STI s Solution for High Quantity Production of Stirling Coolers

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1 1 STI s Solution for High Quantity Production of Stirling Coolers Amr O Baid, Andreas Fiedler, Abhijit Karandikar Superconductor Technologies Incorporated Santa Barbara, CA ABSTRACT The mission of Superconductor Technologies Inc. (STI) is Improving the Quality of Wireless. Through the use of its patented High Temperature Superconductor (HTS) technology, STI has created a cryogenic receiver front-end (CRFE) used by major wireless operators to enhance their network performance. The Sapphire cryocooler is a key component in the STI SuperLink product family. Since 1999, more than 4,000 Sapphire cryocoolers have been deployed with over 44 million hours of accumulated run time. The current generation of cryocooler has an efficiency of 16.7W/W, 6W lift at 77K with 100W input power and a 35 C heat reject temperature. STI s Sapphire cryocooler is not only suitable for wireless industry products, but for other applications where low cost, high efficiency, and mass production cryocoolers with field-proven reliability are decisive components for enabling product commercialization. The wireless communication industry demands products that are cost effective and extremely reliable, typically with MTBF requirements in the hundreds of thousands of hours. Field data confirms that the Sapphire cryocooler has met and even exceeded these requirements. Performance data will be presented along with reliability data from the field and in-house testing. Various methods of thermal management of the cryocooler in systems will also be discussed. INTRODUCTION For seven years, STI has been producing Stirling cycle cryocoolers as part of their CRFE products for the wireless communication industry. The free piston, gas-bearing cryocooler is a critical component that contributes to the overall receiver system s lifetime and reliability. Cryocooler reliability data, therefore, is continuously tracked and updated. The product has an MTBF of greater than 500,000 hours and is based on reliability data from the field and in-house lifetime test results. This has far surpassed STI s original goal of 50,000 hours. The extraordinary MTBF of > 500,000 hours has remained consistent over the last two years even with an increased number of deployed systems. The Invisible Cooler, which is small, lightweight, and highly reliable has become a reality. STI s unique cryocooler design has a new name: Sapphire.

2 Sapphire Cryocooler 2 Dewar, HTS RF Filters Electronics Board Figure 1. STI s flagship product, SuperLink Rx 850 Figure 1 shows STI s flagship product, the SuperLink Rx 850, one of over 4,000 maintenance-free SuperFilter and SuperLink Rx units deployed worldwide. In this figure, the Sapphire cryocooler is surrounded with a shroud to optimize forced convection cooling of the unit using a fan. Cryocooler Performance The Sapphire cryocooler is used in conjunction with a Dewar and HTS RF Filters based on thin-film technology that operates at a temperature of 77K and 3.5W maximum heat load. The unit is designed to operate over the complete ambient temperature range of -40 C to +60 C. The coldfinger temperature is regulated by an electronics board as shown in Figure 1. STI s cryocooler has more than sufficient heat lift even under the worst-case conditions. The performance and physical characteristics of the Sapphire cryocooler are listed as follows: Heat lift 6W of lift at 77K and 35 C heat reject temperature Nominal input power 100W input power at 6W, 77K and 35 C Maximum input power 140W, 60 Hz Weight 7.5 lbs. Dimensions 3.5 inches OD x 11.5 inches length Operating orientation Any Ambient temperature -40 C to +60 C Vibration output 2.2N with passive balancer Figure 2 shows the heat lift of the Sapphire cryocooler as a function of input power, and at various heat reject temperatures.

3 3 12 Cryocooler Lift at 77K, W C 42 C 75 C Input Power, W Figure 2. Cryocooler lift at 77K vs. input power for various heat reject temperatures RELIABILTY DATA FROM THE FIELD STI has more than 4,000 units operating in the field. The combined run time is over 44 million hours. Of these units, 460 coolers have > 26,280 accumulated hours (three years). Table 1 shows a summary of STI units in the field. Figure 3 shows the accumulated run time and the number of coolers in the field as a function of time. Infant mortality No Wear out seen to date No Total number of coolers > 4,000 Average hours per cooler 18,000 Cryocoolers with > 1 year run time >1,600 Cryocoolers with > 3 years run time 460 Cumulative hours > 44 million hours MTBF > 500,000 hours Results of Continuous Improvement/Refinement After Q Number of coolers > 2,600 Cumulative hours 15 million hours MTBF Unknown (only 3 units failed) Table 1. Summary of coolers in the field as of March 2004

