- Email: [email protected]
The X-ray microcalorimeter spectrometer (XMS): a reference cryogenic instrument for Constellation-X
Cryogenics 44 (2004) 543–549 www.elsevier.com/locate/cryogenics
The X-ray microcalorimeter spectrometer (XMS): a reference cryogenic instrument for Constellation-X Paul L. Whitehouse a b
a,*
, Peter J. Shirron a, Richard L. Kelley
b
NASAGoddard Space Flight Center, Code 552, Greenbelt, MD 20771, USA NASAGoddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
Abstract The first two of four observatories in the constellation will be launched together in 2013 and followed a year later by the launch of the remaining pair. The four will independently orbit the Sun–Earth Lagrange point L2. An instrument compliment resides in the focal plane module (FPM) of each observatory 10 m from the optics module and consists of three hard X-ray telescope (HXT) detectors, a reflection grating spectrometer (RGS) focal plane CCD camera (RFC) and an X-ray microcalorimeter spectrometer (XMS). Instrument awards are scheduled for early 2006. The reference detector system for XMS is a 32 · 32 array of microcalorimetric superconducting transition edge sensors (TES) with SQUID based multiplexed readout and amplification. A multi-stage continuous ADR will provide the stable 50 mK desired for the TES array and a stable 1 K for the SQUID amplifiers while also lifting thermal parasitic and inefficiency loads to a 6 K cryocooler interface. The 6 K cryocooler is expected to emerge from the jointproject advanced cryocooler technology development program (ACTDP) in which Constellation-X is an active partner. Project preformulation activities are marked by extensive technology development necessitating early, but realistic, thermal and cooling load requirements for ADR and ACTDP-cryocooler design points. Such requirements are driven by the encompassing XMS cryostat and ultimately by the thermal environment imposed by the FPM. It is further desired that the XMS instrument be able to operate horizontally in the laboratory, with a warm vacuum shell, during an extensive calibration regime. It is that highly integrated reference instrument (microcalorimeter, ADR, cryocooler and cryostat) that will be examined here. Published by Elsevier Ltd. Keywords: Microcalorimeter; ADR; Cryocooler; Cryostat; Space cryogenics
1. Introduction 1.1. General description Constellation-X is being planned to inherit the X-ray sky from Chandra, XMM-Newton and Astro-E. Two pair of observatories are scheduled for separate launches in 2013 and 2014. Each pair will be mounted side by side within the single fairing of an Atlas V launch vehicle. Each observatory will execute a number of phasing loops in separate orbits about Earth and then take advantage of a lunar assist to send it on a cruise trajectory to the Sun–Earth Lagrange point L2. Once on station in their Lissajous orbits about L2, the observatories will observe in concert to achieve high throughput, but will not fly in formation.
*
Corresponding author. E-mail address: [email protected] (P.L. Whitehouse).
0011-2275/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.cryogenics.2004.03.011
Each observatory (Fig. 1) is divided into a spacecraft bus and a telescope module (TM). The full mission bandpass is covered by two telescope systems within the TM of each observatory: • Spectroscopy X-ray telescope (SXT) with bandpass from 0.25 to 10 keV. • Hard X-ray telescope (HXT) with bandpass from 6 to 40 keV. The telescope module is further subdivided into three modules: • The optics module (OM) consists of the SXT flight mirror assembly (including mirrors, reflection grating assembly, and collimators), the HXT mirrors, star tracker, associated kinematic mounts, and supporting structure. • The focal plane module (FPM) includes the detectors and associated coolers, electronics, focus mechanisms, support structure, and sunshade.
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P.L. Whitehouse et al. / Cryogenics 44 (2004) 543–549
Spacecraft Bus 1.6 m dia. Spectroscopy X-ray Telescope (SXT) Flight Mirror Assembly (FMA) and Reflection Grating Assembly (RGA)
High Gain Antenna
Spacecraft Bus Component Compartments
Telescope Module
Optical Bench Instrument Electronics Bay and external radiators
Solar Panel
Optics Module
Hard X-ray Telescope (HXT) Mirrors (3) Hard X-ray Telescope (HXT) Detectors (3)
Focal Plane Module
X-ray Microcalorimeter Spectrometer (XMS)
RGS Focalplane Camera (RFC)
Sunshade
Fig. 1. Exploded view of a single Constellation-X observatory presenting the component nomenclature.
