Investigation on the internal thermal link of pulse tube refrigerators

Within a pulse tube refrigerator (PTR) in coaxial configuration the pulse tube is located inside the regenerator matrix in axial direction. An internal thermal contact between these two main components of the coldfinger occurs. The experimental investigation of the direction and the quantity of transferred heat is in focus of this paper. Intermediate cooling of the regenerator by the corresponding part of its own pulse tube can improve the cooling performance of a PTR. Therefore, a well-adapted geometrical arrangement between the pulse tube and the regenerator is essential, considering the temperature distribution inside the coldfinger. We deduce design parameters to optimise the configuration of coaxial PTRs.     

Integrated Resonant Self-Pumped Loop and pulse tube cryocooler for cryogenic cooling distribution

Cooling distribution is a vital technology concerning cryogenic thermal management systems for many future space applications, such as in-space, zero boil-off, long-term propellant storage, cooling infrared sensors at multiple locations or at a distance from the cryocooler, and focal-plane arrays in telescopes. These applications require a cooling distribution technology that is able to efficiently and reliably deliver cooling power (generated by a cryocooler) to remote locations and uniformly distribute it over a large-surface area. On-going efforts by others under this technology development area have not shown any promising results.This paper introduces the concept of using a Resonant Self-Pumped Loop (RSPL) integrated with the proven, highly efficient pulse tube cryocooler. The RSPL and pulse tube cryocooler combination generates cooling power and provides a distributive cooling loop that can be extended long distances, has no moving parts, and is driven by a single linear compressor. The RSPL is fully coupled with the oscillating flow of the pulse tube working fluid and utilizes gas diodes to convert the oscillating flow to one-directional (DC) steady flow that circulates through the cooling loop. The proposed RSPL is extremely simple, lightweight, reliable, and flexible for packaging. There are several requirements for the RSPL to operate efficiently. These requirements will be presented in this paper. Compared to other distributive cooling technologies currently under development, the RSPL technology is unique.     

Two-stage pulse tube refrigerator in an entire coaxial configuration

In this paper, we introduce a new kind of two-stage pulse tube refrigerators. The chosen entire coaxial configuration combines the advantages of the coaxial design with the two-stage pulse tube concept. Lead coated screens build the inhomogeneous regenerator matrix of the second stage. Without any rare earth compounds the refrigerator reaches a no load temperature of 6.6 K at the second stage cold tip. The active type of phase shifting is generated by a rotary valve combined with two needle valves at the hot end of each pulse tube (compressor Leybold RW 6000, 6 kW input power). This paper focuses on the design parameters and first performance measurements.     

20-50 K and 40-80 K pulse tube coolers: Two candidates for a low temperature cooling chain

Following its important cryogenics heritage for the European Space industry for both Ariane launcher and Orbital programs, Air Liquide - Advanced Technology Division (AL/DTA) is proposing different pulse tube cryocoolers all over the temperature range to answer the needs of earth observation and scientific missions.This paper presents recent performance improvement of the large heat lift 40-80 K pulse tube cooler (LPTC). Four units have been manufactured and tested. Three units are dedicated to lifetime testing in the framework of French Military Space Program (under CNES contract) and Meteosat Third Generation program (ESA contract). The batch performances are described and the product maturity is discussed in this paper.To lower the temperature range and to complete our cryogenic chain, we developed in partnership with CEA/INAC/SBT, a heat intercepted 20-50 K pulse tube cryocooler. This cooler has been developed in the framework of an ESA contract (ESA/ESTEC No 20497/0/NL/PA-20-50 K pulse tube cooler). A development phase has been performed to test and optimize different cold head architectures to reach the 300 mW@20 K specification. A no-load temperature of 12.5 K has been demonstrated on breadboard model. The outputs of the trade-off, the resulting design and the performances are described.In complement to the dilution cooler similar to the one developed for the PLANCK mission, those two pulse tube coolers are potential candidates for a very low temperature cooling chain. By optimizing the capabilities of the 20 K stage for low temperature operation (no-load in the range of 8 K) the coupling of the three independent stages becomes possible.     

