Miniature cryocooler developments for high operating temperatures at Thales Cryogenics

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1 Miniature cryocooler developments for high operating temperatures at Thales Cryogenics R. Arts a, J-Y Martin b, D. Willems a, C. Seguineau b, S. Van Acker b, J.C. Mullié a, A. Göbel a, M. Tops a, J. Le Bordays b, T. Etchanchu b, A.A.J. Benschop a ; a Thales Cryogenics B.V. (Netherlands) b Thales Cryogenie S.A.S. (France) ABSTRACT In recent years there has been a drive towards miniaturized cooled IDCA solutions for low-power, low-mass, low-size products (SWaP). To support this drive, coolers are developed optimized for high-temperature, low heat load dewardetector assemblies. In this paper, Thales Cryogenics development activities supporting SWaP are presented. Design choices are discussed and compared to various key requirements. Trade-off analysis results are presented on drive voltage, cold finger definition (length, material, diameter and sealing concept), and other interface considerations, including cold finger definition. In parallel with linear and rotary cooler options, designs for small-size high-efficiency drive electronics based on state-of-the-art architectures are presented. Keywords: Stirling, Cryocooler, SWaP, Linear, Rotary, Cold finger, HOT 1. INTRODUCTION The size, weight, and power of cooled infrared cameras have, for the most part, been limited by the size, weight and power requirements of the cryocooler. These aspects are regularly summarized as the SWaP considerations Size, Weight, and Power. For a given type of cooler technology, size, weight, and input power tend to correlate, meaning that a cooler drawing less power, is usually a small cooler as well. The theoretical lower limit for power consumed by a Stirling cryocooler is determined by the total heat load, as well as the Coefficient of Performance (COP): [W] of cooling per [W] of electrical input power. The theoretical upper limit to the COP is the Carnot efficiency, given by COP T l C =. (1) Th Tl As illustrated in Figure 1, this COP scales quite dramatically with cold temperature. By increasing the cold temperature from 77 K to 150 K, the theoretical COP improves by almost a factor 3. A true breakthrough in the area of low power SWaP cooled infrared cameras therefore depends on the maturity of infrared detectors capable of operating at increased cryogenic temperatures ( HOT or High Operating Temperature ). The increasing maturity of this new generation of detectors predominantly operating in the medium wave or MWIR spectral range has led to a new demand for miniature cryocoolers. In addition to enabling smaller, lower-power IDCA modules, the increase in focal plane temperature has also simplified the use of large-format arrays, enabling megapixel infrared detector without requiring an large cooler. This paper will however focus on smaller systems.

2 Figure 1: COP C versus cold temperature. 2. EXAMPLE SWAP APPLICATION One typical application where a HOT detector could be used, is the handheld thermal imager. Due to the size and power consumption of the traditional cooled infrared detector, handheld thermal imaging has long been dominated by uncooled (microbolometer) detectors. However, attaining good sensitivity (low NETD) with an uncooled detector requires a large lens (low F/# number = F/1 is typical) while a cooled MWIR detector is typically sufficiently sensitive even with an F/4 lens [4]. This means that the advantage of having an uncooled detector, which is a reduction in mass due to not having to include a cryogenic cooler, is countered by requiring a larger lens. This difference becomes especially pronounced when considering long-range applications, where telescopes with long focal lengths are used. For example: a full VGA uncooled thermal imager (VOx 17 µm pitch array) with a FOV of 4, will require a (F/1) lens with a diameter in the order of 200 mm. This will add significantly to the size, mass and cost of the camera, compared to the smaller F/4 lens diameter of 50 mm needed with a cooled imager for the same FOV and same pixel pitch. In addition, a further reduction in lens diameter is possible due to the smaller pixel pitch attainable with cooled MWIR detectors, but discussion of this is outside the scope of this paper.

