Page: 1 of 7 Conference naam Cryogenic Engineering Conference Conference year 2005 Title of paper J. C. Mullié 1, P. C. Bruins 1, T. Benschop 1, Authors of paper I. Charles 2, A. Coynel 2, L. Duband 2 1 THALES Cryogenics, Eindhoven, 5600, the Netherlands 2 CEA/DSM/DRFMC/Service des Basses Températures Grenoble, 38054, France ABSTRACT In order to provide cryogenic cooling for applications that are extremely sensitive to mechanical vibration, Thales Cryogenics has been delivering U-shape pulse tube cryocoolers since 2001. The disadvantage of the U-shape design is that the available regenerator volume is too limited if the application puts constrains on the overall diameter of the cold finger, thus limiting the coolers efficiency. As presented at CEC/ICMC 2003, Thales Cryogenics and CEA/SBT have achieved very good results with a large concentric pulse tube delivering 4W @ 77K driven by a flexure bearing compressor. Furthermore, the same team, together with Air Liquide DTA, developed a very efficient 1W pulse tube cooler for the ESA MPTC project. Based on the experiences obtained with those programs, Thales Cryogenics and CEA/SBT have now developed a small concentric pulse tube that is driven by a flexure bearing compressor. The result is a very compact and reliable cooler, with an efficiency that is nearly doubled compared to the U-shape version with the same overall external diameter dimensions. This paper describes the trade-offs that have been considered in the design phase, and gives a detailed overview of the test results, the status of the qualification program and a comparison with a comparable Stirling cold finger. INTRODUCTION When the Thales LPT9110 U-shape pulse tube cryocooler was first introduced in 2001, it was noted that the efficiency was relatively low compared to Stirling cryocoolers of comparable size. One of the reasons for this was the fact that the U-shape configuration, combined with the constraints of the application on the overall cold finger diameter, limited the cross sectional area of the regenerator. It was clear that a concentric configuration of the pulse tube and regenerator would give a more compact cold finger, thus enabling a larger regenerator within the constraints of the application. At the time, there were a number of thermodynamic and manufacturing difficulties that prevented introducing a concentric pulse tube for production. The work performed by the Air Liquide/Thales/CEA team for the ESA MPTC program 1 indeed revealed that for comparable pulse tube dimensions, a larger cross sectional area of the regenerator would increase the cooling power. As presented at CEC-ICMC 2003 3, the CEA-SBT team had established a correct definition for a 4W concentric pulse tube resulting in the 4W LPT9310 cooler, where a concentric pulse tube is driven by a Thales flexure bearing compressor. It was thus decided to develop a commercial version of the MPTC pulse tube, in a concentric layout. The concentric pulse tube is combined with a second generation Thales Cryogenics flexure compressor, and as such bears the name LPT9510. The target specification is 1000mW@ 80 K @ 60 W.
Page: 2 of 7 COOLER DESIGN AND TRADE OFFS In this section, the results achieved with the concentric version of the MPTC are given. It is explained how the differences between the space version and the civil version affect the performance of the cooler. Pulse Tube design description During the design of the LPT 9510 coaxial pulse tube, the regenerator volume and pulse tube volume were kept very close to the ones used in the MPTC cooler. The regenerator is placed around the cold finger, giving an overall diameter of 18mm. For cost reasons, stainless steel tubes replace the titanium regenerator and pulse tube walls used in the MPTC. Figure 1 shows the result of a breadboard test to compare the performance of the concentric PT compared to the (titanium) U shape and an in-line configuration with same characteristic dimensions. A dismountable prototype was designed and built as a tool to compare different configurations and different heat exchangers. In the dismountable prototype, all flow patterns are identical to the final product. In order to facilitate laser welding of the final cold finger, the warm end has a stainless steel envelope. Copper EDM (Electric Discharge Manufacturing) heat exchangers are placed in the warm and cold end. As motivated in an earlier publication 5, an inertance, wound inside the buffer, is used to provide the phase shifting, and there is no double inlet. A picture of the prototype and the final LPT 9510 cold finger is shown in Figure 2. Figure 1: Performance comparison of In-line, U-shape and Concentric Pulse tubes Figure 2: Dismountable pulse tube prototype and cold finger of LPT9510 In going to volume production of the pulse tube, there are a number of design choices that had to be made to enable efficient production. A number of these choices have a negative impact on the performance of the cooler:
