Presented at the 16th International Cryocooler Conference, held May 17-20, 2008 in Atlanta, Georgia.As an experienced supplier of tactical IR-technology, AIM is now involved in several IR space programs covering applications at different wavelengths. To the meet demands for compactness and weight, AIM has focused on Integrated Detector Cooler Assemblies (IDCA). This approach provides for the elimination of the detector - cooler interfaces on the customer side. Cryocoolers used in the IDCA configuration are based upon the designs of the AIM Flexure Bearing Moving Magnet family. Due to volume production, all relevant components and manufacturing processes have achieved a high level of maturity and thus enabling accurate reliability prediction. To apply standard processes for the fabrication of space IDCA, the cold finger needs to fit into a standard dewar interface. Therefore, the pulse-tube cold finger was developed with standard Stirlingtype dimensions. To retain maximum flexibility, the cold finger is designed for use with different sizes of flexure bearing compressors. The buffer volume and inertance tube are optimized for individual compressor types and performance requirements. The qualification activities for two different space programs are ongoing and their respective results will be presented in this paper. Furthermore, detailed performance characteristics at variable conditions will be presented

Mai, M.

Rühlich, I.

Schreiter, A.

Zehner, S.

Scholarly Materials And Research @ Georgia Tech

AIM-Space Cryocooler ProgramsM. Mai, I. Rühlich,  A. Schreiter, S. ZehnerAIM Infrarot-Module GmbHHeilbronn, GermanyABSTRACTAs an experienced supplier of tactical IR-technology, AIM is now involved in several IR spaceprograms covering applications at different wavelengths.To the meet demands for compactness and weight, AIM has focused on Integrated DetectorCooler Assemblies (IDCA). This approach provides for the elimination of the detector - coolerinterfaces on the customer side. Cryocoolers used in the IDCA configuration are based upon thedesigns of the AIM Flexure Bearing Moving Magnet family. Due to volume production, all relevantcomponents and manufacturing processes have achieved a high level of maturity and thus enablingaccurate reliability prediction.To apply standard processes for the fabrication of space IDCA, the cold finger needs to fit intoa standard dewar interface. Therefore, the pulse-tube cold finger was developed with standard Stirling-type dimensions. To retain maximum flexibility, the cold finger is designed for use with differentsizes of flexure bearing compressors. The buffer volume and inertance tube are optimized for indi-vidual compressor types and performance requirements.The qualification activities for two different space programs are ongoing and their respectiveresults will be presented in this paper. Furthermore, detailed performance characteristics at variableconditions will be presented.INTRODUCTIONThe demand for infrared imaging in space has increased significantly over the last decade.Particular emphasis has been placed upon civilian applications such as weather forecasting, geo-logical observations and atmospheric monitoring. Depending on the application, imaging systemsneed to be sensitive to a specific wavelength, later referred to as the spectral range. Whilst SWIRsystems employ detectors to capture “Short Wave Infrared”, MWIR and LWIR systems are sensi-tive to “Medium-” and “Long Wave Infrared”. Table 1 addresses the application of the spectralrange with their respective purpose.AIM as a leading manufacturer of IR-detectors and modules has recently been chosen as thesupplier for two infrared space imagers: one to be incorporated in the Korean Multipurpose Satel-lite referred to as KOMPSAT-3A and the other to be integrated into the German EnvironmentalMapping and Analysis Program (EnMAP). The KOMPSAT-3A, and EnMAP imagers operate atdifferent spectral ranges, necessitating different detector temperatures. The following sections willintroduce the above-mentioned programs and describe a suitable cooling arrangement.133134 SMALL 50-80 K SINGLE-STAGE CRYOCOOLERSSPACE PROGRAMSKOMPSAT-3AThis module is intended to operate at an altitude of approximately 700 km (lower earth orbit) captur-ing geospatial, environmental and agricultural information by scanning the earth’s surface in a “pushbroom” mode. The satellite incorporates an individual detector for both IR and panchromatic spectralrange. The infrared imager operates within a wavelength of 3,000 to 5,000 nm at high spatial and thermalresolution. The sensing array is made of