As key components in pulse tube refrigerators (PTRs), heat exchangers have great influence on the performance of the PTRs, especially the cold end heat exchangers which dominate the cooling effect between the cold gas and heat load. Filling copper screens are widely used to improve the performance of heat exchange and laminar flow. Whereas, the heat transfer rate of copper screens is still not good enough for the actual requirements of PTRs. Furthermore, the flow resistance of the copper screen is growing up quickly with the increase of screen mesh. In this paper, we propose a new type of copper foaming metal with high heat transfer area and low flow resistance in the heat exchanger instead of the copper screens. The heat transfer performances of the copper screens and the copper foaming metal are firstly compared by theoretical calculation, which shows that the performance of the copper foaming metal with 600 lm pore size is better than that of 20 and 80 mesh copper screens, verified by experimental results. A four valve pulse tube refrigerator (FVPTR) with copper foaming metal of 600 lm pore size as filling material of the heat exchanger achieved 69.5 K, 2.5K lower than that of using 20 mesh copper screens, 1.7K lower than that of using 80 mesh copper screens.

A brief review is presented on previous experimental results and correlations on the friction factor of cryocooler regenerators operating at oscillating flow and pulsating pressure conditions, for different mesh sizes of packed woven screens, focusing on the effects of different operating frequencies ranging from 20 to 80 Hz, at room and cryogenic temperatures. A comparison of the friction factor data with those of other studies is presented to clarify the reason for the difference. Finally, a new oscillating flow correlation of regenerators, in terms of several nondimensional parameters, is discussed and compared.

We are developing a new cryogenic neutrino detector: electron bubble chamber, using liquid helium as the detecting medium, for the detection of low energy p-p reaction neutrinos (<420 keV), from the Sun. The program focuses in particular on the interactions of neutrinos scattering off atomic electrons in the detecting medium of liquid helium, resulting in recoil electrons which can be measured. We designed and constructed a small test chamber with 1.5 L active volume to start the detector R&D, and performed experimental proofs of the operation principle. The test chamber is a stainless steel cylinder equipped with five optical windows and ten high voltage cables. To shield the liquid helium chamber against the external heat loads, the chamber is made of double-walled jacket cooled by a pumped helium bath and is built into a LN2/LHe cryostat, equipped with 80 K and 4 K radiation shields. A needle valve for vapor helium cooling was used to provide a 1.7-4.5 K low temperature environments. The cryogenic test chamber has been successfully operated to test the performance of Gas Electron Multipliers (GEMs) in He and He+H2 at temperatures in the range of 3-293 K. This paper will give an introduction on the cryogenic solar neutrino detector using electron bubbles in liquid helium, then present the cryogenic design and operation of liquid helium in the small test chamber. The general principles of a full-scale electron bubble detector for the detection of low energy solar neutrinos are also proposed.

The performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne, He+H2 and Ne+H2 was studied at temperatures in the range of 3-293K. This paper reports on previously published measurements and additional studies on the effects of the purity of the gases in which the GEM performance is evaluated. In He, at temperatures between 77 and 293 K, triple-GEM structures operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved through the Penning effect as a result of impurities in the gas. At lower temperatures the gainvoltage characteristics are significantly modified probably due to the freeze-out of these impurities. Double-GEM and single-GEM structures can operate down to 3K at gains reaching only several tens at a gas density of about 0.5g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be re-established in Penning mixtures of Ne+H2: very high gains, exceeding 104, have been obtained in these mixtures at 30-77K, at a density of 9.2g/l which corresponds to saturated Ne vapor density at 27K. The addition of small amounts of H2 in He also re-establishes large GEM gains above 30K but no gain was observed in He+H2 at 4K and a density of 1.7g/l (corresponding to roughly one-tenth of the saturated vapor density). These studies are, in part, being pursued in the development of two-phase He and Ne detectors for solar neutrino detection.

A new two-phase cryogenic neutrino detector using electron bubble (e-bubble) specifically in liquid helium is proposed and being developed for real time, high rate measurements of low-energy p-p reaction neutrinos from the sun. The e-bubble detector is a time projection chamber-like (TPC) tracking detector. The task of such a neutrino detector is to detect the ionization of the elastically scattered target electrons by incident neutrinos, and then to characterize their energy and direction and to distinguish them from radioactive backgrounds. The ionization signals are expected to be small and hence undergo avalanche amplification in the saturated vapor above the liquid phase by gas electron multipliers (GEMs) at high gain. Higher granularity and intrinsically suppressed ion feedback give a good spatial resolution and are the major advantages of this technology. It should be possible to construct such a detector to track charged particles down to 100-200 keV in a massive liquid helium target with fractional millimeter spatial resolution in three-dimensional space, using the GEM-based TPC with a high-resolution CCD camera, for both the electronic and light readout.