• The optical bench (OB) is a five-sided optical metering structure between the FPM and OM which provides the 10 m focal length between the mirrors and detectors, and also includes thermal control, X-ray baffling, and electrical harnesses. The HXT consists of three mirror assemblies located in the OM, each with a dedicated detector at its focus in the FPM. The SXT consists of a single flight mirror assembly (FMA) in the OM shared by the CCD camera a reflection grating spectrometer (RGS) and the microcalorimeter detector of an X-ray microcalorimeter spectrometer (XMS). Coincident with the beginning of its pre-formulation phase in 1996, Constellation-X embarked on a comprehensive technology development program. In fact, the preponderance of pre-formulation activities center around technology development––all in preparation for instrument awards in early 2006. Technologies needed for the detector package of XMS are among these first development efforts. We begin with an overview of those technologies and then those of the cooling system, to be added to Constellation-X funding beginning in fiscal year 2004. The remaining considerations will be given to the cryostat and the instrument system––to the integrated dance between evolving requirements and developing technology.
detector. Semiconductor thermistor microcalorimeters (specifically, neutron transmutation doped [NTD] Ge) are being developed as an alternate implementation [1] and paramagnetic based microcalorimeters have recently come under consideration. Each pixel casts a variable resistance in a superconducting quantum interference device (SQUID) based multiplexed readout circuit (MUX) [2] which is coupled to series SQUIDarrays for amplification and final read-out by external electronics. A series of test arrays of increasing size are planned on the road to the final reference 32 · 32 pixel detector. Later demonstration arrays will be married with the continuous ADR, discussed below and elsewhere in these proceedings, to create an XMS demonstration sub-unit that will double as a cold test platform for testing components and systems. 2.2. Continuous adiabatic demagnetization refrigerator Cooling the detector package to a continuous, stable 50 mK will be a 4-stage adiabatic demagnetization refrigerator (CADR) [3]. An additional continuous and stable intermediate stage will anchor the series SQUIDarrays near 1 K. The warmer stages of the CADR are sequentially linked through heat switches and cycled to transfer heat to a relatively warm (6 K) cryocooler interface. The ‘‘1 K’’ stage has an added benefit of guarding lower temperature stages from temperature extremes in the cycles.
2. XMS instrument technologies 2.3. Cryocooler 2.1. Microcalorimeter Superconducting transition-edge sensor (TES) microcalorimeters are in development for the XMS baseline
A key component of the XMS is a mechanical cryocooler that provides several stages of active cooling inside the instrument cryostat. Its primary purpose is to
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amounted to an inside-out packaging exercise, utilizing a strut support system, intended to form initial thermal load estimates for cryocooler and ADR development requirements.
provide a 6 K heat-sink for the CADR. A secondary purpose is to actively cool a stage at the 90–100 K ‘‘warm’’ end of high temperature superconducting ADR power leads. A tertiary requirement is for heat-sink stages intermediate to those just mentioned. A cryocooler to meet the needs of Constellation-X is part of a cooperative effort within NASA to develop several 6 K cooling technologies through the Advanced Cryocooler Technology Development Program (ACTDP).
3.1. Objectives and requirements While the scientific heritage will be accepted with some pride, we are reluctant to accept certain aspects of the application heritage. We dream of creating a more benevelant integration and testing environment with a much shorter close-out, cool-down, warm-up and opening turn-around time. It is desired, that after cryocooler integration, all instrument components and leads be easily accessible from one end of the cryostat. The XMS will be calibrated based on the Astro-E/E2 model with X-ray monochrometers attached to the cryostat in a horizontal configuration. It will be very
3. XMS system A block diagram of the TES based XMS is shown in Fig. 2. A much different cryostat would be needed for an NTD based XMS since its JFETs require a cold-bias heat sink near 110 K and will not be discussed here. The initial realization of the baseline concept (Fig. 3) x-rays gate valve
main shell Legend
filters fil te rs thermal sink detector package
Microcalorimeter/CADR 6 K insert
thermal strap
50 mK Microcalorimeter array
fluid lines readout amplifier (SQUIDs)
Anticoincidence detector
heat switch superconducting cable
CADR Stage 1
50 mK 2nd stage amplifier (SQUID arrays)
45 mK CADR Stage 2
command
0.275 K CADR Stage 5
0.25 K CADR Stage 3
1K
Microcalorimeter Controller/Amplifier
1K
data
Microcalorimeter Digital Electronics
1K CADR Stage 4
0.9 K 6.5 K
CADR Control Electronics
LVPS
6K 18 K Cryocooler Control Electronics
35 - 40 K 90 - 100 K 125 K (on orbit)
cold head
300 K (on ground)
loop heat pipe to radiator
28 V from s/c bus
science data condenser
compressor
Cryocooler
command from and engineering data to s/c 1553 bus
Fig. 2. Block diagram of the X-ray microcalorimeter spectrometer (XMS).