Mercury-Cadmium-Telluride (MCT) and hybridized with theRead-Out-Integrated-Circuit (ROIC) forming the Focal Plane Array (FPA). With a total heat load of 800mW, the KOMPSAT MWIR MCT detector is to be operated at 80 K. For those given conditions, theAIM SF400 compressor was chosen in conjunction with a ½” pulse-tube cold finger.EnMAPUnder the auspices of the German hyperspectral satellite EnMAP, AIM is developing the dedi-cated 1,024x256 MCT-Focal Plane Array push-button imaging detector module for the SWIR spectralrange. The EnMAP-satellite will operate at an altitude of 643 km and scan the earth from a polar orbitat a spatial resolution of 30 x 30 meters with a 10 nm radiometric resolution from 900 nm to 2450 nm.The main objective of EnMAP is to investigate a wide range of ecosystem parameters encompassingagriculture, forestry, soil and geological environments, coastal zones and inland waters. This willsignificantly increase the understanding of the coupled biospheric and geospheric processes and thus,methods can be developed to improve the sustainability of our vital resources. Because of the noisesensitivity of a SWIR detector is comparatively low, the system can be operated at comparativelyhigh temperatures for a cryogenic application and has smaller cooling requirements. The EnMAPdetector dewar has a thermal load of approximately 750 mW at 150 K. For this program, the SF100compressor with a ¼” pulse-tube cold finger is used.  Table 2 presents the cryocooler configurationsfor the AIM space programs.TACTICAL LONG LIFE COOLERSSplit inear Flexure Bearing ooler (SF-series)The general requirements for long-life performance stability and reliability are satisfied by theAIM Flexure Bearing cooler series. The common cooler design prevents piston coating abrasion byproviding a radially stiff piston suspension which permits steady concentric movement withoutrandom contact to the surrounding assemblies.Considering the contamination of the working gas by outgassing organic material, the com-pressor was designed so that all materials with outgassing potential are separated from the workingfluid. AIM compressors of the SF-family use a moving magnet design, which isolates the coil, amajor contaminator, to be separated from the working gas.Table 1.   Spectral range and applications for different IR-SystemsL CFigure 1 shows the schematic design of the SF100 and the SF400 compressor. The SF100compressor was originally designed as a form fit function replacement for a “One Watt class tacti-cal Linear cooler (OWL) and fulfills all requirements of the OWL specification with Stirlingcoldfinger. In this configuration the cooler achieves 20,000 hr MTTF. This mean time to failurevalue is caused by the wear-out of the Stirling expander surface only. The compressor does notincorporate a relevant lifetime-limiting mechanism and is thus suitable to meet MTTF figures inexcess of 50,000 hr.Pulse-Tube ColdfingerWhen using a flexure bearing Stirling-type cooler, the coldfinger is the lifetime-limiting compo-nent. Due to its moving displacer and mechanical abrasion, the achievable cooling capacity of theStirling coldfinger degrades over time. Additionally, the displacer significantly contributes to the cooler'sexported vibrations. Without any moving parts, pulse-tube cold fingers provide for an  unlimitedlifetime, theoretically, by having neither abrasion nor contamination potential. In combination withthe Flexure Bearing compressor, pulse-tube coolers achieve lifetimes above 50,000 hr MTTF.AIM has developed a pulse-tube coldfinger in the ½’’ standard dimension as used in severaltactical cooler programs (Figure 2). The pulse-tube coldfinger was developed to fit into an AIMdeveloped dewar. All dimensions and materials used at the dewar interface are identical to theStirling type coldfinger. Consequentially all production processes, well known from Stirling-typebatch production, can be used to build up the detector dewar assembly.Figure 1. Flexure Bearing compressor setup (principle sketch)Table 2. Overview of AIM space programs135AIM SPACE CRYOCOOLER PROGRAMS136 SMALL 50-80 K SINGLE-STAGE CRYOCOOLERSSpace apability for ryocoolersThe AIM Flexure Bearing cooler series provides for high reliability and long life performance.The experience of more than one hundred SF100 cooler units provides the basis for the develop-ment of a space-qualified cryocooler family to be offered at a reasonable cost. Reliability and longlife performance are supported by life time testing of several SF100 Stirling and pulse-tube coolers.Tested coolers demonstrate constant cool-down and steady state performance over a duration