Characterization of a low frequency magnetic noise from a two-stage pulse tube cryocooler

Magnetic noise of a two-stage pulse tube cryocooler (PT) was measured by a fundamental mode orthogonal fluxgate magnetometer and by a LTS Double Relaxation Oscillation SQUID (DROS) first-order planar gradiometer. The magnetometer was installed in a dewar made of aluminum at 12 cm distance from a section containing magnetic regenerative materials of the second pulse tube. The magnetic noise spectrum showed a clear peak at 1.8 Hz, which is the fundamental frequency of the He gas pumping rate. The 1.8 Hz magnetic noise registered a peak, during the cooling down process, when the second cold-stage temperature was around 12 K, which is well correlated with the 1.8 Hz variation of the temperature of the second cold stage. Hence, we attributed the main source of this magnetic noise to the temperature variation of the magnetic moments resulting from magnetic regenerative materials, Er₃Ni and HoCu₂, in the presence of background static magnetic fields. We have also pointed out that the superconducting magnetic shield of lead sheets reduced the low frequency magnetic noise generated from the magnetic regenerative materials. With this arrangement, the magnetic noise amplitude measured with the LTS DROS gradiometer, mounted at 7 cm horizontal distance from the magnetic regenerative materials, in the optimum condition, was lower than 500 pT peak-to-peak, whereas the noise level without lead shielding was higher than the dynamic range of DROS instrumentations which was around .     

High-power Stirling-type pulse tube cooler working below 30 K

High-power Stirling-type pulse tube coolers (PTCs) are promising candidates for cooling HTS devices and gas liquefaction or separation applications. Nevertheless, till now most high-power Stirling-type PTCs are not able to reach a refrigeration temperature below 35 K. Here, a high-power two-stage Stirling-type PTC was designed, manufactured and experimentally investigated. In order to realize a convenient coupling with a thermal load, U-shape configuration is adopted in both stages, which makes it more challenging to distribute the gas flow and reduce dead volume in the cold end heat exchanger. By optimizing operating conditions, flow straightener, and double-inlet opening, the cooler has reached no-load refrigeration temperatures of 29.6 K and 27.1 K at 55 Hz and 40 Hz, respectively. Furthermore, the cooler is able to provide cooling powers of 50 W at 45.6 K and 100 W at 59.3 K when input pV powers are 4.77 kW and 4.59 kW, respectively.     

Development of No-Vent™ liquid hydrogen storage system for space applications

A number of technological advances required to store and maintain normal-boiling-point and densified cryogenic liquids, including liquid hydrogen, under zero boil-off conditions in-space, for long periods of time, have been developed. These technologies include (1) thermally optimized compact cryogen storage systems that reduce environmental heat leak to the lowest-temperature cryogen, which minimizes cryocooler size and input power, and (2) actively-cooled shields that surround the storage systems and intercept heat leak. The processes and tools used to develop these technologies are discussed. A zero boil-off liquid hydrogen storage system technology demonstrator for validating the actively-cooled shield technology is presented.     

Investigation on a thermoacoustically driven pulse tube cooler working at 80 K

The pulse tube cooler (PTC) driven by a thermoacoustic engine can completely eliminate mechanical moving parts, and then achieves a simpler and more reliable device. A Stirling thermoacoustic heat engine has been constructed and tested. The heat engine can generate a maximal pressure ratio of 1.19, which makes it possible to drive a PTC and get good performance. Frequency is one of the key operating parameters, not only for the heat engine but also for the PTC. In order to adapt to the relatively low design frequency of the PTC, the operating frequency of the thermoacoustic heat engine was regulated by varying the length of the resonance tube. Driven by the thermoacoustic engine, a single stage double-inlet PTC obtained the lowest refrigeration temperature of 80.9 K with an operating frequency of 45 Hz, which is regarded as a new record for the reported thermoacoustically driven refrigerators.     

