- Email: [email protected]
Space infrared telescope facility mission and cryogenic design
Space infrared telescope facility mission and cryogenic design* P.V. Mason, D. Petrac, S.W. Petrick and D.M. Strayer NASA Jet Propulsion Laboratory, Pasadena, California, USA The Space Infrared Telescope Facility (SIRTF) is the last of the Great Observatory missions. It is presently scheduled for launch in 2001. The mission will study the infrared spectrum from 2 to 1200 #m with three imaging and spectral instruments. The observatory will have a 5 year lifetime and will be placed in a 100 000 km earth orbit. The cryogenic system is based on a 4000 I superfluid helium cryostat. The mission and the cryogenic system are described, and the cryogenic technology issues are discussed.
Keywords: infrared astronomy; space cryogenics; superfluid helium
The infrared sky was first explored from ground based, aircraft-based and balloon-based telescopes. All suffered from an inability to achieve performance limited only by sky background at wavelengths longer than 10/zm because (1) the atmosphere is largely opaque and radiates strongly in the infrared, (2) the detectors cannot achieve adequate signal-to-noise ratio due to thermally generated noise, and (3) the telescope optics and baffle cannot be cooled sufficiently to eliminate black-body radiation. The Infrared Astronomical Satellite was the first mission to perform IR astronomy in space. It was primarily a survey mission, with a limited time allotted to study of specific objects. It performed an all-sky survey in four bands from 8 - 1 2 0 / z m . This survey is serving the same function as the well-known Palomar Sky Survey visible spectrum all-sky survey performed with a 48in Schmidt telescope; it serves as a map for high resolution, high sensitivity missions. The IR Space Observatory (ISO) will provide imaging and spectral resolution capabilities, and will perform high-resolution studies of specific objects. Its role in IR astronomy corresponds to that of medium-size visiblelight telescopes. SIRTF will correspond to the large visible-light telescopes such as the Palomar 200in, and the Hubble Space Telescope. While it will have considerably less angular resolution because of its 1 m primary mirror, its sensitivity and spectral resolution will put it at the forefront of the art. It will have a sensitivity at least 10 000 times that of large ground-based near-IR telescopes. SIRTF will cover the wavelengths from 2 - 1 2 0 0 / z m with its various instruments. Its angular resolution will be better than 0.5 arcsec at the shortest wavelengths and will decrease in proportion to wavelength. The pointing " P a p e r p r e s e n t e d at t h e 1991 S p a c e C r y o g e n i c s W o r k s h o p , 1 8 - 2 0 J u n e 1 9 9 1 , Cleveland, OH, USA
and control systems will be compatible with this resolution. The SIRTF observatory is intended to last a minimum of 5 years and to achieve an 85 % viewing efficiency. A change from a low earth orbit to a 100 000 km orbit will enable both of these objectives to be met. Because of the lower earth and sky background radiation, parasitic and aperture heat loads will be much smaller. And since there will be no requirement for manoeuvres to avoid looking at the earth, the viewing efficiency will increase from about 40% to about 85%. The overall requirements imposed on SIRTF are given in Table 1. The instrument complement is given in Table 2. The Infrared Array Camera (IRAC) is an imaging instrument covering the wavelengths from 2 . 5 - 2 7 ~m. The Multiband IR Photometer for SIRTF (MIPS) is also an imaging instrument. It operates in two modes; one diffraction limited and the other with a wide field of view. It includes a long wavelength bolometer which operates at 0.1 K. An adiabatic demagnetization refrigerator in this instrument provides this temperature. The IR spectro-
Table 1
Basic requirements on the SIRTF system
Mirror diameter Wavelength coverage Background limited performance Diffraction limited wavelength Angular resolution Field of view Sensitivity 10 #m 60 #m Number of detectors Spectral resolution, X/AX Lifetime
> 9 0 cm 1.8 1200 #m 1.8 2 0 0 # m 3 #m 0.9 arcsec at 3/~m 7 arcmin 6 #Janskys 1O 0 / J a n s k y s > 200 0 0 0 > 2000 > 5 years
O01 ] - 2 2 7 5 / 9 2 / 0 2 0 1 0 7 - O 4 ;,~ q 9 9 2 B u t t e r w o r t h - H e i n e m a n n ktd
Cryogenics 1992 Vol 32, No 2
107
Space infrared telescope facility mission: P.V. Mason et al. Table 2
SlRTF instruments
Name
Wavelength ( / ~ m )
Infrared Array Camera
1.8 - 27
Multiband IR Photometer
1.8-5.3 5.3-27
InSb 2 5 6 x 256 array Si:As IBCa 128 x 128 array
Temperature (K)
Resolution
10-12 5
Angular (arcsec) Narrow Wide 0.3 × 0 . 3 1.2 × 1.2 0 . 6 3 x 0 . 6 4 2.4 x 2.4
30 - 1200 30-55 5 0 - 120 120-200 200-500 5 0 0 - 1200
Infrared Spectrometer
Detectors
Angular (arcsec) Ge:Be 16 × 32 array Ge:Ba 32 x 32 array Ge:Ga stressed 2 x 8 Ge bolometer 2 × 2 array Ge bolometer 1 ea.