3 3. SWAP REQUIREMENTS The various manufacturers of HOT detectors each have differing dewar and detector parameters. At the start of Thales Cryogenics cooler development, a baseline set of parameters was identified. A generalized set of SWaP parameters is given below. Table 1: Target SWaP requirements Operating temperature [K] 150 Total heat load [mw] Includes: - Cold finger wall parasitics - Dewar load (radiative) - Wire load (conductive) - Active load power dissipated by detector 150 Thermal mass [J] 100 While defining a generalized set of requirements is always somewhat dubious, as different FPA manufacturers and technologies each have their own working points, it is our belief that the 150 K operating point is the current target in the trade-off between performance and power consumption. Various different cryocooler solutions can be envisioned that are suitable for these parameters. As a generalized trade-off between requirements cannot be made, both an integral rotary and split linear solution are developed at Thales Cryogenics. Both cryocooler technologies having their specific pro s and con s. A common mechanical interface is used in Thales Cryogenics SWaP coolers, allowing for a case-by-case selection of the optimal cooler for a given application, without requiring a redesign of the infrared detector-dewar assembly Optimizing the RM1 and RM2 for HOT 4. PRIOR WORK As presented in 2012 [6], initial work was performed to optimize the RM1 and RM2 rotary cryocoolers for elevated cold temperatures. At that time, the filling pressure was decreased in order to optimize the coolers for steady-state power draw at cold temperatures of 150K and above. The main benefits at system level were expected to be found on the total input power, start-up time and overall reliability. The first point is identified as one of the key driver for the development of new ultracompact handheld thermal imager insofar as the battery remains one of the largest contributors in terms of size and weight and thus operational time on one battery block should be as high as possible without losing system performance. Nonetheless, optimization of current rotary coolers shows some limitation insofar as the rotation speed tends to decrease with higher cold temperatures. At extreme low rotation speed, driving instabilities were found leading to a degradation of the stability of the thermal regulation. This is the result of an oversized cooling capacity of the current cryocoolers for operating at HOT temperature (150K and above) with current thermal detectors (same size and pitch). RM1 and RM2 are currently used for cold temperatures up to 110K. RM1 could be used for temperature rising to 130K with relevant benefits on reliability, power consumption and cool-down time [6].

4 4.2. LSF9997 linear cryocooler Initially presented in 2010, the UP8497 and LSF9997 cryocoolers were developed as a first attempt to meet demands for smaller linear cryocoolers, for continuous operation. The LSF9997 cryocooler was developed using a moving magnet, flexure bearing compressor, in order to take advantage of Thales Cryogenics heritage to achieve an extremely high reliability. At that time, development was still focused on a working point of 77 to 120K tip temperature. The LSF9997 has since matured into series production, and is routinely used for K applications UP8497 linear cryocooler The UP8497, on the other hand, was developed specifically to achieve a high power density and efficiency. The initial 2010 prototype [3] did not yet reach a significant advantage over the LSF9997. For this reason the initial design was shelved and not launched for series production. In recent years, however, the demand for higher power densities in miniature linear cryocoolers has increased, which has resulted in a redesign effort on the UP8497. As the drive frequency and dewar interface definition between LSF9997 and UP8497 are identical, only the compressor was redesigned. The redesign effort was focused on enabling higher input powers to the cooler, resulting in higher cooling powers while maintaining the high efficiency (high COP). The transfer line connection was moved to the center of the compressor, while the electrical connections to both linear drive motors were separated, to allow for potential future use of Active Vibration Reduction [7] in case induced vibrations of the overall cooler at system level should be minimized. A side-by-side comparison of the two versions of the 8497 is shown below. Figure 2: UP version (left) and 2015 version (right). There is a trade-off to be made between efficiency and power density. By increasing the filling pressure, the power density of the UP8497 can be improved, at the expense of efficiency. This was tested using a thermal mockup dewar, supplied by IRCameras LLC, and shown in Figure 2. Cooldown curves were measured for two different filling pressures, namely 20 bar and 25 bar. The results can be seen in Figure 3. It is apparent that the higher cooling power available at 25 bar filling pressure immediately results in a significant reduction in cooldown time (from 245 seconds to 205 seconds), while the influence on efficiency is negligible. At +71 C ambient temperature, the effect is similar 25 bar results in a lower cooldown time (415 seconds instead of 450) but an increase in steady-state power consumption. At this ambient temperature, the influence on steady-state power draw is no longer negligible an increase of 20% was observed. Depending on the application requirements, a trade-off can be made between efficiency and cooldown time.