Page: 3 of 7 Stainless steel tubes were used instead of titanium tubes. The warm end envelope is made of stainless steel. The impact of the lower heat conductivity of the steel is seen in the left graph of Figure 3. For safety reasons related to the strength of the pressure vessel, the filling pressure is reduced from the optimum (40 bar) to 30 bar. The effect is shown in Figure 3 (right). Figure 3: Impact of warm end material and filling pressure on PT performance Figure 4: MPTC compressor and LPT95xx compressor Compressor design The difference between the ESA MPTC compressor and the LPT 9510 compressor is shown in Figure 4. It can be identified from the pictures that the ESA-MPTC compressors is sealed by bolting both compressor halves to an aluminum integrated heat-sink construction. This section describes the differences between the ESA-MPTC compressor and the used LPT 9510 compressor. The LSF95xx 2 nd generation flexure bearing compressor is a modular design that can be combined with several cold fingers 1. The present day design of the linear motor, however, puts constraints on the achievable piston diameter, moving mass and spring constant, thus limiting the range of cold fingers that the compressor can drive with maximum efficiency. In order to reduce development and qualification effort and thus costs, it was decided not to change the linear motor, i.e. to drive the concentric pulse tube with the best match compressor from the existing LSF95xx range. The previous paragraph has shown that for identical pdv work, the concentric pulse tube has the same cooling power as the U-shape MPTC pulse tube. Due to the motor design of the LSF95xx
Page: 4 of 7 compressor, there are a number of reasons why the resulting LPT9510 cooler is less efficient than the MPTC cooler in terms of cooling power as a function of electrical input power. These reasons are summarized below: The optimum filling pressure of both pulse tubes is 40 bar. However, in order to stay within the constraints put on pressure vessels for safety (MIL-STD1522 requirements and CE requirements), the LSF95xx compressor is limited to 30 bar filling pressure, which yields a slightly lower efficiency of the Pulse Tube (Figure 3). The estimated loss in efficiency due to the lower charging pressure is 6%. The design of the LSF95xx has a minimum piston diameter that is larger than the optimum for the pulse tube. Thus the mechanical damping of the linear motor is higher resulting in a lower compressor efficiency compared to the ESA-MPTC compressor. The estimated loss of efficiency due to the larger piston damping is 7 %. The MPTC compressor has an extremely high filling factor in the motor wiring, due to the use of flattened silver wire. For reasons of cost-effectiveness, the LSF95xx compressor uses a more standard wiring with a lower filling factor. Thus increasing the Ohmic losses in the wiring at equivalent pdv power. The estimated loss in efficiency due to the larger piston damping is 4 %. Due to the weight and thermal management requirements for the MPTC, it is equipped with a coated aluminum center part. This creates a very efficient conductive path for the compression heat, which gives the cooler a better efficiency. The estimated loss in efficiency due to the larger piston damping is 2 %. COMPARISON BETWEEN STATE-OF THE ART PULSE TUBE AND STIRLING In discussions with (potential) customers, it is often noted that the customer does not really know when to choose a pulse tube cooler, and when to choose a Stirling cooler. In order to provide a rough guideline, this section compares an LSF9589 Stirling cooler with an LPT9510 pulse tube cooler, for a number of important requirements. Since especially in military infrared applications the dewar diameter is the main driver for the cooler choice, the LSF9189 with its 13mm cold finger is chosen here as a comparison, as its dimension is the closest to the 18 mm cold finger of the LPT9510. Cooling performance and efficiency As the graph in Figure 5 reveals, the efficiency of the pulse tube is lower than the efficiency of the Stirling cooler. At 80 K, there is a difference of almost a factor of 2. On top of that, the pulse tube is much more sensitive to increases in the ambient temperature. This implies that for applications where a high efficiency is required (limited electrical power or limited warm end heatsinking), the Stirling cooler is the best choice if no other requirements are driving the choice of the cooler.