2.5 2.0 1.52 O. 1 O. 1
47 4.7 187 90 240
2.5 - 2 0 0 2.5-4.0 4-28 28-50 50-120 120 - 2 0 0
InSb 256 × 2 5 6 array Si:As IBC a 128 x 128 array Ge:Be 4 × 32 array Ge:Ga 4 x 32 array Ge:Ga, stressed 2 × 16 array
Spectral resolution Low X/~X = 100 High X/z~X 2000
8 5 5 1.9 1.4
=
alBC = impurity band conductor
meter (IRS) has two ranges; a low spectral resolution position with a resolution of 1:100, and a high resolution range of 1: 2000. The National Research Council has recently completed a review and recommendations for astronomy and astrophysics for the next decade t. This report (referred to as the Bahcall report after the chairman of the committee which wrote it) places the SIRTF first on its list of large missions. SIRTF will be launched on a Titan4/Centaur launch vehicle. Programme start is planned for October 1994, with launch in 2001.
Description of the cryogenic system
DEWAR SUPPORT SYSTEM CRYOGEN
TANK
Description A sketch of the SIRTF telescope is shown in Figure 1, and a cross-section in Figure 2. As in IRAS, the helium is in the superfluid state. It is contained in an inner toroidal tank containing 40001 of superfluid helium. The inner tank is surrounded by three shields cooled by the gaseous helium boiled off from the inner tank. The superfluid helium is retained in the inner tank by a porous plug.
108
Cryogenics 1992 Vol 32, No 2
~
i
~
z
FINE GUIDANCE T~\ SENSOR
BAFFLES PRIMARY MIRROR
INSTRUMENTS TERTIARY MIRROR
Background Three space missions using superfluid helium as a cryogen have flown to date; the Infrared Astronomical Satellite in 1983, the Superfluid Helium Experiment and the Shuttle IR Telescope on Spacelab 2 in 1985, and the Cosmic Background Explorer in 1989. At least four more are planned in the next decade; Lambda Point Experiment on Shuttle in 1992, the European Space Agency Infrared Space Observatory in 1993, the Superfluid Helium On-Orbit Transfer experiment in 1993 and the Space Infrared Telescope Facility in 2001. As currently planned, all used or will use the basic tensioned-strap support system first developed at Beech Aircraft in the 1970s.
APERTURE ADE
SECONDARY MIRROR
Figure 1
Sketch of the SIRTF telescope
For purposes of ground test, it is planned to provide LN2 cooling to the outer vapour-cooled shield. This will maintain it at 80 K, about the temperature it will have on orbit. Thus the inner VCSs and the instrument will be at flight temperature. This is essential for test of the instruments because many of the detectors cannot operate at temperatures more than a degree above these values. The requirements on SIRTF are compared with those of IRAS in Table 3. From a cryogenic point of view, the major differences are the much longer lifetime and the lower instrument temperature. The 5 year lifetime requirement is met by minimizing instrument and parasitic heat loads. The outer shell will operate at 115 K or lower, as compared to 200 K, and the aperture heat load will be only about 1.9 mW. A summary of the thermal performance of the baseline design is given in Figure 3. It will be noted that about half of the load is instrument and fine guidance sensor heat loads, while half arises from parasitic leaks. The predicted helium mass flow is 2.91 mg s -1, corre-
Space infrared telescope facility mission: P.V. Mason et al.
