International Workshop on Cooling System for HTS Applications (IWC-HTS) 14th (Wed.) to 16th (Fri.) October, 2015 at Kunibiki Messe (Shimane Prefectual Convention Center), Matsue, Japan

HighHigh----reliabilityreliability and highhigh----efficiencyefficiency cryocoolers for HTS Applications

Daniel Willems*, Jeroen Mullié, Tonny Benschop, Garmt de Jonge Thales Cryogenics BV, PO box 6034, 5600HA Eindhoven, The Netherlands

Thales Cryogenics BV has developed a large range of cryocoolers. Both Stirling and pulse-tube coolers are available for a wide range of tactical, industrial, and scientific applications. These systems provide cooling at cryogenic temperatures, for which typically liquid cryogens are used. The most obvious reason for using cryocoolers in HTS systems is ease of use. There is no dependence on the use of liquid cryogens that need to be topped up. An actively cooled system can run autonomously without interference for long periods of time. One should consider life time requirements and vibrations when selecting a cooler solution for HTS systems. The lifetime of a cooler could be an important factor in the overall lifetime of the entire system, and the moving parts in coolers could potentially generate vibrations that disturb the device that needs to be cooled.

Figure 111: 1: Schematic layout of a linear Stirling cooler (left) and a pulsepulse----tubetube cooler (right). The pulsepulse----tubetube cooler is shown without the compressor.

Both Stirling and Pulse-tube coolers are so-called regenerative cryocoolers. They rely on the cyclic compression and expansion of a working gas in a hermetically sealed system. The thermodynamic cycle of the Stirling cooler is well known. In a Stirling cooler, two moving pistons create a phase difference between gas pressure and flow to generate a heat pumping effect. The thermodynamic cycle of a pulse-tube cooler is comparable, but a network of flow impedances is used instead to create the necessary phase difference.

The focus in the development of these coolers has always been on improving reliability, efficiency, and integration aspects.

The main advantage of a pulse tube is that there are no moving parts in the cold head of the cooler. As a result, there is no wear associated with moving parts. Furthermore, no vibrations are generated in the cold head. For a similar cooling power, pulse-tube-coolers are typically larger and the efficiency is typically lower than Stirling coolers. But the lifetimes are significantly higher. Reliability of coolers is further improved by removing the main failure mode, wear, in the

Cryogenics and Superconductivity Society of Japan (CSSJ) International Workshop on Cooling System for HTS Applications (IWC-HTS) 14th (Wed.) to 16th (Fri.) October, 2015 at Kunibiki Messe (Shimane Prefectual Convention Center), Matsue, Japan compressor as well. This has been achieved by using so-called flexure bearings in linear compressors. Flexure bearings are springs, with a defined stiffness in the axial direction, and a nearly infinite stiffness in the radial direction. As a consequence, the pistons can be placed in the exact center of the cylinder, without contact. Thales Cryogenics has mastered the production process such that these kinds of compressors, including the required piston alignment steps, can take place in large series production.

The reliability of linear coolers has thus significantly increased. In the early years of linear coolers, typical lifetime of linear Stirling coolers was approximately 2000 hours. Nowadays, pulse-tube coolers have average lifetimes of 135000 hours, Stirling cryocoolers with flexure bearing compressors have average lifetimes 60000 hours, and Stirling coolers based on contact seal compressors have lifetimes of 30000 hours and the statistics are still increasing for all these cooler types. Because the coolers are closed systems, no maintenance is required during these lifetimes.

Because of the absence of moving parts in the cold head, a pulse-tube cooler generates very few vibrations, and is thus very suitable for cooling vibration sensitive sensors. The residual vibrations are generated in the compressor. In linear compressors, typically dual opposed pistons are used. Two identical motor halves operate in opposite direction, cancelling the vibrations caused by the moving masses. Residual vibrations are the result of tolerances; small changes in parts and process tolerances lead to small residual vibrations. These vibrations can be further reduced. The most simplest method is to decouple vibration with flexible links. If this is not possible, active reduction is a very effective option to further reduce the induces vibrations. With AVR, induced vibrations are reduced in an active feedback loop. It requires more complex drive electronics, with two independent output stages for each motor half. An accelerometer measures the residual vibrations, and its signal is used to define the required drive for each motor. Recently, this technology is expanded to Stirling coolers as well. If all moving masses are placed along the same axis of motion, the induced vibrations of the entire cooler can be reduced to levels comparable to pulse-tube coolers.

As a result of these developments, actively cooled systems are present in many applications. Space applications benefit from the high reliability. In space, guaranteed operation for many years is usually required. In earthly applications low vibrations are required for any application that uses sensitive electronics, and long lifetimes are relevant for all applications, especially those who need long uninterrupted and unattended operation, such as HTS applications.

In the presentation, we will further present the current state-of-the-art of readily available cooling solutions for HTS applications. TCBV has coolers available in the temperature ranges between 30 and 120 K and input powers below 500 W.

In the outlook on the development of these miniature mechanical cryocoolers for the coming five years will be presented in order to be able to line up cryocooler development with HTS user requirements.

Keywords : Cryocooler, Stirling, Pulse-tube coolers, efficiency, reliability, vibrations Corresponding author *: [email protected]

Cryogenics and Superconductivity Society of Japan (CSSJ)