4 STI Field Summary -- 3/1/04 4 # Units Shipped 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1, /99 12/99 3/00 6/00 9/00 12/00 # Shipped Cum. Hrs. Run-Time # Shipped = 4,096 Cum. Hrs Run-Time = 44,500,000 3/01 6/01 9/01 12/01 3/02 6/02 9/02 12/02 3/03 6/03 9/03 12/03 50,000,000 45,000,000 40,000,000 35,000,000 30,000,000 25,000,000 20,000,000 15,000,000 10,000,000 5,000,000 0 Cumulative Hrs Figure 3. Field summary number of units shipped vs. time As part of STI s rigorous reliability and continuous improvement program, a thorough Pareto analysis is conducted on a regular basis to identify potential cryocooler design and reliability improvements. This resulted in cryocooler design changes in Q Since that time over 2,600 coolers have been built, accumulating over 15 million hours in the field. Only three of these units have failed, and these failures were not wear-related. There are various ways to determine MTBF, such as Wiebull, Exponential Distribution, and lognormal. The method chosen depends on the nature of the product, and the types of failures that result. As part of the qualification of the cooler, tests were conducted to determine the Hazard Rate or Instantaneous Failure Rate (IFR) as a function of time. Commonly known as the bathtub curve, the results clearly show no infant mortality in Figure 4. In addition, the IFR does not change over time so there is no indication of wear out. By inspecting the units returned from the field as well as from on-going reliability testing at STI (see next section), this has been validated. Typical MTBF calculations are done using the exponential distribution. This methodology only applies if the product shows no infant mortality and constant failure rate (no wear out). Since the Sapphire cryocooler has met these two main criteria, this methodology is applicable. The equation for calculating MTBF using the exponential distribution methodology becomes simply: MTBF (hours) = Total accumulated run time (hours)/number of failures "Bathtub Curve" Coolers shipped 8/99 thru 12/00 Hazard Rat/IFR No increase in failure rate = No wear out Month Figure 4. Bathtub Curve Hazard rate or Instantaneous Failure Rate (IFR) vs. time (Hazard rate or Instantaneous Failure Rate (IFR) = # failing in next time period, having survived up to that time period)

5 For units produced since Q1 2002, the MTBF is nearly 5 million hours. While this clearly demonstrates a highly reliable unit, ironically, not enough failures of this configuration have occurred to determine a statistically significant MTBF. However, the 500,000-hour MTBF number for the entire population of coolers in the field is statistically significant and is the figure claimed by STI. 5 IN-HOUSE LIFE TEST A few years ago, STI began additional in-house lifetime tests in order to determine the run time until wear out, thereby evaluating the latest design improvements. Ten optimized design coolers, introduced in Q2 2002, are currently operating at or close to worst-case conditions, defined as elevated cryocooler heat rejection temperature (between 50 C and 60 C), and maximum heat load or input power. 5 of the 10 lifetime coolers are operating with a regulated coldfinger temperature of 77K. The remaining coolers are tested with the maximum possible input power achievable by the limitations of the electronics board. Figures 5 and 6 show the test setup and measurement results for the five temperature-regulated cryocoolers. Electronics Board Fan Cryocooler Power Supply Heat Load Power Supply Vacuum System Figure 5. One of STI s in-house lifetime test stations

6 6 17 Performance [% Carnot] SN 854 SN 664 SN 1009 SN 1028 SN Run Time [hrs.] Figure 6. Performance results of temperature regulated lifetime coolers The heat reject temperature, input power, heat load, and cold-end temperature data for these lifetime coolers is recorded every 10 minutes on a continuous basis. This allows any variation in cryocooler performance to become immediately apparent. Figure 6 shows a stable performance of all coolers within a range of 14% to 16 % of Carnot. All units with a current run time of more than 12,000 hours have shown nearly no performance variations during the last 2,000 hours, and therefore result in no wear out. OUTDOOR CRFE SYSTEM SUPERLINK RX 1900 SuperLink Rx 1900 is STI s newest product, an integrated duplexed CRFE for outdoor applications. The fully weatherized unit includes a SuperLink receiver and up to six dual duplexers. The key requirement for SuperLink Rx 1900 (see figure 7) is that it operates outdoors without a fan for thermal management. This resulted in developing a Sapphire cryocooler with a modified heat rejecter and heat pipes as shown in Figure 8, and described in the next section. Figure 7. STI s SuperLink Rx 1900 for outdoor applications