Six straps
I
MIRROR
/
/
/
/ \
\
) DRAIN
~RD RING
IIRTN
~UPPORT
Figure 2
Cross-section of the telescope
Plumbing. (6Kg. 1%)
Launch (27Kg. 5%) Instruments (14gKg, 27%)
MI (46Kg,
\
Figure 4
Aperture (16Kg, 3%) Mirro (45Kg,
;S
g, 20%)
Figure 3
Cryogenic budget of SIRTF on orbit
Table 3 A comparison of SIRTF and IRAS requirements and design features
Orbit Bath temperature I n s t r u m e n t temperature
Arrays Bolometer Design lifetime Outer shell
SIRTF
IRAS
100 0 0 0 km 1.25 K
900 km
1.4 K O. 1 K
2.4 K None
Dewar support strap configuration
\
1.8 K
5 yr
1 yr
Ground test
115 K LN 2 cooled OVCS
200 K Uncooled
I n s t r u m e n t thermal
Copper strap
AI mounting ring
interface Aperture cover
Passive
Supercritical helium
sponding to a heat load of 62.2 mW. The lifetime is 5.68 years, based on a 95 % fill at final top-off and a 2 day ground hold. The details of the thermal design and analysis will be described in a paper at the 1991 SPIE Optical Applied Science and Engineering Conference 2. The inner tank is supported by 12 glass fibre/resin straps; six each at top and bottom of the tank (see Figure 4). These are spread in a vee configuration to maximize stiffness while minimizing the area-to-thickness ratio and hence the heat leak. In the baseline design, they are 1 m long x 51 mm wide by 1.19 mm thick and have a cross-sectional area of 121 mm 2. The fundamental axial natural frequency is 15.1 Hz and the lateral is 18.5 Hz, both meeting the launch vehicle requirements. Strap limit loading is 2575 kg, yielding a fatigue life of 30 000 cycles, well above the required 6800 cycles. The ejectable aperture cover has three shields which are passively cooled by radiation from the interior of the main cryogen tank. A decision is to be made concerning the use of thermal links between the vapour-cooled shields in the cover and those in the main cryogen system as in ISO 3. A 1 m fiat mirror will be mounted on the aperture cover to aid in determining the alignment of the instruments while the telescope is cold. It will be cooled below 5 K by a helium flush line during certain phases of instrument test on the ground. A unique feature of the design is the use of vapourcooled shields in tension. This allows the VCS thickness to be limited only by the thermal requirements, and
Cryogenics 1992 Vol 32, No 2
109
Space infrared telescope facility mission: RV. Mason et al. hence their weight is minimized. Their current total mass is 88 kg, v e r s u s 236 kg for the original design with the shields in compression.
Technology development From a cryogenic point of view there are three major technology drivers; the low temperature of the bath, the required long life, and the need for the lightest possible mass. Certain IR sensor arrays in the IRS will require a temperature no higher than 1.4K. The cryostat designers have elected to operate the bath at 1.25 K and to allow a temperature rise of no more than 0.15 K from the bath to the array. Both of these require careful design, backed by a development programme to ensure that they can be met. The SIRTF bath temperature of 1.25 K is well below that of any missions flown to date. As is well known, the fountain pressure is given by Ap = pSAT and hence approaches zero with the entropy as the temperature drops. At 1.25 K it is only about 12 Pa mK -1. While the vapour pressure is also low, about 115 Pa, requiring only a 10 mK drop across the porous plug, there is concern that small accelerations may cause breakthrough. In early mission operations there will be about 2 m of liquid helium in the tank, and at a density of 0.143 g cm-3, an acceleration of 4 mg will be enough to overcome the fountain pressure generated by 1 mK of temperature difference. A careful theoretical and experimental investigation of the fountain effect at these pressures and temperatures will be conducted. The effect of pore sizes, pore geometry, pore length and temperature will be examined to allow selection of the optimum parameters. The SIRTF cryogenic system must provide cooling for milliwatt heat loads for detectors operating at 1.4 K, and for other detectors operating at temperatures near 2 K with heat loads of 1 5 - 2 5 mW. With the main bath temperature operating near 1.25 K, only a 0.15 K temperature rise is allowed between the bath and the colder sensors. JPL has therefore planned two cooling links to the bath: a low temperature, low heat load link; and a higher heat load link to cool the other sensors. Our search of the literature indicates that wide ranges of thermal conductivity and interface thermal resistance are reported at the temperatures for SIRTF operation. We shall perform measurements of temperature drops across joints and links that can be assembled relatively easily, and which will provide the required small AT. The candidate links are copper straps, with the copper consisting of high residual resistance ratio (RRR) material. Besides data for the optimally annealed