5 Figure 3: UP8497 cooldown at different filling pressures. Figure 4: Heat lift of UP8497 (25 bar) vs LSF9997.

6 5. RECENT DEVELOPMENTS In order to meet the SWaP requirements outlined in Table 1, two new cryocoolers were developed - a linear cooler by Thales Cryogenics B.V., Eindhoven, The Netherlands, hereafter referred to as UP an integral rotary cryocooler by Thales Cryogenie SAS, Blagnac, France, hereafter referred to as RM-SWaP Which cooler is optimal for a given application depends on the exact requirements, but as is the case for the existing Thales cryocooler portfolio, both type of products have their respective application fields Defining a common interface To maximize flexibility for Thales Cryogenics customers, a common dewar interface was designed for both the RM- SWaP miniature rotary cooler and the UP8197 miniature linear cooler. As both cryocoolers were designed with the same detector parameters in mind, this will ensure that a customer will only need a single dewar design to enable the production of detector assemblies containing either cooler. A number of key requirements for a common dewar interface were identified early on: - Flange diameter, bolt hole pattern diameter - Length (while still allowing for sufficient effective regenerator length) - Low thermal loss As the basis for the cold finger dewar interface, the standard Thales ¼ definition, present on RM2, RM4, and the UPxx97 and LSFxx97 coolers was used. A larger displacer diameter would result in an increased size of the cooler mounting interface, especially with the RM-SWaP cooler, while a smaller displacer diameter would introduce some technical risks. These considerations have resulted in a cold finger length of 30 mm (internal) and a bolt hole pattern with a diameter of only 15.2 mm. For the initial test dewars, the cold finger was made out of stainless steel. To further reduce heat load future infrared dewars should preferably be based on TA6V material. A sketch of the common cold finger as well as a mounted prototype can be seen in Figure 5. Figure 5: Cold finger sketch (left), photo (right) 5.2. UP8197 The linear cryocooler for the SWaP requirement is named UP8197. During the initial design trade-off, it was decided to use a moving-magnet dual opposed-piston compressor. This has the advantage of eliminating the need for an external balancer while maximizing efficiency and power density. By using two independent drive motor wires, future application of Thales Active Vibration Reduction is possible for highly vibration-critical applications. Combining a dual opposed-piston compressor with a standard free displacer cold finger has proven to be extremely effective in the application of active vibration reduction, allowing for very low vibration levels without sacrificing efficiency [7]. The dimensions of the compressor are 60 x 32 mm and the unit weighs less than 250 grams (excluding heat sink and dewar).

7 Performance measurements of the first unit can be seen in Figure 7. It should be noted that these initial performances were measured with a laboratory test dewar, not yet optimized for heat load (estimated parasitic loss of 200 mw). Performances given are active loads (power to heater resistor in test dewar) only. Figure 6: UP8197 photograph. Figure 7: UP8197 performances, room temperature ambient. The UP8197 cooler is planned to be qualified mid Start of production is currently planned for the end of 2015.

8 5.3. RM-SWaP The initial design trade-off was made to emphasize size and weight reductions, and to keep a high efficiency at High Operating Temperatures. By using the preliminary results obtained on RM1 and RM2 coolers [2], design steps were made to meet requirements specified in Table 1. A specific mock-up was designed to test such a configuration by using a RM1 external casing. The experimental design verification is in line with the previous performed experiments [6] (three filling pressures, several cold temperatures and several displacer lengths) [6]. Tests were performed with a laboratory test dewar, and results adapted to the characteristics of a dewar representative of HOT applications (see Table 1 for indicative figures). The current results are shown below. In particular, the impact of the filling pressure is first studied. In a second part, several max rotation speeds are proposed. Third, several displacer with varying length were mounted successively. At last, the performances are compared when working at other cold temperatures. Figure 8: RM-SWaP Impact of filling pressure on the input power (left) and the cooldown time (right) at room temperature for a thermal regulation at 150K. The sensitivity of the input power toward the filling pressure is close to 4.7% per bar in the range considered. The use of low filling pressure is thus relevant for the optimization of the input power. Nonetheless, the main drawback is noticed on the cooldown time, where the cooldown time rises of 6.4% per bar. It is noticeable however that the CoolDown Time (CDT) remains below 2 minutes, even at low filling pressure. In order to compensate a degradation of the CDT or to improve on the CDT in general, the maximum rotation speed during the cooldown can be increased. Figure 9: RM-SWaP Impact of maximum rotation speed on the cooldown time at room temperature (150K) Indeed, the sensitivity of the cooldown time towards rotation speed is 1.2% per Hz, with no impact on the input power in regulation. A slight rise of the max input power during cooldown necessary occurs (also being 1.2% per Hz). The electronic driver design must take into account the maximum rotation speed to be used in the application.