A new computational model for entire pulse tube refrigerators: Model description and numerical validation

In present paper, a new modeling approach for the performance of pulse tube refrigerator is proposed. The new approach combines one-dimensional and two-dimensional models (1-D and 2-DCC model) together, and can be used to simulate the fluid flow and heat transfer processes of the basic type, orifice type and double-inlet type pulse tube refrigerators (PTRs). With the present model, the complicated fluid flow and heat transfer characteristics in the PTR system can be efficiently depicted and the computational time can be greatly reduced. Then based on the approach, the distribution characteristics of the flow and temperature fields of the three types of PTR are numerically analyzed. The complicated fluid flow and heat transfer phenomena in PTR, such as DC flow, velocity and temperature annular effects are presented vividly. The numerical results show that the 1-D and 2-DCC model is reliable and practical, which can be used to explore the physical mechanism of the thermodynamic processes of the PTR system and optimize the design of the PTR system and its components.     

Experimental research of Stirling type pulse tube refrigerator with an active phase control

A Stirling type pulse tube refrigerator with an active phase control has been experimentally investigated. A phase shifter, which controls the phase angle between the mass flow and the pressure inside a pulse tube, plays a key roll in the performance of pulse tube refrigerators. In this study, an electrically driven and mechanically damped linear compressor, which is directly connected at the warm end of the pulse tube using a connecting tube, is used as the active phase controller (APC). Therefore, this active phase control pulse tube refrigerator (APCPTR) has no reservoir. Amplified electric signals of a function generator are supplied to both the main linear compressor, which is used as the pressure wave generator (PWG), and the APC. The type of these two linear compressors is a dual-opposed piston. The advantage of this phase sifter is easy to control the electric input power and the phase angle between the PWG and the APC. In order to clarify the characteristics of the APCPTR, the cold end temperature and the gas pressure have been measured.     

Design and performance of the dilution cooler system for the Planck mission

We report the design, main features and on-ground qualification of the dilution cooler qualification model for the Planck mission. We mainly emphasize the innovative cryogenics and structural aspects developed in order to meet the severe requirements of the mission. We also report the results of the dilution cooler itself in the 100 mK range after successful tests of the qualification model on a dedicated test bench.     

Analysis of DC gas flow in GM type double inlet pulse tube refrigerators

In a GM type double inlet pulse tube refrigerator, a DC gas flow is an intrinsic phenomenon. It is important to understand the characteristics of the DC gas flow. In this paper, the relation between the DC gas flow, valve operating time intervals, and flow patterns in the bypass of the GM type double inlet pulse tube refrigerator is studied with a numerical simulation when a symmetric bypass is used. The governing equations of the numerical simulation based on the nodal analysis are discretized with an implicit finite volume method. The simulation result shows that the valve opening angle difference is the main parameter having influence on the DC gas flow, and the effect depends on the flow patterns in the bypass.     

A single-stage pulse tube cooler reached 12.6 K

A single-stage G-M type pulse tube cooler (PTC) was designed and tested to explore the lowest attainable refrigeration temperature and to further improve the cooling performance in the temperature range of 15-40 K. The magnetic material Er₃Ni was used as part of the regenerative material besides the phosphor-bronze and the lead so as to improve the efficiency of the regenerator. With an input power of 6 kW, a lowest no-load refrigeration temperature of 12.6 K was obtained, which is a new record for the single-stage PTC. The cooling capacity at 15-40 K was also significantly improved, which may extend the application of the single-stage PTC for the cooling of superconductors and cryopumps.     

Developments in Turbo Brayton technology for low temperature applications

A single stage reverse Brayton cryocooler using miniature high-speed turbomachines recently completed a successful space shuttle test flight demonstrating its capabilities for use in cooling the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope (HST). The NICMOS CryoCooler (NCC) is designed for a cooling load of about 8 W at 65 K, and comprises a closed loop cryocooler coupled to an independent cryogenic circulating loop. Future space applications involve instruments that will require 5–200 mW of cooling at temperatures between 4 and 10 K. This paper discusses the extension of Turbo Brayton technology to meet these requirements.     