of20,000 operating hours. Life test evaluation is ongoing.The internal compressor design (coil, magnet, cylinder sleeve, piston, spring, etc.) of the tactical ver-sion remains unchanged for space applications. However, further quality assuring steps are added to thestandard manufacturing processes and test procedures to fulfill the higher standards for space applications.For the space versions of the compressor and the pulse-tube coldfinger, several external con-struction changes  have been made to meet the space interface requirements. Figure 3 shows acomparison of the standard SF400 compressor housing with pulse-tube and the modified space-ready chassis. The compressor and the pulse-tube warm end provide surfaces for heat pipe mount-ing. The new housing also includes the mechanical interface for the mounting the compressor to thebase plate of the satellite.SPACE CRYOCOOLER PERFORMANCE The cryocooler performance specifications are already given in the space programs section.For both programs, a pre-qualification has already been performed to assure product adequacy andcompatibility. The compressor, the pulse-tube warm end and also the buffer volume were connectedto a thermal interface assuring the missions environmental temperature conditions.Figure 2.   Flexure Bearing compressor SF100B with ½’’ Pulse-Tube coldfinger and buffer volumewith integrated Inertance tube.Figure 3. SF400 standard compressor housing (left), space modified housing (right)C CIn Figure 4, the cooling capacity of the SF100 is plotted over input power at different cold headtemperatures for Tamb=25°C. The power consumption at the nominal operating point is 17.5 WDC.For the tests, standard cooler control electronics were used.In Figure 5, performance data for the SF400 driven by an AC power source is shown for differ-ent cold tip temperatures.In general, space applications demand very low vibration output levels. EnMAP and KOMPSAThave specified vibration output levels in the typical range of tactical cryocoolers. Therefore novibration control electronics are needed. Figure 6 shows the vibration output signals of the standardSF100 compressor with a pulse-tube coldfinger for all three dimensions.The graph of the compressor longitudinal axis shows the vibration output distribution withsignal peaks at the compressor operating frequency and higher harmonics. With a peak value lessthan 1 NRMS, the standard SF100 Pulse-Tube cooler clearly meets the requirements for the EnMap.The vibration of the coldfinger longitudinal axis is characterized by noise without any additionalpeaks at operating frequency or the higher harmonics resulting from the coldfinger. For the qualifi-cation and the flight model, motor halves of the compressor will be matched according to theirdriving forces, which will reduce the vibration output level.Figure 4. Performance data for SF100 at stated cooling temperaturesFigure 5. Performance of SF400 with 60 WAC input power137AIM SPACE CRYOCOOLER PROGRAMS138 SMALL 50-80 K SINGLE-STAGE CRYOCOOLERSQUALIFICATION PROCEDURESLifetime QualificationThere are several failure mechanisms which can lead to a failure. AIM has extensive experi-ence in linear cryocoolers. Field data from more than 30,000 cryocoolers provide the statisticalbasis. Table 3 shows the failure probability of the AIM Flexure Bearing pulse-tube cryocoolers forspace applications. The values are based on Ross1 and field data.Organic ContaminationOrganic contamination is a lifetime limiting factor for cryocoolers. This process is stronglytime dependent but because of the long lifetime a real time testing is not deemed as feasible. In-stead, an accelerated evaluation scheme had been developed. This procedure foresees two indepen-dent test series described in Table 4.Figure 6. Vibration output data of SF100 Pulse-Tube coolerTable 3. Failure mechanisms of space cryocoolersAfter finishing 5,000 hours of real time testing, a set of qualification models is to be evaluatedwith respect to contamination by means of gas chromatography. Knowing the average contamina-tion after 5,000 hrs. A trend can be establish which can be linearly extrapolated  to the specifiedlifetime as is 60,000 hrs for typical space profiles.  