110
Cryogenics 1992 Vol 32, No 2
copper, we shall also measure the effects of strains and vibrations of large amplitude on the measured conductivity. We have also considered using superfluid links from the bath to the sensors. While such links are calculated to have satisfactory performance with even modest diameters, their implementation poses problems that JPL is reluctant to address if other satisfactory thermal links are available. While soft metals like lead and indium provide good interface thermal conductance in bolted metal-metal joints at higher temperatures, operating the joints so far below the superconducting transition temperatures of these joint fillers raises doubts concerning their effectiveness at these lower temperatures. We intend to characterize the temperature drops across aluminiumcopper and copper-copper joints at the temperatures relevant to SIRTF and at appropriate heat flow levels. Measurements will be made for joints at various pressures, with a range of finishes on the metals, and with and without soft-metal gap fillers such as indium and gold. The current leads for the magnet of the adiabatic demagnetization refrigerator add substantially to the heat leak. Even with Nb3Sn superconducting leads, the wire heat load from the IVCS to the bath is 3 to 4 mW. Their thermal conductance could be considerably reduced if high temperature superconductors were used between vapour-cooled shields and from the inner vapour-cooled shield to the instrument package. The best high temperature superconductors are ceramics and are quite brittle. Addition of silver improves the mechanical properties, but increases the thermal conductivity. Trade-off studies will be done to determine the benefit. If they are substantial, a development programme to solve the mechanical problems may be undertaken. Aluminium-lithium alloys offer the possibility of reducing tank weight by about 10%. If weight becomes a critical problem, an investigation of AI - L i alloys will be undertaken.
Acknowledgement The work described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
References 1 The Decade of Discovery in Astronomy and Astrophysics (Ed. Bahcall, J.) National Academy Press, Washington, DC, USA (1991) 2 Bhandari, P. and Petriek, W. Proc 1991 SP1E Optical Applied Science and Engineering Conference, to be published 3 Seldel, A. and Passvogel, Th. The ISO cryostat; its qualification status. Presented at CEC/ICMC, June 1991. To be published in Advances in Cryogenic Engineering (1992)
Keywords: infrared astronomy; space cryogenics; superfluid helium
The infrared sky was first explored from ground based, aircraft-based and balloon-based telescopes. All suffered from an inability to achieve performance limited only by sky background at wavelengths longer than 10/zm because (1) the atmosphere is largely opaque and radiates strongly in the infrared, (2) the detectors cannot achieve adequate signal-to-noise ratio due to thermally generated noise, and (3) the telescope optics and baffle cannot be cooled sufficiently to eliminate black-body radiation. The Infrared Astronomical Satellite was the first mission to perform IR astronomy in space. It was primarily a survey mission, with a limited time allotted to study of specific objects. It performed an all-sky survey in four bands from 8 - 1 2 0 / z m . This survey is serving the same function as the well-known Palomar Sky Survey visible spectrum all-sky survey performed with a 48in Schmidt telescope; it serves as a map for high resolution, high sensitivity missions. The IR Space Observatory (ISO) will provide imaging and spectral resolution capabilities, and will perform high-resolution studies of specific