Advantages of high-frequency Pulse-tube technology and its applications in infrared sensing

R. Arts, D. Willems, J. Mullié, T. Benschop

Thales Cryogenics B.V. (Netherlands)

ABSTRACT

The low-frequency pulse-tube cryocooler has been a workhorse for large heat lift applications. However, the high-frequency pulse tube has to date not seen the widespread use in tactical infrared applications that Stirling cryocoolers have had, despite significant advantages in terms of exported vibrations and lifetime. Thales Cryogenics has produced large series of high-frequency pulse-tube cryocoolers for non-infrared applications since 2005. However, the use of Thales pulse-tube cryocoolers for infrared sensing has to date largely been limited to high-end space applications. In this paper, the performances of existing available off-the-shelf pulse-tube cryocoolers are examined versus typical tactical infrared requirements. A comparison is made on efficiency, power density, reliability, and cost. An outlook is given on future developments that could bring the pulse-tube into the mainstream for tactical infrared applications.

Keywords: Pulse-tube, high frequency, cryocooler, IDCA

1. INTRODUCTION

A key point in cryocooler development over the last two decades has been reliability. For many applications, the life time and reliability of the cryocooler determine system life time and reliability. In tactical cryocooler applications, this has resulted in significant focus on various aspects related to wear and reliability, including fundamental tribological research into dry-bearing methods, both for rotating and linear motion, as well as new developments on contactless approaches for generating the pressure waves required for Stirling cooler operation. Around the turn of the century, the flexure-bearing compressor gained popularity in use in tactical split Stirling cryocoolers. Thales Cryogenics invested heavily in flexure-bearing, moving-magnet compressor technology [1], which has since resulted in the flexure-bearing Stirling cooler design of the LSF series, in which the compressor is no longer a factor of significance in limiting cryocooler (and system) reliability. The flexure-bearing revolution has all but eliminated compressor-related failure mechanisms from affecting cryocooler life time and reliability, leaving the cold head itself as a potential limiting factor in life time. Various cold head design concepts have been made that eliminate the dominant failure mechanisms, bringing cold head reliability to the same level as that of the flexure-bearing compressor. One such design is the pulse-tube cold finger, a design first proposed by Gifford and Longsworth in the 1960’s [11]. The elegance of the pulse-tube principle is that it completely eliminates the need for moving solid parts in the cold head, resulting in significant advantages in terms of life time and exported vibrations when compared to competing technologies. The specific version of pulse tube in use by Thales Cryogenics is the high-frequency - or Stirling-type - pulse-tube. In this specific cryocooler version, an approximately sine-shaped pressure wave is used to drive the cold head. The other variant of the pulse-tube that has seen widespread use is the low-frequency, or Gifford-McMahon type, pulse-tube. This design, manufactured by several companies makes use of a DC compressor to create a constant pressure difference, with a rotary valve alternating the pulse-tube inlet between high and low pressure at a significantly lower frequency.

In this paper, we will give a brief overview of this technology and the acceptance in the market to date, and will examine the current state of the industry. We will conclude with an outlook on potential future advances and uses of pulse-tube technology.

Figure 1: Pulse tube versus Stirling cold finger.

2. BACKGROUND – PULSE-TUBES AT THALES CRYOGENICS

The first large-scale commercial deployment of pulse-tube technology at Thales occurred in 2002, not long after the flexure-bearing revolution [2]. This pulse-tube design, the LPT9110, consisted of a flexure-bearing compressor together with a u-shaped pulse tube. At the time, reliability was only one of the key requirements that resulted in the selection of pulse-tube technology for the application; the other was induced vibration. As a pulse-tube cold finger does not contain any moving solid masses (such as the displacer of a Stirling cold finger), this technology is eminently suitable for vibration-sensitive applications. Electronic control methods can be used to reduce compressor vibrations, further reducing the amount of vibrational force applied to the cold tip – and therefore the cooled object. This technology was successfully series-produced for commercial (laboratory) customers as early as 2002 [2]. In Figure 2, an early example is shown of a pulse-tube cryocooler optimized for low vibration. In this configuration, the compressor and its heat sinks are mounted in suspensions, resulting in a high accelerometer feedback signal for electronic vibration suppression. Furthermore, the transfer line geometry is chosen such that minimal residual compressor vibrations are transferred to the cold tip.