9 In a third step, several displacer lengths have been mounted and tested on the same cooler. Two lengths were tested, and figures for the RM-SWaP target length were interpolated. The two lengths are currently used on RM1-short version and RM2 (one short version and one normal length version). Figure 10: RM-SWaP - Power in regulation versus displacer As illustrated, a huge impact was found on the power in regulation. The power in regulation varies of 2.4% per mm. This means that a 5 mm extension of the cold finger will result in an improvement of the input power up to 12%. Insofar as the thermal mass of the cold finger is bigger for longer cold finger, the cooldown time is expected to degrade with increasing length. Nonetheless, the experiments show sensitivity lower than 0.5% per mm. In any case, the CDT remains under At last, other cold temperatures have been tested: 140K and 170K, in order to evaluate how the RM-SWAP should manage to deal with a varying cold temperature. At 140K, the power regulation increases slightly above 2.5W. Up to 170K, the power regulation decreases down to 1.8W in a typical configuration with a 30mm cold finger. Such variations mean that the power regulation varies of 1.1% per K. The Cooldown time in the same configuration varies from 92s at 170K up to 116s at 140K, that is to say a variation of 0.7% per K. The performances of the RM-SWaP remain close to the target even if working at 140K, and are improved for higher temperatures. The RM-SWaP has also been tested for current operational temperature, i.e. 110K. Even if an increase of the regulation power is observed, one can note that the cooling power is still enough to work down to 110K. The Cooldown time remains below 150s. Figure 11: RM-SWaP - Power in regulation versus Cold Temperature, ranging from 110K to 170K. As it was previously described elsewhere [2], the optimization of the cryocooler design is based on a trade-off mainly made at system level between start-up time, power consumption and reliability needs.

10 It occurs that a 2.2W power consumption can be reached with a 30mm cold finger length when operating at 150K. The filling pressure, the rotation speed and the cold finger length have a huge impact on the performances of the RM-SWaP. The first two parameters can be easily tuned depending on mission profile and trade-off made at system level, the cold finger length may still be considered as an important additional parameter to adjust performances to meet the requirements at system level especially for detector temperatures below 140K. The modification of such a parameter involves a redesign of the displacer length, and an adaptation of the dewar bore length. Nonetheless, because of the large impact of the length on the input power regulation, such a modification should be considered when the priority is given to power consumption and miniaturization at system level. By taking into account the required engine dimensioning, a first mock-up was then designed for technological risk mitigation. The current sizes of the RM-SWaP are 37 mm along the motor direction, and 61.5 mm along the optical direction. Figure 12: RM-SWaP dimensions The actual weight is 137gr. To reach this miniaturization while keeping high level of efficiency, all the usual functions of the cooler have been optimized. In particular, thanks to the high level of maturity of our existing products, it has been decided to weld the external cooler envelope instead of the existing assembling method with screws. The external casing is laser welded in a final stage after mounting and system verification steps. Figure 13: RM-SWaP Comparison of the RM-SWaP mock-up (left) with RM2 (center) and RM3 (right). Figure 14: RM-SWaP Picture of the RM-SWaP mock-up.