7 Passive thermal management design of SuperLink Rx 1900 The indoor units (e.g. SuperLink Rx 850) operate in semi-protected, air-conditioned environments in cellular service provider shelters. This conditioned air safely circulates through the unit and the fins on the cryocooler heat rejecter. The use of forced convection greatly simplifies the thermal management while minimizing the system weight. In contrast, the outdoor units (e.g. SuperLink Rx 1900) are exposed to a variety of ambient environmental conditions, including temperature extremes such as rain, snow, high humidity, and wind-blown dust. The requirement to operate reliably for a long period of time under these conditions eliminates the possibility of circulating air through the unit enclosure, pushing the design toward a sealed, insulated thermal heat management concept. Most of the heat generated by the system (65%) comes from the Stirling cryocooler. The remainder is generated from other unit components such as the second stage amplifiers, PCB, AC/DC power converter, and the amplifier power supply. All of these components are designed to mount directly on the flat surface of the heat sink to expedite heat removal. Some of the system components are shown in Figure 8. To reject heat from the Stirling cryocooler to the outside environment, a temperature delta of no more than 20 C must be maintained between the cryocooler heat rejecter zone and the ambient air. To accomplish this requirement without circulating ambient air through the enclosure, a heat pipe/heat sink combination is used to remove the heat passively through conduction and free convection. A heat sink with fins is used to transfer heat to the ambient air. To replace the finned heat rejecters with an efficient heat pipe interface (< 1 C temperature delta), a variation of the Sapphire cryocooler was designed. A heat pipe structure thermally connects the cryocooler heat rejecter zone to the finned heat sink, which transfers heat to the ambient air. The use of a heat pipe/heat sink combination in place of relatively low reliable fans results in a product with long life and high reliability. STI shipped over 50 SuperLink Rx 1900 systems last year, incorporating into its products the use of highly reliable passive cooling to serve the outdoor needs of the cellular industry. 7 NEMA Standard The outdoor enclosure unit, including a thermal management system, is designed for National Electrical Manufacturers Association (NEMA) 4X standard. NEMA 4X enclosures are constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment. In addition, the NEMA 4X enclosures provide a degree of protection against falling dirt, rain, sleet, snow, wind-blown dust, splashing water, hose-directed water, corrosion, and external formation of ice. Modified heat rejecter Cryocooler Sapphire Heat Pipe Base plate with fin structure Figure 8. Heat pipe version of the cryocooler Sapphire

8 ALTERNATIVE CRYOCOOLER APPLICATIONS The Sapphire cryocooler can be used for a variety of applications where low cost, high reliability, high performance, and long life are important or critical. Some examples include, but are not limited to the following: 8 Medical devices Cold electronics: microprocessors, detectors for focal plane arrays or gamma rays, and infrared detectors Freezers for biomedical specimens Micro liquid Oxygen and liquid Nitrogen generators SUMMARY STI s Sapphire cryocooler has demonstrated field-proven reliability of greater than 500,000 hours MTBF, combined with high efficiency, high power density, and cost effective manufacturability. These critically important achievements are necessary to ensure STI s success in the wireless communication industry. To date, STI has delivered more than 4,000 cryo-cooled receiver units, accumulating over 44 million hours of run time. 460 coolers have been operating for more than three years. The latest generation, of which over 2,600 units have shipped since Q2 2002, has demonstrated even higher MTBF. This conclusion is also based on field test results as well as in-house tests at STI. The SuperLink Rx 1900 is equipped with a modified version of the Sapphire cryocooler where heat removal is managed by using heat pipes and convection cooling to serve the outdoor needs of the cellular industry. Sapphire can be used for a variety of applications where low cost, high reliability, high performance, and long life are important or critical. REFERENCES 1. Hanes, M. and O baid, A., Performance & Reliability Data for Production Free Piston Stirling Cryocooler Cryocoolers 12, Kluwer Academic/Plenum Publishers, New York (2003), pp