ratio about 220 at 20 K in agreement with calculations obtained from a simple model. These characteristics can be further optimized working on the geometry.     

Gas gap thermal switches using neon or hydrogen and sorption pump

The very low pressure obtained thanks to adsorption phenomenon at low temperature can be used to build cryogenic heat switches, which offer the possibility to make or break thermal contact between two parts of a cryogenic system. The ON (conducting) and OFF (insulating) states of the switch are obtained by varying the gas pressure between two copper blocks separated by a gap of 100 μm. This pressure is controlled by acting upon the temperature of a small sorption pump (activated charcoal) connected to the gap space. For a “high” sorption pump temperature, the gas previously adsorbed in the sorption pump is released to the gap between the two blocks, allowing a good thermal conduction through the gas (ON state). On the opposite, cooling the sorption pump allows a very good vacuum between the copper blocks, which efficiently break the thermal contact (OFF state). Experimental thermal characteristics (Conductance in the ON and OFF state, ON-OFF switching temperature) of such a “Gas Gap Heat Switch” are described using hydrogen or neon as exchange gas and are compared with theoretical calculations.     

A multipurpose He refrigerator

We introduce a mini ³He refrigerator, operating at ∼300 mK starting from 4.2 K without pumping on the main ⁴He bath. The innovative idea is that the present one is suitable for a very fast operation; for its use, it is sufficient a storage ⁴He Dewar. In this way we drastically reduce the time required to cool it down, because there is no need for a classic cryostat. This prototype is particularly aimed for all those operations in which it is necessary to test a large number of samples that do not require long duration measurements at low temperature.     

3He Impurities in 4He Systems Adsorbed on Curved Substrates

It is shown that ³He impurities in sufficiently large ⁴He systems adsorbed onto substrates with curved geometries form surface bound states, analogous to the Andreev state on a planar liquid--vapor interface. We report the analysis performed for superfluid ⁴He adsorbed on the external surface of the nano-fullerene C₆₀ and on cylindrical nano-wires of Au. It is found that a single ³He impurity diluted into such adsorbed structures behaves as on films on planar substrates and as on pure ⁴He clusters.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.     

Scaling Results for Superfluid 3He in 98% Open Aerogel

We present experimental observations of the suppressed superfluid transition temperature, T _ca , superfluid fraction, ρ _s /ρ and Leggett frequency of ³He-B in aerogel, Ω _Ba . We determine T _ca from mass decoupling and the vanishing of the frequency shift away from the Larmor frequency in our different samples and different laboratories. We find that the suppressed transition temperature for ³He in aerogel occurs at a sample dependent, but approximately pressure independent, length, , where T _c and ξ ₀(P), are the transition temperature and the pressure dependent zero temperature coherence length for bulk ³He. T _ca also occurs at a pressure independent value of the Leggett frequency of bulk ³He-B. Further, we find that when the superfluid fraction and square of the Leggett frequency are plotted against T _ca −T (and not (T _ca −T)/T _ca ), the results of each measurement nearly collapse on to a pressure independent but sample dependent plot, with no further scaling. When plotted on a log–log scale, both measurements exhibit power laws in the range 1.33–1.45.      

Balloon-borne He cryostat for millimetre bolometric photometry

A cryostat including a ³He refrigerator has been designed, built, coupled to a multiband far infrared (FIR) photometer and successfully operated on a balloon flight. Here the cryostat, which is able to cool three bolometers to 0.3 K and one to 1.5 K, is described. The total holding time of the cryostat exceeds 5 days, being limited by exhaustion of the main ⁴He and N₂ baths. The autonomy of the ³He cryostat is more than 50 h, and the recycling time is ≈3 h. The performance of the system in the ARGO 1993 flight is also reported.     

The specific heat and thermal conductivity of Eccosorb CR-124 between 800 mK and 6 K

Eccosorb CR-124 is a castable iron-loaded epoxy resin with excellent absorption properties for electromagnetic waves in the GHz range. It is commonly used in low temperature instruments to absorb thermal radiation. The specific heat c(T) of Eccosorb CR-124 was measured between 800 mK and 6 K with the heat pulse method. The results were fitted to the function , with T being the temperature in Kelvin. In addition, the thermal diffusivity D(T) of Eccosorb CR-124 was determined. At a temperature of 2.3 K, a peak is observed in D(T). Consequently, the thermal conductivity λ(T) of Eccosorb CR-124 features two distinct regimes. Below 2.3 K, it fits the function . Above 2.3 K, it fits the function .     