objects. Its role in IR astronomy corresponds to that of medium-size visiblelight telescopes. SIRTF will correspond to the large visible-light telescopes such as the Palomar 200in, and the Hubble Space Telescope. While it will have considerably less angular resolution because of its 1 m primary mirror, its sensitivity and spectral resolution will put it at the forefront of the art. It will have a sensitivity at least 10 000 times that of large ground-based near-IR telescopes. SIRTF will cover the wavelengths from 2 - 1 2 0 0 / z m with its various instruments. Its angular resolution will be better than 0.5 arcsec at the shortest wavelengths and will decrease in proportion to wavelength. The pointing " P a p e r p r e s e n t e d at t h e 1991 S p a c e C r y o g e n i c s W o r k s h o p , 1 8 - 2 0 J u n e 1 9 9 1 , Cleveland, OH, USA
and control systems will be compatible with this resolution. The SIRTF observatory is intended to last a minimum of 5 years and to achieve an 85 % viewing efficiency. A change from a low earth orbit to a 100 000 km orbit will enable both of these objectives to be met. Because of the lower earth and sky background radiation, parasitic and aperture heat loads will be much smaller. And since there will be no requirement for manoeuvres to avoid looking at the earth, the viewing efficiency will increase from about 40% to about 85%. The overall requirements imposed on SIRTF are given in Table 1. The instrument complement is given in Table 2. The Infrared Array Camera (IRAC) is an imaging instrument covering the wavelengths from 2 . 5 - 2 7 ~m. The Multiband IR Photometer for SIRTF (MIPS) is also an imaging instrument. It operates in two modes; one diffraction limited and the other with a wide field of view. It includes a long wavelength bolometer which operates at 0.1 K. An adiabatic demagnetization refrigerator in this instrument provides this temperature. The IR spectro-
Table 1
Basic requirements on the SIRTF system
Mirror diameter Wavelength coverage Background limited performance Diffraction limited wavelength Angular resolution Field of view Sensitivity 10 #m 60 #m Number of detectors Spectral resolution, X/AX Lifetime
> 9 0 cm 1.8 1200 #m 1.8 2 0 0 # m 3 #m 0.9 arcsec at 3/~m 7 arcmin 6 #Janskys 1O 0 / J a n s k y s > 200 0 0 0 > 2000 > 5 years
O01 ] - 2 2 7 5 / 9 2 / 0 2 0 1 0 7 - O 4 ;,~ q 9 9 2 B u t t e r w o r t h - H e i n e m a n n ktd
Cryogenics 1992 Vol 32, No 2
107
Space infrared telescope facility mission: P.V. Mason et al. Table 2
SlRTF instruments
Name
Wavelength ( / ~ m )
Infrared Array Camera
1.8 - 27
Multiband IR Photometer
1.8-5.3 5.3-27
InSb 2 5 6 x 256 array Si:As IBCa 128 x 128 array
Temperature (K)
Resolution
10-12 5
Angular (arcsec) Narrow Wide 0.3 × 0 . 3 1.2 × 1.2 0 . 6 3 x 0 . 6 4 2.4 x 2.4
30 - 1200 30-55 5 0 - 120 120-200 200-500 5 0 0 - 1200
Infrared Spectrometer
Detectors
Angular (arcsec) Ge:Be 16 × 32 array Ge:Ba 32 x 32 array Ge:Ga stressed 2 x 8 Ge bolometer 2 × 2 array Ge bolometer 1 ea.
2.5 2.0 1.52 O. 1 O. 1
47 4.7 187 90 240
2.5 - 2 0 0 2.5-4.0 4-28 28-50 50-120 120 - 2 0 0
InSb 256 × 2 5 6 array Si:As IBC a 128 x 128 array Ge:Be 4 × 32 array Ge:Ga 4 x 32 array Ge:Ga, stressed 2 × 16 array
Spectral resolution Low X/~X = 100 High X/z~X 2000
8 5 5 1.9 1.4
=
alBC = impurity band conductor
meter (IRS) has two ranges; a low spectral resolution position with a resolution of 1:100, and a high resolution range of 1: 2000. The National Research Council has recently completed a review and recommendations for astronomy and astrophysics for the next decade t. This report (referred to as the Bahcall report after the chairman of the committee which wrote it) places the SIRTF first on its list of large missions. SIRTF will be launched on a Titan4/Centaur launch vehicle. Programme start is planned for October 1994, with launch in 2001.
Description of the cryogenic system
DEWAR SUPPORT SYSTEM CRYOGEN
TANK
Description A sketch of the SIRTF telescope is shown in Figure 1, and a cross-section in Figure 2. As in IRAS, the helium is in the superfluid state. It is contained in an inner toroidal tank containing 40001 of superfluid helium. The inner tank is surrounded by three shields cooled by the gaseous helium boiled off from the inner tank. The superfluid helium is retained in the inner tank by a porous plug.