11 The performances of such a design have still to be evaluated. The RM-SWaP cooler is planned to be qualified at the end of 2015 and beginning of 2016, while the production is planning to start at the end of Miniaturized drive electronics The low power draw of SWaP cryocooler products necessitates low-power digital control logic as well. Instead of aiming for the smallest possible cooler driver, our design philosophy has been to minimize the size for a full-featured cooler driver. This means that the SWaP cooler driver should have functionality on par with the latest generation of Thales digital cooler drive electronics, while providing optimal size and efficiency. As a basis for the drive electronics for the UP8197 and the RM-SWaP cooler, recent developments in miniaturized rotary drive electronics were used, namely the RDE1232 [5]. The first product to be developed was drive electronics for driving miniature linear coolers. One notable feature that is expected to become more important for SWaP applications is the ability to have dual programmable set points. As was demonstrated by various manufacturers of IDCAs (see, e.g., [4]), a usable reduced-performance image can now be obtained at temperatures exceeding 200 K. This means that having two separate set points in cooler drive electronics is now expected to become a very real advantage in an operational scenario. In usage scenarios where a high infrared photon flux is present, a rapid cooldown is required, or where reduced image quality is acceptable, a higher FPA set point can be used. This will result in a steady-state power draw that is further reduced, increasing battery life in a handheld application. The Thales LPCDE1220 linear cooler driver incorporates a dual programmable set-point function, next to having the comprehensive feature set of the existing Thales drive electronics portfolio. The following key functions are available: - Mounting holes and connector for easy integration - Suitable for driving multiple types of linear coolers - Serial link for programming and communication - Discrete Cooler Ready output - Discrete Cooler ON/OFF input - Discrete Cooler STANDBY input (dual set point) - Slow start curves - Ambient temperature correction µa diode bias current - High temperature stability - High efficiency - Drive frequency up to 140 Hz - Input voltage from 8 V to 28 V While the same architecture is used for both the rotary and linear drive electronics, it was decided to develop two separate driver boards rather than making a universal design suitable for driving both coolers, as this allows for optimization in terms of size and cost. The linear and rotary drive electronics will be made to have maximum commonality regarding interfacing. The LPCDE1220 will be used as the basis for developing a driver for the RM-SWaP cooler, start of development planned directly after launch of the LPCDE1220. The board that is currently under qualification is shown in Figure 15, board dimensions of 37.5 x 28.0 mm. During preliminary tests, this board has been successfully used for driving either the UP8197 or the UP8497.

12 Figure 15: LPCDE CONCLUSION With the addition of the UP8197 and the RM-SWaP, the Thales Cryogenics product portfolio covers the present gamma of requirements for HOT cooled detectors. Preliminary testing shows excellent performances for RM-SWaP for applications where an absolute minimum in size and weight is required, while the UP8197 has shown relatively high cooling powers. In order to optimize cost efficiency, both solutions are designed with a common cold finger interface and two electronic drivers with the same core design. Nonetheless, the results obtained on RM-SWaP show a huge impact of the displacer length on the input power in regulation: for specific applications where the main drivers are autonomy and miniaturization (new generation of HandHeld Thermal Imager), a longer version of the current cold finger should be considered. For lower cold temperatures or larger heat loads (larger arrays), the existing previously Thales portfolio, including RM1, RM2, LSF9997 and the recently-redesigned UP8497, has now reached a high level of maturity. REFERENCES [1] Van der Weijden, H., Benschop, A., Van den Groep, W., Willems, D., Mullié, J., High reliable linear Cryocoolers and miniaturization developments at Thales Cryogenics, Proc. SPIE 7660 (2010) [2] Benschop, A., Van de Groep, W., Mullié, J., Willems, D., Clesca, O., Griot, R., Martin, J.-Y., Cryocoolers developments at Thales Cryogenics enabling compact remote sensing, Proc. SPIE 7834 (2010) [3] Van den Groep, W., Mullié, J., Willems, D., Van Wordragen, F., Benschop, A., The development of a new generation of miniature long-life linear coolers, Cryocoolers 16 (2011), pp [4] Pillans, L., Baker, I., McEwen, R., Ultra-low power HOT MCT grown by MOVPE for handheld applications, Proc. SPIE 9070 (2014) [5] Willems, D., Tops, M., Bots, F., Arts, R., De Jonge, G., Benschop, A., Etchanchu, T., Developments in advanced cooler drive electronics, Cryocoolers 18 (2014), pp [6] Martin, J-Y., Cauquil, J-M., Benschop, A., Freche, S., Thales Cryogenics rotary cryocoolers for HOT applications, Proc. SPIE 8353 (2012) [7] Arts, R., De Bruin, A., Willems, D., De Jonge, G., Benschop, A., Adaptive vibration reduction on dual-opposed piston free displacer Stirling cooler, Proc. SPIE 9070 (2014).