Figure 2: LPT9110 cold head (left) and vibration-optimized cooler configuration (right).

Around the same time that the U-shaped LPT9110 cooler (Figure 2) was developed, Thales Cryogenics participated in ESA-funded development projects for pulse-tube cryocoolers, with Thales developing cryocooler compressors for space, suitable for use with coaxial pulse-tubes developed by CEA-SBT, and Air Liquide taking responsibility for the full cryocooler. (see, for example, [3]) To date, this collaboration has resulted in the MTPC and LPTC cryocoolers [10], of which the LPTC has since been selected for a variety of European missions. Apart from the LPTC- and MPTC cooler designs which resulted from ESA-funded development, the coaxial pulse-tube designs by CEA have been adapted and industrialized for commercial applications, resulting in the Thales LPT9310 and LPT9510 cryocoolers. These coolers could be regarded as the non-Space siblings of the LPTC and MPTC coolers (see [4] and [5]). In addition, Thales has developed the LPT9710 high-power cryocooler which will be discussed in more detail in Section 6. The LPT9510 and LPT9310 cryocooler models have since been produced in large series. The properties of a pulse-tube cooler, as compared to other cooler technologies, makes this technology eminently suitable for various applications, for example the cooling of Germanium detectors for gamma ray detection [6], where 24/7 operation is required with a strict limit on exported vibrations. In the following sections, we will compare the properties of pulse tube coolers to other cooler types in more detail.

3. RELIABILITY

As outlined in the opening paragraph of this paper, flexure bearing technology (as well as competing technologies such as gas bearings) have all but eliminated compressor bearing wear as a failure mechanism in cryocoolers. But how does the pulse-tube stack up to other competing technologies? When the current baseline MTTF figures were presented in 2012 [7], a baseline MTTF of 90000 hours for the LPT series of coolers was claimed, versus 45000 hours for the LSF series, which makes use of the same type of compressor. Based on the same type of analysis today, an MTTF of 119000 hours can be claimed. If the actual statistics of fielded units are taken into account, this MTTF figure can be assumed to be far higher still [6], with over 300 units in the field that have accumulated over 5 years of operation. By all indications, even the off-the-shelf build standard of the Thales LPT series is sufficient for life times exceeding 10 years of continuous operation, with an extremely low failure probability over that period. A competing technology that could potentially allow the same reliability characteristics to be reached with conventional Stirling technology, is applying a flexure bearing on the displacer ([8], [9]). This competing technology combines the advantages of a conventional Stirling cooler, most notably the higher efficiency, with the reliability of a near-contactless system. Results obtained on Thales LSF9330 life time test units – which have passed the 10 year mark – so far indicate that flexure bearings on the displacer can bring similar life times within reach of conventional Stirling technology.

Figure 3: Flexure-bearing displacer (left) / LSF9330 (center) / LPT9310 (right).

However, this does not negate the other advantages of pulse tubes. Apart from the lower manufacturing complexity of a pulse tube compared to a flexure-bearing cold finger, the exported vibrational force (or lack thereof) is a significant advantage. This will be detailed in the next section.

4. VIBRATIONS

The extremely low exported vibrations signature of a pulse tube cold head remains a unique selling point of this technology. The principal point remains the absence of a moving solid mass (displacer) to expand the working gas and generate the heat lift. This was already reported in some detail previously [5], a typical comparison is shown in Figure 4 for reference.

Figure 4: Comparison of LPT9510 and LSF9589 exported vibrations.