108
Cryogenics 1992 Vol 32, No 2
~
i
~
z
FINE GUIDANCE T~\ SENSOR
BAFFLES PRIMARY MIRROR
INSTRUMENTS TERTIARY MIRROR
Background Three space missions using superfluid helium as a cryogen have flown to date; the Infrared Astronomical Satellite in 1983, the Superfluid Helium Experiment and the Shuttle IR Telescope on Spacelab 2 in 1985, and the Cosmic Background Explorer in 1989. At least four more are planned in the next decade; Lambda Point Experiment on Shuttle in 1992, the European Space Agency Infrared Space Observatory in 1993, the Superfluid Helium On-Orbit Transfer experiment in 1993 and the Space Infrared Telescope Facility in 2001. As currently planned, all used or will use the basic tensioned-strap support system first developed at Beech Aircraft in the 1970s.
APERTURE ADE
SECONDARY MIRROR
Figure 1
Sketch of the SIRTF telescope
For purposes of ground test, it is planned to provide LN2 cooling to the outer vapour-cooled shield. This will maintain it at 80 K, about the temperature it will have on orbit. Thus the inner VCSs and the instrument will be at flight temperature. This is essential for test of the instruments because many of the detectors cannot operate at temperatures more than a degree above these values. The requirements on SIRTF are compared with those of IRAS in Table 3. From a cryogenic point of view, the major differences are the much longer lifetime and the lower instrument temperature. The 5 year lifetime requirement is met by minimizing instrument and parasitic heat loads. The outer shell will operate at 115 K or lower, as compared to 200 K, and the aperture heat load will be only about 1.9 mW. A summary of the thermal performance of the baseline design is given in Figure 3. It will be noted that about half of the load is instrument and fine guidance sensor heat loads, while half arises from parasitic leaks. The predicted helium mass flow is 2.91 mg s -1, corre-
Space infrared telescope facility mission: P.V. Mason et al.
Six straps
I
MIRROR
/
/
/
/ \
\
) DRAIN
~RD RING
IIRTN
~UPPORT
Figure 2
Cross-section of the telescope
Plumbing. (6Kg. 1%)
Launch (27Kg. 5%) Instruments (14gKg, 27%)
MI (46Kg,
\
Figure 4
Aperture (16Kg, 3%) Mirro (45Kg,
;S
g, 20%)
Figure 3
Cryogenic budget of SIRTF on orbit
Table 3 A comparison of SIRTF and IRAS requirements and design features
Orbit Bath temperature I n s t r u m e n t temperature
Arrays Bolometer Design lifetime Outer shell
SIRTF
IRAS
100 0 0 0 km 1.25 K
900 km
1.4 K O. 1 K
2.4 K None
Dewar support strap configuration
\
1.8 K
5 yr
1 yr
Ground test
115 K LN 2 cooled OVCS
200 K Uncooled
I n s t r u m e n t thermal
Copper strap
AI mounting ring
interface Aperture cover
Passive
Supercritical helium
sponding to a heat load of 62.2 mW. The lifetime is 5.68 years, based on a 95 % fill at final top-off and a 2 day ground hold. The details of the thermal design and analysis will be described in a paper at the 1991 SPIE Optical Applied Science and Engineering Conference 2. The inner tank is supported by 12 glass fibre/resin straps; six each at top and bottom of the tank (see Figure 4). These are spread in a vee configuration to maximize stiffness while minimizing the area-to-thickness ratio and hence the heat leak. In the baseline design, they are 1 m long x 51 mm wide by 1.19 mm thick and have a cross-sectional area of 121 mm 2. The fundamental axial natural frequency is 15.1 Hz and the lateral is 18.5 Hz, both meeting the launch vehicle requirements. Strap limit loading is 2575 kg, yielding a fatigue life of 30 000 cycles, well above the required 6800 cycles. The ejectable aperture cover has three shields which are passively cooled by radiation from the interior of the main cryogen tank. A decision is to be made concerning the use of thermal links between the vapour-cooled shields in the cover and those in the main cryogen system as in ISO 3. A 1 m fiat mirror will be mounted on the aperture cover to aid in determining the alignment of the instruments while the telescope is cold. It will be cooled below 5 K by a helium flush line during certain phases of instrument test on the ground. A unique feature of the design is the use of vapourcooled shields in tension. This allows the VCS thickness to be limited only by the thermal requirements, and
Cryogenics 1992 Vol 32, No 2
109
Space infrared telescope facility mission: RV. Mason et al. hence their weight is minimized. Their current total mass is 88 kg, v e r s u s 236 kg for the original design with the shields in compression.