Page: 5 of 7 Figure 5: Performance comparison of LSF 9189 and LPT 9510 (shown on picture) The lower steady state cooling performance in combination with the higher internal thermal mass of the pulse tube cold finger will also result in a significantly longer cooldown time. Lifetime Careful consideration based on FMECA analysis 2 indicates that the lifetime of a Stirling cooler with a moving magnet flexure bearing compressor is limited by the cold finger. Lifetime tests at Thales Cryogenics are still ongoing. An update of the MTTF calculation 2 that was performed in 2003, today gives an MTTF of 41946 hours for an LSF Stirling cooler. Based on the lifetime tests performed, a similar FMECA analysis for the pulse tube cooler indicates that a lifetime of more than 5 years (43800 hours) in an application is achievable. Vibration output Tests have been performed to compare the induced vibrations in axial direction generated by the Pulse tube and the Stirling cold finger. The resulting graphs are depicted below. As Figure 6 shows, the vibration output in axial direction is dramatically reduced using a pulse tube cooler, especially in the low frequency range. Around the operating frequency, the pulse tube vibration level is reduced by a factor of more than 20 in comparison to a LSF 9189 Stirling cold finger. Both vibration levels have been measured at an electrical input power to the compressor of 80 W. As described in an earlier publication 5, achieving low vibration in an application usually requires active control of the compressor vibration as well. Robustness of cold finger For a Stirling cold finger, the maximum allowable side load is limited by the deformation of the cold finger, causing misalignment of cold finger and displacer and thus a decrease in performance due to increase of the friction between displacer and cold finger. For an LSF9189 cold finger, the side load is limited to 5 N. As shown in [4], a pulse tube cold finger can support a considerable mechanical load. Deformation of the cold finger is not a problem for performance, which implies that the only limit is (plastic) deformation of the cold finger. Figure 7 shows that the stresses inside the cold finger are still acceptable under a 50 N side load. The calculation has been made without taking into consideration the significant support of the regenerator Matrix material that is present in the cold finger. This difference between Stirling and pulse tube is of little importance for a slip-in configuration where the detector is supported by the dewar wall, but may be an important consideration for applications where the cold finger has to support a relatively large mass to be cooled.
Page: 6 of 7 Figure 6: Comparison of Pulse Tube (left) and Stirling (right) induced vibration. Note the different scales of the vertical axis. Figure 7: FEM representation of cold finger deformation under a radial load of 50 N CONCLUSION It is shown that concentric pulse tube technology has matured to a degree where the cooling capacity is equal to that of an in-line or U-shape pulse tube of comparable dimensions. In the transfer from small quantity EM (Engineering Model) manufacturing to volume production, some production optimizations have resulted in a slight performance degradation of the concentric pulse tube cooler. The article shows that these degradations are well understood, and could be overcome if the market requires a higher cooling efficiency. The article compares essential properties and performances of the concentric pulse tube with a stirling cryocooler, in order to enable end users to choose the right cooling technology for their application.
REFERENCES Page: 7 of 7 1. Trollier, T., Ravex, A., Charles, I., Duband, L., Mullié, J.C., Bruins, P.C., Benschop, A.A.J., Linder, M., Performance test results of a Miniature 50 to 80 K Pulse Tube Cooler for Space Applications, Cryocoolers 13, pp 93-100 (2004) 2. Mullié, J. C., Bruins, P.C., Benschop, A. A. J. and Meijers, M., Development of the LSF95xx 2 nd generation flexure bearing coolers, Cryocoolers 13, pp 71-76 (2004) 3. Charles, I., Duband, L., Martin, J-Y., Mullié, J.C., Bruins, P.C., Experimental characterization of a pulse tube cryocooler for ground applications, Advances in Cryogenic Engineering vol 49, pp 1373-1379 (2003). 4. Trollier, T., Ravex, A., Charles, I., Duband, L., Mullié, J.C., Bruins, P.C., Benschop, A.A.J., Linder, M., Design of a Miniature 50 to 80 K Pulse Tube Cooler for Space Applications, Advances in Cryogenic Engineering vol 49, pp 1318-1325 (2003) 5. Bruins, P.C., Koning, A. de, Hofman, T., Low Vibration 80 K Pulse Tube Cooler with flexure bearing compressor, Cryocoolers 12, (2002)