Heat transfer through subcooled He I layer from distributed heat source in a pressurized He II channel

The subcooled He I layer, in contact with a large heated surface in a channel filled with the pressurized superfluid He II (He II_p), expands the non-boiling region above the Kapitza region up to q_n, above which nucleate boiling sets in. As the bath temperature decreases, q_n is increased more rapidly than q_λ at which the superfluidity is broken at the centre of the heated surface. The value of q_n is increased as the channel gap increases, and is independent of the channel orientation as well as q_λ. Metastabilization of superconducting coils may be enhanced by taking the non-boiling limit q_n into account.     

Physical Mechanisms for Effective Mass Enhancement in 3He

We use structural information from simulations and from variational ground state calculations for calculating the effective mass of ³He at zero temperature. It is found that the relatively large effective mass is due to a combination of several physical effects: Density fluctuations cause an effective mass enhancement due to predominantly hydrodynamic backflow. This effect is, around the Fermi momentum, a smooth function of the single particle wave number; its magnitude is consistent with the effective mass of ⁴He impurities in ³He. Spin-fluctuations, on the other hand, cause a pronounced peak of the effective mass around the Fermi wave number. We also find, consistent with earlier work, an instability of the single particle spectrum at about 2.5 k _F, this is due to the coupling to density fluctuations in the maxon region.     

Bose-Einstein Condensation of Magnons in Superfluid 3He

The possibility of Bose-Einstein condensation of excitations has been discussed for a long time. The phenomenon of the phase-coherent precession of magnetization in superfluid ³He and the related effects of spin superfluidity are based on the true Bose-Einstein condensation of magnons. Several different states of coherent precession has been observed in ³He-B: homogeneously precessing domain (HPD); persistent signal formed by Q-balls at very low temperatures; coherent precession with fractional magnetization; and a mode of the coherent precession in compressed aerogel. The coherent precession has been also found in ³He-A in compressed aerogel. Here we demonstrate that all these cases are examples of a Bose-Einstein condensation of magnons, with the magnon interaction term in the Gross-Pitaevskii equation being provided by different types of spin-orbit coupling in the background of the coherent precession.      

Development of a low-temperature He compressor for superfluid He-He mixtures

The development, testing and modeling of a “compressor” that is capable of increasing the concentration of the ³He component of a liquid superfluid ³He-⁴He mixture is discussed. This compressor was developed to drive refrigeration cycles for cooling below 1 K. The compressor design and performance testing is described in detail. The compressor was operated at 1.2 K and ³He molar flow rates of 130 μmol/s were achieved. Compression ratios in excess of 6 were also demonstrated. The theoretical models presented are used to estimate the expected efficiencies of the compressor as well as the effect of the ⁴He component on the power required to drive the compressor.     

Frictional Effects in Thin 3He Films

The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid ³He showed that the film decouples from the substrate with a time constant which is proportional to T ⁻¹ where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute ³He-⁴He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that varies as T ⁻¹. The variation of with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.     

A-B Phase Conversion and Coexistence of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He at 2.4 MPa in cylindrical aerogel by NMR method. When the liquid is cooled down from the normal state, the A-like phase appears below superfluid transition temperature T _c ^a which is suppressed in comparison with the transition temperature of the bulk liquid. With further cooling below the certain temperature T _ab,c ^a , the A-like phase is converted into the B-like phase gradually. Both phases stably coexist within about 90 μK. When you keep the temperature constant in which both phases coexist, the A-B phase conversion stops. With furthermore cooling, the whole liquid becomes the B-like phase. The cwNMR spectra at the coexistence state suggest that the B-like phase is not uniformly distributed in the A-like phase like a large number of small bubbles in a liquid, but separated as a whole from the A-like phase. By applying a field gradient which changes as a function of square of radius, we found that the A-like phase is in the edge part with a cylindrical shape and the B-like phase is in the central part with a columnar shape.      

