TRATAMIENTO ISOLOMA-CENTRO

As mentioned in Section 1.4.3.2 above, the two most important practical mechanical cooler technologies for use in space are the Stirling cycle refrigerator and the pulse tube refrigerator.

Suitable Stirling cycle systems have been developed in the UK by Matra-Marconi Space (MMS) with Rutherford Appleton Laboratories and in the United States by Ball Aerospace"^^. To reach 4 K, these systems typically employ a Stirling cycle refrigerator (SR) that cools to 20 K, and a Joule-Thomson (JT) cycle cooler using helium-4 as the working fluid, to cool from 20 K to 4 K. A brief description o f Stirling cycle and Joule-Thomson cooling is presented below.

1.4.3.2.2.1 Stirling cycle cooler

Stirling cycle coolers have been flown on several successful missions, having been first employed upon an orbital satellite by the IS AMS instrument in 1991, when the 80 K cooler operated successfully for 642 days in orbit.

The Stirling cycle is a practical refrigeration cycle that is theoretically capable o f achieving the Carnot efficiency. Heat taken from the fluid during constant volume cooling is stored in a regenerator o f large heat capacity and reversibly returned to the fluid during constant volume heating later in the cycle. The net heat flow out o f the system is therefore simply the heat rejected to the heat sink during the isothermal compression (1-2 below). The perfect Stirling cycle’s COP is therefore equal to the Carnot cycle COP.

The SR requires two reciprocating parts, usually a piston and a displacer. The piston compresses and expands the working gas (usually helium) before the displacer moves the gas through the regenerator from the warm compressor to the cold part o f the refrigerator where it is allowed to expand. The heat transferred from the gas to the regenerator in moving from the compressor to the displacer is ideally recovered by

Chapter 1 Introduction to Space Cryogenics 71

the gas on its return journey. Figure 1-26 is a photo o f the space-qualified Ball Aerospace two-stage Stirling cycle cooler, which can cool to 30 K.

■

Figure 1-26: Ball Aerospace Stirling cycle cooler^^

Figure 1-27 shows a schematic o f a Stirling cooling cycle. The ideal Stirling refrigeration cycle is as follows:

1-2 Isothermal compression

Heat generated during compression is rejected to a heat sink

2-3 Constant volume cooling

Hot working fluid passes through the cold regenerator matrix giving up heat Ç23 at constant volume

3-4 Isothermal expansion

Chapter 1 Introduction to Space Cryogenics 72

4-1 Constant volume heating

Cold fluid is passed back through the hot regenerator matrix absorbing heat Q4] at constant volume, completing the cycle. For an ideal

regenerator with large heat capacity, Q23 ^ ^4 1.

Regenerator

Matrix Compressor

Figure 1-27: Schematic o f Stirling refrigeration cvcle

In the first stage (1-2) o f the cycle, the entropy is reduced whilst maintaining the temperature at a constant level. In the cooling stage (2-3), heat is lost from the working fluid as the temperature is reduced, so the entropy falls further. The pressure and volume o f the working fluid can be represented graphically in a 'p-v plot’, which has pressure on the y-axis and volume on the x-axis (Figure 1-28 below). This illustrates the constant volume processes (2-3) and (4-1). A temperature-entropy (T-S)

diagram (Figure 1-29) is also shown for comparison with the Carnot cycle (Figure 1-23).

Chapter 1 Introduction to Space Cryogenics 73

P

Q\2

V

Figure 1-28: p-v diagram for Stirling refrigeration cycle

T

Th

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Unfortunately, although the SR can theoretically achieve the Carnot cycle COP, the Stirling cycle, like other regenerative cycles, is inefficient below around 20 K This is primarily due to losses associated with the regenerator. To operate efficiently, the regenerator material needs a high volumetric heat capacity compared to the helium gas, so that it can absorb a lot o f heat without its temperature rising significantly. As the temperature decreases, however, the heat capacity o f the helium rises rapidly whilst that o f most metals (fi-om which the regenerator matrix is made) falls according to the Debye law (C ocT^). The regenerator therefore becomes inefficient at low temperatures. In practice, temperatures below 15 K cannot be achieved by SRs alone. SRs are typically used to cool from 300 K to 20 K. For this temperature change, a good SR operates at around 2.3% o f the Carnot efficiency"^"*.

In order to provide cooling at temperatures below 20 K, a non-regenerative cycle is required, such as that employed by the JT refrigerator.

1.4.3.2.2.2 Joule-Thomson cooler

In the Joule-Thomson (JT) cooler, a gas is expanded adiabatically through an orifice with the enthalpy, H, o f the gas being conserved in moving from one side o f the orifice to the other. For an ideal gas, the enthalpy is a function o f temperature only, so the temperature does not change on passing through the restriction. For real gases, however, 77 is a function o f both temperature and pressure:

We can plot curves o f constant enthalpy on a graph o f temperature against pressure, such as Figure 1-30 below. The slope o f the curves o f constant enthalpy, (^2T/4?)//, is called the Joule-Thomson coefficient, //.

