G. Thummes

A two-stage pulse tube cooler operating below 4 K

Abstract A two-stage double-inlet pulse tube cooler for cooling below 4 K is designed and constructed by the aid of numerical analysis. The hot end of the 2nd stage pulse tube is connected to the phase shifting assembly at room temperature without the use of a regenerative tube. A commercial helium compressor with input power of 6 kW and a redesigned commercial rotary valve serve to generate the pressure oscillation. Using a three-layer 2nd stage regenerator filled with ErNi 0.9 Co 0.1 , ErNi and lead spheres, a lowest temperature of 2.23 K and cooling powers of 370 mW at 4.2 K and 700 mW at 5 K are obtained. The operating frequency is 1.0 Hz, the average helium pressure is 1.55 MPa, and the pressure ratio at the main inlet is 2.0. The cold head of the 2nd stage reaches 4.2 K after 100 min and 2.23 K after 270 min.

Performance improvement of a pulse tube cooler below 4 K by use of GdAlO3 regenerator material

Abstract The cooling power and coefficient of performance of a two-stage pulse tube cooler below 4 K have been increased greatly by using the newly developed ceramic magnetic regenerative material GdAlO 3 (GAP). Cooling powers of 200 mW at 2.8 K, 300 mW at 3.13 K, and 400 mW at 3.70 K have been achieved with a compressor input power of about 4.8 kW. The results show that the cooling power near 3.0 K increases by 150% compared to that of the same pulse tube cooler (PTC) employing only conventional HoCu 2 and ErNi regenerator materials.

Approaching the 4He lambda line with a liquid nitrogen precooled two-stage pulse tube refrigerator

Abstract We report on the design and test of a three-stage refrigerator, consisting of a liquid nitrogen bath with heat exchangers and two subsequent pulse tube stages. The liquid nitrogen bath serves for precooling the oscillating 4 He-gas flow. The PTR stages are operated in orifice and double-inlet mode with the orifice valves, reservoirs and second-inlet valves located at ambient temperature. Er 3 Ni shot is used for the regenerator matrix in the coldest stage. A minimum temperature of 2.13(1) K has been achieved, which, at the average operating pressure of 19.5 bar, is only 0.19 K above the λ-line for the superfluid phase transition of 4 He.

Effects of DC gas flow on performance of two-stage 4 K pulse tube coolers

DC flow was found to exist in most double-inlet pulse tube coolers from theoretical simulations. DC flow effects in a 4 K pulse tube cooler were studied by numerical analysis and experiment. Numerical predictions show that a negative DC flow from the hot end of the pulse tube to the inlet of regenerator has two positive effects on the cooler performance: (1) increasing cooling capacity of the pulse tube; and (2) dissipating the enthalpy flow at the hot end of the pulse tube. Several functions which cause DC flow, such as valve timing and geometry of double-inlet valve and DC flow bypass, were tested in the two-stage 4 K pulse tube cooler. The predictions and experiment both show that the negative DC flow leads to change of temperature profiles in pulse tube and regenerator. Experiments verify that cooling power at 4.2 K of the two-stage pulse tube cooler can be considerably improved with aid of a suitable negative DC flow.

Thermodynamic performance prediction of pulse tube refrigeration with mixture fluids

Abstract A refrigeration cycle with two isentropic and two isobaric processes, referred to as the modified Brayton cycle, is introduced for predicting the thermodynamic performance of pulse tube refrigeration with a binary mixture refrigerant. The corresponding theoretical expressions of cooling power, thermodynamic efficiency and required work of a refrigeration cycle are established. Based on the prediction calculations for a number of cryogenic fluids, promising mixture pairs of working refrigerants are recommended. The computed results show that 9.5% of the coefficient of performance (COP) and 6.7% of the cooling power at 80 K could be gained if a mixture pair of 10% nitrogen and 90% helium is used instead of pure helium for the pulse tube refrigeration. The minor mixture pairs which have a positive effect on pulse tube refrigeration at temperatures near 80 K, including hydrogen–helium, argon–helium and neon–helium, are also discussed.

Performance Study on a Two-Stage 4 K Pulse Tube Cooler

Systematic investigations towards performance improvement of a two-stage 4 K pulse tube cooler, concerning structural parameters, staging configuration and compressor input power, are presented in this paper. Theoretical analysis and experiment reveal that a proper negative DC flow through the double-inlet bypass has positive effects on cooling performance. With a modified cooler design a highest cooling power of 0.5 W at 4.2 K and 1 W at 5 K have been achieved. With heating load of 20 W at 67 K on the first stage, the 2nd stage can provide 0.42 W at 4.2 K. All these results have been obtained by using a 6 kW G-M compressor combined with a redesigned GM rotary valve. By using a compressor with measured input power of 1.7 kW the same cooler has a net cooling power of 170 mW at 4.2 K. The corresponding COP at 4.2 K is 1.0×10-4, which reaches the same order of magnitude as that of GM or GM+JT coolers at the same temperature range.