A continuous He cryostat with pulse-tube pre-cooling and optical access

We have designed and constructed a continuously operating ³He cryostat with windows for laser ablation and spectroscopy. A two-stage pulse-tube refrigerator cools two platforms to base temperatures of approximately 40 K and 4 K respectively. The platforms are equipped with heat exchangers that cool separate streams of ⁴He and ³He. The ⁴He stream is used to run a thermally isolated evaporative refrigerator with a base temperature of approximately 1.5 K and a cooling power of 20 mW. The ⁴He refrigerator is used to condense ³He, which is used to run a ³He evaporative refrigerator on the experimental cell. The cryostat runs continuously at temperatures from 10 K to 0.4 K with a cooling power of 1.5 mW at 0.5 K.     

Long-Range Order in the A-like Phase of Superfluid 3He in Aerogel

A mutual action of the random anisotropy brought in the superfluid ³He by aerogel and of the global anisotropy caused by its deformation is considered. Strong global anisotropy tends to suppress fluctuations of orientation of the order parameter and stabilizes ABM order parameter. In a limit of vanishing anisotropy fluctuations of ABM order parameter became critical. It is argued that still in a region of small fluctuations the order parameter changes its form to be less sensitive to the random anisotropy. For a favorable landscape of the free energy of superfluid ³He the fluctuations remain small even in a limit of vanishing global anisotropy and the long-range order is maintained.      

Novel and efficient B lined tubelet detector as a replacement for He neutron proportional counters

This paper presents a novel and robust proportional detector which addresses the well publicized shortage of ³He gas by using a ¹⁰B lining applied to a tubelet configuration. The advantage of the tubelet structure is that it yields a detector maintaining the form factor of a conventional ³He tube whilst achieving a sensitivity of up to 75% of a 3 atm ³He device. The design and fabrication of the tubelet detector is presented and discussed with test data comparing the new detector to existing ³He and BF₃ tubes. The application of the tubelet design to security and industrial applications including retro-fitting to existing portals and suitability for high integrity oil and gas installations is addressed.     

Magnetic Distortion of the B-like Phase of Superfluid 3He Confined in Aerogel

We present measurements of the response of the B-like phase of superfluid ³He in aerogel to an applied flow. The measurements are made using a cylindrical piece of 98% silica aerogel attached to a vibrating wire resonator. The resonator is immersed in superfluid ³He at 16 bar pressure and at low temperatures. A variable magnetic field is applied such that the aerogel-confined superfluid may exist in the A-like or B-like phase, while the surrounding fluid is always in the bulk B-phase. The resonator response reveals a velocity dependence of the inferred aerogel-confined superfluid fraction. We discuss measurements of the temperature and magnetic field dependence of the response in the B-like phase. We find a significant field dependence indicating a strong magnetic distortion of the B-like phase order parameter.      

Measurements of the Superfluid Joule–Thomson Refrigerator Using High Concentration 3He–4He Mixtures

The superfluid Joule–Thomson refrigerator (SJTR) uses a liquid superfluid ³He–⁴He mixture to provide cooling below 1 K. Performance measurements of the SJTR using 5% and 11% ³He concentration mixtures are reported. High concentration operation shows higher cooling powers at high temperature. Ultimate temperatures are seen to increase with increasing concentration due to a pinching of the temperature defect in the recuperative heat exchanger. This pinching effect is due to the variation of the heat capacity of the ³He–⁴He mixture with temperature and concentration and is discussed in detail and design changes are suggested to mitigate it.     

Temperature and Concentration Relaxation in Phase-separated Superfluid 3He–4He Mixtures

The kinetics of the temperature and concentration variations in the superfluid ³He–⁴He mixtures with initial concentration of 9.8% ³He, and heated from below, was studied experimentally under the pressure of 0.38 bar over a temperature range of 150–400 mK. It is found that in contrast to homogeneous liquids, the temperature and concentration relaxation in phase-separated mixtures can be described by a superposition of two exponential processes in which the time constants of temperature and concentration variations coincide. If the initial mixture was homogeneous and phase separation was triggered by a heat flow, the temperature and concentration vary non-monotonically and exhibit anomalous features at the moment of phase separation. In this case the phase transition starts in the metastable superfluid, formed out of a quite supersaturated mixture where the nucleation of the new phase may be caused by quantized vortices. The results are analyzed in terms of two possible mechanisms of relaxation–the acoustic mechanism with the second sound velocity and the diffusive one connected with dissipative flows of impurity and thermal excitations. It is shown that the measured relaxation times agree with a prediction of the theory.