While the use of active vibration reduction is not unique to Pulse-tube coolers (see e.g. [15]), the use of this technology on a split cycle Stirling cooler places a number of additional constraints on integration:

1. In the case the compressor is used as an actuator for vibration cancellation, as in [15], a rigid connection between compressor and cold finger is needed – which means that off-axis compressor vibrations cannot be effectively decoupled from the application.

2. In the case a separate actuator is used, both the mechanical and the electronic design will increase in complexity (and cost).

This means that by carefully managing the mechanical and electrical design, an optimized pulse-tube configuration has a clear advantage over an optimized Stirling configuration in terms of exported vibrations.

5. CRYOGENIC PERFORMANCE

5.1 Heat lift

When compared to similar-sized Stirling coolers, the high frequency pulse tube is at a disadvantage when it comes to efficiency and power density. This difference is illustrated in Figure 5, where two Thales coolers of equal size and similar build standard are compared.

Figure 5: Comparison of LSF9340 (Stirling) and LPT9310 (Pulse-tube) typical heat lift vs AC input (left) and ambient temperature

(right).

From this figure it can be quite clearly seen that, for two cryocoolers of the same size and similar build standard, the Stirling alternative is almost twice as powerful as its pulse-tube counterpart. This means that in order to achieve the same level of heat lift, a pulse-tube cooler will require approximately twice as much input power as a Stirling cooler. It should be noted that both cooler models shown in Figure 5 are of an off-the-shelf build standard, designed to fit the cost requirements of industrial applications. The potential performance improvements that can be made to the LPT9310 pulse-tube without changing the base design, will be examined in section 6. 5.2 Cooldown time

When comparing cooldown performances between pulse-tube and Stirling, it is not enough to simply examine heat lift characteristics. The amount of mass that needs to be cooled down in a pulse tube (mass of the cold head itself) is much larger than in a Stirling cooler, which adversely affects cooldown times. In a typical tactical infrared application the total energy that needs to be removed from the infrared detector to cool down from room temperature to 80 K, is in the order of 300 J. By comparison, the energy that needs to be removed from a typical Pulse-tube in order to cool down the cold tip from room temperature to 80 K, will already exceed 1 kJ in most cases, excluding the object that needs to be cooled down. As cooldown time is a function of heat lift as well as the total amount of energy that needs to be removed (both the cold head itself and the cooled object), this means that a given pulse-tube with the same maximum heat lift as another given Stirling cooler, can be expected to take 4 times as long to cool down a 300 J infrared detector. However, this disadvantage will not be as pronounced in the case of a large-area, heavy detector – obviously, as the contribution of the detector thermal mass to the total thermal mass increases, the influence of the pulse-tube thermal mass will decrease. It can therefore be concluded that, in order to assess the suitability of a pulse-tube for a given infrared application, one will need to look at the thermal mass of the cooled object as well as the required cooldown time. In the case there is a strict cooldown time requirement and the use of a pulse-tube is considered, an optimization will have to be performed.

6. EFFICIENCY IMPROVEMENTS

As briefly mentioned in the preceding section, the LPT9310 is largely an optimized-for-cost design. The pulse-tube cold finger consists of stainless steel, with all outer parts either stainless steel or copper. This makes the parts for the pulse-tube relatively inexpensive compared to designed-for-space pulse-tubes, and greatly simplifies the production processes – all bonds can be made by standard welding and vacuum brazing processes. When material- and process cost and complexity become less of a driver for design, as is the case in space applications, a number of design options open up for consideration, see Table 1. Most of these design options can be found in designed-for-space cryocoolers (see [10]).

Table 1: Options for improving pulse-tube performances Design option Advantage Disadvantage Optimized regenerator designs [10]

Optimize pressure drop versus heat exchange

Materials cost and complexity

Low conductance material between hot and cold [10]

Reduce the parasitic losses Materials cost and complexity – standard welding and brazing cannot be used

High-conductance material on warm side [12]

Reduced temperature difference to working gas on warm side

Materials cost and manufacturing complexity – metal seal may be required

Optimized cold heat exchanger [12]

Reduced temperature difference to working gas on cold side

Parts machining cost & complexity

Optimized warm heat exchanger [12]

Reduced temperature difference to working gas on warm side

Parts machining cost & complexity

Higher bend radius for inertance

Reduced flow losses in inertance Manufacturing and integration complexity. Potential mass penalty.