Technology development From a cryogenic point of view there are three major technology drivers; the low temperature of the bath, the required long life, and the need for the lightest possible mass. Certain IR sensor arrays in the IRS will require a temperature no higher than 1.4K. The cryostat designers have elected to operate the bath at 1.25 K and to allow a temperature rise of no more than 0.15 K from the bath to the array. Both of these require careful design, backed by a development programme to ensure that they can be met. The SIRTF bath temperature of 1.25 K is well below that of any missions flown to date. As is well known, the fountain pressure is given by Ap = pSAT and hence approaches zero with the entropy as the temperature drops. At 1.25 K it is only about 12 Pa mK -1. While the vapour pressure is also low, about 115 Pa, requiring only a 10 mK drop across the porous plug, there is concern that small accelerations may cause breakthrough. In early mission operations there will be about 2 m of liquid helium in the tank, and at a density of 0.143 g cm-3, an acceleration of 4 mg will be enough to overcome the fountain pressure generated by 1 mK of temperature difference. A careful theoretical and experimental investigation of the fountain effect at these pressures and temperatures will be conducted. The effect of pore sizes, pore geometry, pore length and temperature will be examined to allow selection of the optimum parameters. The SIRTF cryogenic system must provide cooling for milliwatt heat loads for detectors operating at 1.4 K, and for other detectors operating at temperatures near 2 K with heat loads of 1 5 - 2 5 mW. With the main bath temperature operating near 1.25 K, only a 0.15 K temperature rise is allowed between the bath and the colder sensors. JPL has therefore planned two cooling links to the bath: a low temperature, low heat load link; and a higher heat load link to cool the other sensors. Our search of the literature indicates that wide ranges of thermal conductivity and interface thermal resistance are reported at the temperatures for SIRTF operation. We shall perform measurements of temperature drops across joints and links that can be assembled relatively easily, and which will provide the required small AT. The candidate links are copper straps, with the copper consisting of high residual resistance ratio (RRR) material. Besides data for the optimally annealed
110
Cryogenics 1992 Vol 32, No 2
copper, we shall also measure the effects of strains and vibrations of large amplitude on the measured conductivity. We have also considered using superfluid links from the bath to the sensors. While such links are calculated to have satisfactory performance with even modest diameters, their implementation poses problems that JPL is reluctant to address if other satisfactory thermal links are available. While soft metals like lead and indium provide good interface thermal conductance in bolted metal-metal joints at higher temperatures, operating the joints so far below the superconducting transition temperatures of these joint fillers raises doubts concerning their effectiveness at these lower temperatures. We intend to characterize the temperature drops across aluminiumcopper and copper-copper joints at the temperatures relevant to SIRTF and at appropriate heat flow levels. Measurements will be made for joints at various pressures, with a range of finishes on the metals, and with and without soft-metal gap fillers such as indium and gold. The current leads for the magnet of the adiabatic demagnetization refrigerator add substantially to the heat leak. Even with Nb3Sn superconducting leads, the wire heat load from the IVCS to the bath is 3 to 4 mW. Their thermal conductance could be considerably reduced if high temperature superconductors were used between vapour-cooled shields and from the inner vapour-cooled shield to the instrument package. The best high temperature superconductors are ceramics and are quite brittle. Addition of silver improves the mechanical properties, but increases the thermal conductivity. Trade-off studies will be done to determine the benefit. If they are substantial, a development programme to solve the mechanical problems may be undertaken. Aluminium-lithium alloys offer the possibility of reducing tank weight by about 10%. If weight becomes a critical problem, an investigation of AI - L i alloys will be undertaken.
Acknowledgement The work described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
References 1 The Decade of Discovery in Astronomy and Astrophysics (Ed. Bahcall, J.) National Academy Press, Washington, DC, USA (1991) 2 Bhandari, P. and Petriek, W. Proc 1991 SP1E Optical Applied Science and Engineering Conference, to be published 3 Seldel, A. and Passvogel, Th. The ISO cryostat; its qualification status. Presented at CEC/ICMC, June 1991. To be published in Advances in Cryogenic Engineering (1992)