A He pulse tube cooler operating down to 1.3 K

Regenerative cryocoolers that employ ⁴He as working fluid can only reach a lowest temperature of about 2 K. This limitation can be overcome by the use of ³He as working fluid. Here we report on the performance of a two-stage pulse tube cooler that consists of two parallel stages with independent gas circuits. The pressure oscillation in each stage is generated by means of a separate compressor in combination with a rotary valve. With ⁴He in both stages, the minimum no-load temperature of the 2nd stage was 2.23 K, and cooling powers of 50 W at 53 K and 380 mW at 4.2 K were simultaneously available at electrical input powers of 4.54 and 1.45 kW to the 1st and 2nd stage, respectively. Using ³He as working fluid in the 2nd stage, a minimum stationary temperature of 1.27 K has been achieved, which is, up to now, the lowest temperature obtained by regenerative cryocoolers. At an electrical input power of 1.3 kW, the 2nd stage provides a cooling power of 42 mW at 2.0 K and 518 mW at 4.2 K. With ³He, at the same operating condition, the cooling power at 4.2 K was found to be larger than with ⁴He.     

High frequency two-stage pulse tube cryocooler with base temperature below 20 K

High frequency (30-50 Hz) multi-stage pulse tube coolers that are capable of reaching temperatures close to 20 K or even lower are a subject of recent research and development activities. This paper reports on the design and test of a two-stage pulse tube cooler which is driven by a linear compressor with nominal input power of 200 W at an operating frequency of 30-45 Hz. A parallel configuration of the two pulse tubes is used with the warm ends of the pulse tubes located at ambient temperature. For both stages, the regenerator matrix consists of a stack of stainless steel screen. At an operating frequency of 35 Hz and with the first stage at 73 K a lowest stationary temperature of 19.6 K has been achieved at the second stage. The effects of input power, frequency, average pressure, and cold head orientation on the cooling performance are also reported. An even lower no-load temperature can be expected from the use of lead or other regenerator materials of high heat capacity in the second stage.     

Observation and control of temperature instabilities in a four-valve pulse tube refrigerator

The use of a pulse tube cryocooler in an application requires temperature stability at the cold end. In our four-valve pulse tube refrigerator we have observed long-term temperature instabilities lasting some days and short-term instabilities lasting some hours or even minutes. Investigations have shown that the latter anomaly is caused by the dc-flow. The negative influence on the stability is due to an additional mass flow (dc-flow) to the cold end of the pulse tube, which results in a parasitic heat input.In this paper we present an actively controlled dc-flow suppression device, which uses a temperature gradient in the regenerator as a control parameter. This device enables us to eliminate the temperature instabilities.     

A thermodynamic model based on exergy flow for analysis and optimization of pulse tube refrigerators

A thermodynamic model based on exergy flow through pulse tube refrigerators (PTRs) is developed. An exergetic efficiency parameter representing the losses in the pulse tube itself is proposed. The effects of control parameters representing a general phase shifter and their effect on the system performance are discussed. Analytical solutions representing important parameters in the design of PTRs such as the load curve, cooling power and efficiency in terms of basic system input parameters are developed. It is shown that the analytical model is powerful and convenient for optimization of PTRs and in quantifying its operational bound and important losses. Results indicating a compromise between cooling power and efficiency in PTRs under certain conditions are presented and discussed.     

Analytical treatment of counterflow pulse-tube refrigerators

In our laboratory we work an a new type of pulse-tube refrigerator. A complete set of relations is derived for the operation of counterflow pulse-tube refrigerators. The input parameters depend on the working fluid, the geometries of the pulse tube and the counterflow heat exchanger, and the compressor characteristics. The calculated molar flow in the heat exchanger agree within 5% with the measured value. The calculated and experimental lowest temperature and the temperature difference between the hot and cold gas in the heat exchanger agree within 10%. This larger difference is caused by difference between the modelled and real heat exchanger.     

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.