A set of Stirling-type non-magnetic and non-metallic co-axial pulse tube cryocoolers, intended to achieve portable cryogen-free systems with very low interference for high-Tc SQUIDs operation, have been designed and tested in TIPC/CAS. The key feature is that all cooler components in the vicinity of SQUIDs pick-up loops are made of non-magnetic and non-metallic materials, in order to eliminate complicated interference and realize direct couple with SQUIDs. The cooling options, cooler interference and corresponding solutions are reviewed briefly, and then we focus our attention on the cryogenic design and selection of the materials. Over 30 cooler samples have been fabricated and tested systematically. A typical cooling power of over 100mw at 80K with 70W input electrical power has been achieved. Detailed cooling performance and elementary interference characteristics of the coolers are also analyzed and evaluated.

In this paper we developed and designed, based on theoretical considerations, a new type of 4 K GM/PT hybrid refrigerators. The upper warm stage of the hybrid refrigerator is a typical GM refrigeration cycle, and the cold stage is a pulse tube refrigerator (PTR), on which is thermodynamically coupled the upper warm stage. Four different types of phase shifting assembly: (1) a cold auxiliary piston that is connected to the displacer of the upper GM refrigerator stage, (2) an orifice with reservoir encircled the cold head of the upper stage, (3) an orifice with double-inlet, and (4) in combination with the cold auxiliary piston, orifice and double-inlet, has been proposed and analyzed for the adjustment of the phase shift between the gas mass flow and pressure in the pulse tube. Numerical simulation is performed to understand the unique thermo-physical features, to reveal the time-dependent dynamic parameters and to quantify the overall cooling performance of the hybrid refrigerator. We rely on a one-dimensional, unsteady compressible flow numerical model that is based on a mixed Eulerian–Lagrangian method developed by the present author. The model will be first applied to analyze the cooling performance of the hybrid refrigerator with different types of phase shifting assembly. In what following, it is used to simulate the dynamic parameters in the cold stage of the pulse tube cycle. Next, optimization of the structure parameters and geometrical configuration of the new refrigerator will be presented. Finally, the influences of different hybrid regenerative materials on the cooling capacity of the new hybrid GM/PTR will be also discussed.

We measured the flow resistance and flow inductance of inertance tubes at high acoustic amplitudes for four different inner diameters of 0.6, 1.0, 1.5 and 2.0 mm at various tube length ranging from 100 to 1500 mm, at frequencies of 30, 40, 50, 60 and 70 Hz. The experiments were carried out under system mean pressure of 20 bar at room temperature. The experimental data were compared with the explicit solution to the linear momentum equation for small acoustic amplitudes and were fitted with the modification coefficients in terms of the operating frequency and Reynolds numbers characterized by the amplitude of gas velocities.

A dynamic experimental apparatus was designed and constructed to investigate oscillating flow characteristics in a regenerator subjected to a periodically reversing flow established by means of a self-made linear compressor. Detailed experimental data of oscillating pressure drops and phase shift characteristics for regenerators in a high frequency pulse tube cryocooler with an operating frequency of 50 Hz were given. The correlation equations for the maximum and cycle-averaged friction factors in terms of Reynolds numbers and dimensionless distance X were obtained. It was found that the value of the cycle-averaged pressure drop in the oscillating flow across the regenerator is two to three times higher than that of a steady flow at the same Reynolds numbers based on the cross-sectional mean velocity. In addition, the relationship of the phase shifts between the velocity and pressure wave is also discussed. 0 1998 Elsevier Science Ltd. All rights reserved

We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne and Ne+H2 at temperatures in the range of 2.6-293K. In He, at temperatures between 62 and 293 K, the triple-GEM structures often operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved by the Penning effect in the gas impurities released by outgassing. At lower temperatures, the gain-voltage characteristics are significantly modified probably due to the freeze-out of impurities. In particular, the double-and single-GEM structures can operate down to 2.6K at gains reaching only several tens at a gas density of about 0.5g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be reestablished in Penning mixtures of Ne+H2: very high gains, exceeding 104, have been obtained in these mixtures at 50-60K, at a density of 9.2g/l corresponding to that of saturated Ne vapor near 27K. The results obtained are relevant in the fields of two-phase He and Ne detectors for solar neutrino detection and electron avalanching at low temperatures.