Chapter 1 Introduction to Space Cryogenics 75

Inversion

T

Curve

P

Figure 1-30: Locus o f points for which {âTlà)\n= 0

The significance o f this parameter is that, for // < 0, temperature will decrease following an infinitesimal pressure drop across the orifice whereas, for // > 0, the temperature will increase. The locus o f points for which // = 0 is called the inversion curve and is shown in Figure 1-30 as a dotted line. Given the Joule-Thomson coefficient for the operating point o f a JT refrigerator, we can calculate the temperature drop achievable across the orifice. As noted in Section 1.4.3.2 above, the JT cooler developed by Matra-Marconi and RAL cools fi*om the 20 K provided by the Stirling coolers to 4 K with helium-4 as the working fluid. The cooling power o f the Stirling/JT cooler"^ at 4 K is 10 mW. The COP o f the SR/JT system cooling from 300 K to 4 K is around 0.6% o f the Cam ot cycle COP. If temperatures lower than 4 K are required, for pre-cooling a helium-3 adsorption refrigerator, for example, helium-3 can be used instead o f helium-4 in the JT stage. In this case, 5 mW o f cooling power is available"^"* at 2.5 K

The Joule-Thomson cooler is a proven technology for space and has been baselined for ESA’s major PLANCK mission, due for launch in 2007.

Chapter 1 Introduction to Space Cryogenics 76

An alternative technology to the SR with the Joule-Thomson valve is the pulse tube refrigerator. Although historically less efficient than SRs, recent developments have made them serious contenders for future space missions.

1.4.3.2.2.3 Pulse tube refrigerators

The orifice pulse tube refrigerator^^ (OPTR), which is the only type o f pulse tube refrigerator that achieves cryogenic temperatures, operates on a similar cycle to the Stirling cycle. However, the phase difference between pressure and mass flow is provided by a passive orifice instead o f a moving displacer”^^. Since the pulse tube refrigerator has no moving parts, it has several advantages over the Stirling cycle. These include greater reliability, lower electromagnetic interference (EMI) and lower vibration. Some pulse tube cold heads are shown in Figure 1-31 below"^^.

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Transmission o f vibrations from mechanical coolers has been a constant concern for the designers o f space refrigeration systems, especially with sensitive detectors on the cold finger. With their low vibrations, OPTRs would therefore seem to be an extremely attractive technology. Recent research has shown, however, that even when vibrations or EMI may impair detector operation"^^’"^*, steps can be taken to minimize the transmission o f vibrations through sophisticated control systems"^^ and by distancing the compressors from the detectors^®. I f vibrations can be controlled in this manner, then OPTRs must offer similar cooling performance to Stirling cycle coolers to remain a competitive technology.

In the past, only very large OPTRs were able to achieve reasonable COPs. The first two-stage OPTR developed by the University o f Giessen^’ in Germany, for example, provided 0.42 W o f cooling power at 4.2 K for an input power o f 6.3 kW. This equates to a COP o f around 0.4% o f the Camot value. Furthermore, OPTRs are orientation dependent, with 65 K (1/* stage) cooling power falling by over 50% and 4.2 K (2"^ stage) cooling power falling by around 40% with a 45° rotation from the vertical^^.

Although pulse tube cryocoolers have only recently become commercially available, however, they have improved so much since the early designs that their COPs are now comparable to those o f good Stirling cycle/JT coolers. The pulse tube cooler at MSSL, for example, operates at 0.54% o f Cam ot efficiency at 4 K - providing 0.37 W o f cooling power with an input power o f 5 kW. Furthermore, even though pulse tube refrigerators do not have the proven track record o f Stirling cycle coolers in space, they have been developed for use on the Atmospheric Infrared Sounder (AIRS) instmment^^ by N A SA ’s Jet Propulsion Laboratory (JPL). Part o f N A SA ’s Earth Observing System (EOS) aqua platform, the 55 K AIRS instmment is scheduled for launch around December 2000. The AIRS pulse tube cooler with electronics is shown in Figure 1-32 below.

If detectors become more sensitive to vibrations, and the low-vibration pulse tube technology continues to mature, OPTRs are likely to play a leading role in future o f space cryogenics.

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Figure 1-32: AIRS pulse tube refrigerator for space

Regardless o f future progress, it is clear that long-lifetime closed-cycle coolers exist with the capability o f providing a 4 K cold bath from which further cooling down to

10 mK can commence.

The only cooling technologies that currently promise to be able to reach 10 mK in space are adiabatic demagnetization and nuclear demagnetization. These are discussed below in Sections 1.4.3.3 and 1.4.3.4 respectively.

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