Inertance flow geometry optimization

Optimization of phase shift Cost, complexity

Optimized phase shifter concept

Optimization of phase shift Criticality of manufacturing parameters, complexity and cost

The same argument can be made for cryocooler compressor design – by using higher-grade parts and processes, compressor efficiency can be improved. However, as this subject is outside of the scope of this paper we will not examine this in detail. By carefully selecting design choices from Table 1 and optimizing on the system-level, an optimum can be found for a high performance pulse-tube cooler suitable for cost-effective series production. An example where this kind of work was performed is the Thales LPT9710 15W pulse-tube cooler [13], in which the base pulse-tube design makes use of an aluminium alloy warm end. This cryocooler reaches a COP (Coefficient of Performance) that is 15% of the Carnot efficiency. Work is currently underway to build an upgraded version of the LPT9310 pulse-tube, to enable its use in applications where power and heat sink capacity are limited. This will be done using different options than was the case with the LPT9710 – as the input power and heat lift of the LPT9310 are lower than those of the LPT9710, more is to be gained by reducing parasitic losses as well as regenerator optimization. This work will be presented later in 2016 [14].

7. ROBUSTNESS AND ENVIRONMENTALS

Because of the moving displacer in a Stirling cooler (either split-cycle or driven-displacer), the cold finger tube is thin-walled and contains a moving part (the displacer) which has a tight fit with the cold finger tube. This means that a force applied to the cold tip will quickly lead to an overconstrained condition for the displacer, which will force contact between displacer and wall and will lead to friction (reduced performance in the case of a free-displacer cooler) as well as significantly accelerated wear. This is schematically sketched in Figure 6.

Figure 6: Bending force on Stirling cold finger

Because there is no moving part to worry about, a pulse-tube does not have this particular sensitivity. In fact, the coaxial pulse-tube design schematically shown in Figure 1 contains additional features that further improve robustness against mechanical loads:

- Instead of the single cold finger tube of a Stirling design, the coaxial pulse-tube contains two tubes inside each other, making the structure stiffer

- The regenerator matrix between the inner and outer tube adds further stiffness It should be noted that the mass of the pulse-tube cold tip itself is generally larger than that of a similar-sized Stirling cold finger, which can increase the effects of dynamic loading (random vibration, for example). In cases where severe mechanical loads are present (such as the launch of a spacecraft) it may in some cases still be necessary to add additional features to make the design robust – see, for instance, the snubber design incorporated in the LPTC cryocooler [10], also visible in Figure 9.

8. INTEGRATION ASPECTS

The split cycle linear Stirling cryocooler offers distinct advantages when it comes to integration, both in the area of camera-level integration and detector-level integration. We will examine the two separately. 8.1 Cooler-Detector integration

When examining the integrated detector-dewar-cooler assemby (or IDDCA), a significant fraction of the value is in the detector-dewar part. It has therefore become commonplace to build these detector-dewar assemblies in a way that enables replacement of a cryocooler without exposing the detector itself to atmosphere (breaking dewar vacuum) or risking the detector in any other way. The industry-standard way to handle this for Stirling coolers is to build the dewar assembly around a cold finger sleeve, which is designed to interface with the displacer. After dewar assembly, the dewar is integrated with the cryocooler, typically using a metal seal. This concept is shown schematically in Figure 7.

Figure 7: IDCA concept

As explained in section 3, the failure probability of a pulse-tube cold finger becomes vanishingly small. For an IDCA-type application, the necessity to allow for replacement of the entire cryocooler is therefore eliminated. Instead, what can be proposed is a welded design of the dewar around the pulse-tube itself. This integration method is currently performed by some Thales customers on the LPT9510 cooler (see Figure 8).