This approach implies a linear contaminantincrease with time [Figure 7, dashed line] which is highly conservative as performance degradationtypically declines over time. The linear extrapolation results in a worst case value of gas contamina-tion [Figure 7, notation 5] to be withstood by the second test series carried out under deliberatecontamination.The second test series contrasts the extrapolated worst case contamination [Figure 7, dot] witha contamination level causing a series of identical qualification coolers to fail[Figure 8, filled dot].The test units will be operated under identical conditions except that they are continuously andcontrolled fed with contaminant material, naturally existing within the system. This approach al-lows the slow contaminant diffusion processes to be replicated within a short time frame causingthe test units to degrade in an accelerated way. Eventually the qualification models reach specifica-tion limit [Figure 8, notation 6] and are to be examined by gas chromatography giving the respec-tive, quantitative contamination [Figure 8, notation 7].Assuming that cryocooler performance, widely expressed by rated input power, linearly in-creases with system contamination [Figure 8, dashed line] one can insert the extrapolated worstcase contamination from previous step, i.e. [Figure 8, blank dot] providing the derivation of thecorresponding power consumption [Figure 8, X]. If the examined quantity of contaminants for theaccelerated test specimens is of higher order than the linearly predicted worst case contamination,i.e. positive performance margin [Figure 8, Index 9], the expected cooler lifetime is proven.Table 4.  Accelerated lifetime qualification schemeFigure 7. Extrapolation of degradation established by real time testing139AIM SPACE CRYOCOOLER PROGRAMSWear outDue to the Flexure Bearing design, compressor related wear out is well controlled and thusdoes not limit the lifetime of the cooler. Examinations of endurance test units have shown thatneither compressor nor Pulse-Tube exhibited any sign of wear.FatigueFor highest compactness of the compressor a welding joint for the Flexure Bearing spring wasdeveloped. Flexure bearing springs and supporting weld seams are critical components and havebeen qualified by cyclic testing, resulting in a Woehler diagram given in Figure 9. For each stroke20 springs were tested. During the tests, the actual stroke was monitored with a laser stroke sensor.The results confirm more than 35% margin (in tension) to the endurance level at max geometricalstroke. For nominal stroke at full input power (=65% of max stroke) the safety factor is about 2.1and thus sufficient. Additionally, mentioned displacements have been  performed at varying ambi-ent conditions ranging from 23 to –54°C to include effects of brittleness.Figure 8. Deliberate contamination of qualification modelsFigure 9. Woehler curve for Flexure Spring assembly of AIM SF100 tactical cryocooler140 SMALL 50-80 K SINGLE-STAGE CRYOCOOLERSCONCLUSIONAIM has developed space cryocoolers as part of the IDCA’s for EnMAP and KOMPSAT-3Asatellite mission. Based on tactical cryocoolers from serial production, AIM Flexure Bearing com-pressors in combination with a pulse-tube coldfinger have been adapted for space applications. Theinternal designs of the cooler's components (compressor, Pulse-Tube coldfinger and buffer volume)have not been changed. The mechanical and thermal interfaces were adapted to the satellite struc-ture to meet the mission’s requirements.The prequalification at the cooler level demonstrate that the coolers will fulfill the specifica-tion of EnMap and KOMPSAT. At the moment the qualification model for the KOMPSAT satellitewill be assembled. The qualification will be performed on IDCA level starting in the near future.A performance improvement program will decrease the power consumption of the cooler for futurespace applications. In addition, the development of a digital cooler control electronics for the tacticalcooler is in progress. An enhanced space version of this electronics may provide vibration control.REFERENCES1. Ross, R.G., “Cryocooler Reliability and Redundancy Considerations for Long-Life Space Missions,”Cryocoolers 11, Kluwer Academic/Plenum Publishers, New York (2001),  pp. 637-648.2. M. Mai, I. Ruehlich, C. Rosenhagen, Th. Wiedmann “Development of the Miniature Flexure BearingCryocooler SF070,” Cryocoolers 15, ICC Press, Boulder, CO (2009),  pp. 133-138.3. Rühlich, I., Mai, M., Wiedmann, Th., Rosenhagen, C., “Flexure Bearing Compressor in the One WattLinear (OWL) Envelope," Proc. SPIE, Vol. 6542 (2007), pp. 654221-1 to -7.4. Radebaugh, R., "Pulse Tube Cryocoolers for Cooling Infrared Sensors; Proc. SPIE 26, Vol. 4130 (2000),pp. 363-379.5. Kittel, P., Kashani, A., Lee, J.M., Roach, P.R., "General pulse tube theory," Cryogenics,Volume 36, Issue 10, October 1996, pp. 849-857.141AIM SPACE CRYOCOOLER PROGRAMS

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AIM Space Cryocooler Programs

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