Experimental study of staging method for two-stage pulse tube refrigerators for liquid 4He temperatures

Abstract The first two-stage pulse tube refrigerator, providing a lowest temperature of 2.23 K and a cooling power of 370 mW at 4.2 K, employed a parallel arrangement of the two pulse tubes with phase shifters located at room temperature 1 . With the aim of increasing the COP at liquid 4 He temperatures, three modified staging methods were tested in this paper. All refrigerator versions operate with the same two regenerators as already used in the first two-stage setup 1 and also the same 6 kW He-compressor combined with a redesigned G-M rotary valve. The best performance is achieved with a parallel arrangement two-stage refrigerator by introducing proper negative DC flow and impedance tubes. So far the highest cooling power achieved on the second stage at 4.2 K was 0.5 W. With a heat load of 20 W at 67 K on the first stage, the second stage can provide a cooling power of 0.42 W at 4.2 K. Details of the design of the different refrigerators and a comparison of their performance are presented.

Two-stage pulse tube cooler for operation of a Josephson voltage standard near 4 K

Cryogen-free operation of Josephson voltage standards requires a low-noise cryocooler that provides a cooling power of 100 mW or less at 4.2 K, and about 1 W of precooling at an intermediate temperature near 70 K. Due to its intrinsic low level of mechanical vibrations a pulse tube cooler (PTC) appears to be a suitable candidate for such an application. In this work, a two-stage pulse tube cooler has been developed and optimized for cooling of a 1 V or 10 V Josephson voltage standard near 4 K. The performance of the PTC, which is driven by a compressor with a rated input power of 1.7 kW, is optimized under three different conditions. So far, a minimum temperature of 2.66 K and a maximum cooling power of 202 mW at 4.2 K with coefficient of performance (COP) of 1.15×10−4 have been achieved. The performance of the cryocooler is expected to fully satisfy the cooling requirement of the voltage standard. In addition, the experimental results are also compared with those obtained by driving the same cooler by us...

Valve timing effect on the cooling performance of a 4 K pulse tube cooler

Generally, a compressor together with a rotary valve system generates the pressure oscillation in GM-type cryocoolers. The timing of the rotary valve, which is one of the key operating parameters for cryocoolers, determines the relationship between intake and exhaust processes. A systematic investigation of valve timing effects on cooling performance of a two-stage 4 K pulse tube cooler (PTC) is reported. The experiments show that the optimization of valve timing can considerably improve the cooling performance for both stages. For the same PTC, a performance comparison for operation on different compressors with various input powers ranging from 0.5 to 6.0 kW is also presented.

Effect of Pressure Wave Form on Pulse Tube Refrigerator Performance

We have constructed a pulse tube refrigerator (PTR) test apparatus that can be operated in different modes: as basic, orifice-, or double-inlet-type PTR. A special motor driven multi channel rotary valve in combination with needle valves serves to control the Helium gas flow between compressor and refrigerator inlets. With dynamic pressure and temperature sensors at various positions and a calibrated heat input at the cold end the cooling performance of the refrigerator was studied systematically under various working conditions, such as frequency and amplitude of the pressure wave and the settings of the orifice and second inlet flow. Keeping constant the average mass flow through the rotary valve, particular attention has been directed towards a possible effect of the shape of the pressure wave. A minimum temperature of 42 K has thus far been achieved with double inlet configuration using a frequency of 2.1 Hz, an average pressure of 21 bar, a high to low pressure ratio of 1.32, an average mass flow rate through the rotary valve of 1.0 g/s, and an approximately trapezoidal wave form. Under these conditions, the net cooling power at 80 K is Q = 2.5 W. With a higher pressure ratio of 1.5, corresponding to an average mass flow of 1.56 g/s, Q = 4 W at 80 K is obtained at the expense of a higher no-load temperature of 52 K. In all three modes of operation the net cooling power varies linearly with cold end temperature. For orifice and double-inlet configuration the measured slopes dQ/dT are in qualitative agreement with the enthalpy flow theory of Radebaugh et al.. A significant effect of the pressure wave form on the minimum temperature and the temperature profile along the pulse tube has been observed. While holding all other parameters constant, the minimum temperature increases markedly when the dwell times at high or low pressure are reduced. Regarding high cooling power, the optimum wave form is characterized by a long dwell time at high pressure, which may be ascribed to an increased heat transfer to the hot heat exchanger.