Figure 8: LPT9510 cold head with weld flange

For convenience, it is practical to disconnect the pulse-tube from the compressor during dewar manufacturing. In a typical Thales design, the transfer line between compressor and pulse-tube is brazed, which means the customer would have to perform a brazing themselves after dewar assembly. However, other possibilities have been implemented. A design with a metal seal is an obvious alternative – used, for example, in the Thales LPT9710. For some customers, even off-the-shelf helium-tight couplings, such as those produced by Swagelok, can be considered. One important difference between an IDCA-type Stirling cold finger and a typical pulse-tube, is the cold plate. As thermal contact between working gas and tip is critical for pulse-tube performance, many designs make use of a copper cap as a cold interface.

This is not always the correct choice for an IDCA-type dewar for an infrared application. For these types of dewars, a cold plate material is used that has a coefficient of thermal expansion (CTE) close to that of the cooled object. Invar and Kovar cold plates are frequently used for this reason. These materials can be considered for a pulse-tube cold tip as well in the case direct mounting of an infrared FPA is required. 8.2 Cooler-Housing integration

When examining Figure 1, one difference between a Stirling cold finger and a typical high-frequency pulse-tube design becomes apparent: the presence of a buffer volume on the warm side of the cold finger. For many industrial customers, the presence of this buffer does not pose any problems, as the system (housing) is simply designed around the presence of this buffer. However, for some applications this buffer may be placed at an inconvenient location. In a thermal imaging application, for example, there are many other integration constraints in the vicinity of the infrared detector – optics and proximity electronics all need to be placed close to the detector, and therefore to the cold finger itself. One important advantage of the split cycle linear cooler is that the compressor can be placed at a distance from the cold finger, avoiding interferences with other parts. For the pulse-tube buffer, the same can be done. This was for example done in the LPT9110 configuration shown in Figure 2, where the buffer was placed below the compressor (not visible in the photograph). However, this introduces an additional mechanical interface for the customer to deal with. One alternative that has been proposed to eliminate this interface while still maintaining the flexibility of removing the buffer from the direct vicinity of the cold finger, is integrating the buffer onto the compressor structure. This is regularly done in designed-for-space cryocoolers, such as the current build standard of the LPTC cooler.

Figure 9: LPTC Cooler with buffer on compressor and launch support tube (image: Air Liquide).

In the case space around the cryocooler for integration is not critical, the building blocks of a split cycle pulse tube can be combined to build an integral-concept as well. One such configuration is the designed-for-space LPT6510 cryocooler, currently under development in cooperation with Absolut System SAS [16]. The LPT6510 is based on the Thales MPTC compressor, developed with ESA funding, and the Absolut System SSC80 pulse-tube, originally developed for a terrestrial astronomy application. This cryocooler only has a single warm side interface for mounting and heat sinking, resulting in a compact and efficient solution.

Figure 10: LPT6510 Artist Impression (courtesy of Absolut System SAS)

9. OUTLOOK – DEVELOPMENTS AT THALES CRYOGENICS

A number of ongoing developments at Thales have been briefly mentioned in the preceding paragraphs, such as the designed-for-space LPT6510 integral cooler, as well as a high-performance variant of the LPT9310. However, one development that has not yet been mentioned are plans to build a smaller-scale pulse-tube. Cryocooler requirements in general and cooled infrared applications in particular are moving in the direction of small-size, high operating temperature systems. While pulse-tube technology is at a disadvantage when it comes to compactness, its advantages still indicate there will be a niche for smaller pulse-tube coolers. Preliminary work is currently being performed on defining a smaller-scale pulse-tube cooler for high operating temperatures (HOT). Starting point for development will be the Thales LSF9997 compressor [17]. Thales currently invites feedback, comments and requirements from customers to further define the operating requirements for such a product.

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