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Transfer line & Inertance tube to compressors under platform
Pulse Tube Cold-head
" 6 Kelvin Can "
" 1 Kelvin Can " S-Link ADR/Cryoooler interface Focus Mechanism (1 of 3) " Detector Can " 50 mK superconducting magnetic shield
Instrument Platform (section)
platform aperture
Optical Path
Fig. 3. Conceptual XMS instrument.
desirable to operate the cryostat, ‘‘as is’’, with a room temperature shell. From the Spitzer Space Telescope example we are likely to have sensitive coatings on the cryostat exterior, but hope to require no more than a cover or non-vacuum shell to protect those coatings. We are working on the assumption that we will require the main shell to provide a vacuum enclosure for ground and flight operations. It is a first hope for cryogen-free systems that a vacuum shell would not have to be flown, but contamination concerns and realities may not allow us to fly a system with only thermal shielding or even simple localized vacuum protection. 3.2. Thermal analysis XMS temperatures are controlled within the cryostat; however, the instrument relies on TM thermal control for its external conductive and radiative environment. The instrument section has a relatively open view of space to accommodate passive radiation temperature control of the cryostat main shell and CCD detectors.
A thermal model of the instrument platform was developed in SINDA, fed by the radiation couplings generated by TSS (Fig. 4), to determine the passive cryostat shell temperature before embarking on any optimization efforts. Boundary conditions include white painted cryostat shell, sun loaded sunshade, room temperature platform mounts for external struts and views divided between space and white MLI blankets. These basic conditions yielded a shell temperature of 110 K. There were thoughts at the time of using the shell as an intermediate stage radiator for an integrated cryocooler to dissipate 6–8 W. That power would raise the shell temperature to 125 and 132 K respectively. Some dissipation of this sort will be inevitable for the case of a mounted cold-head from a split pulse tube or stirling cryocooler configuration as in the current baseline. Recent flights of WMAP and SIRTF indicated that a colder shell should be possible with careful selection of outer shell surface preparations. However, the lowest possible shell temperature may not be optimum for a cryostat with an integrated cold-head.
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3.3. Cryostat design considerations
Fig. 4. Graphical representation of TSS/SINDA model for the instrument platform of the focal plane module (FPM) without pulse tube cold head mounted on cryostat. Smaller graphic elements represent conductive and radiative nodes.
Assuming some dissipative loading of the shell, a boundary temperature of 125 K was used to make beginning estimates of the internal heat map of the cryostat including the 6 and 18 K cryocooler heat loads. Strut sizing for a supported mass of 15 kg at 6 K was estimated using the methods of Ross [4]. Effective MLI conductivities were obtained from the Ball ATC model for couplings above 18 K. Radiation loads were estimated from design surface areas and average emissivity for goldized surfaces below 18 K. Estimated detectorlead loads were obtained from the microcalorimeter development team. ADR lead loads were derived from thermal conductivity data for Nordic Superconducting BSSCO/Ag–Au(18%) high temperature superconducting tape. The ADR 6 K load to the cryocooler was extrapolated from 4.2 K sink test data available at the time. The 18 K load to the cryocooler was a straightforward 55 mW, a factor of roughly three lower than the ACTDP requirement. The contribution of the ADR to the 6 K load requires some qualification. The ADR is an entropy mover and will need to eventually dump a certain amount of energy to a cold-load interface with the cryocooler. The cycle period and profile determines the instantaneous load to the cryocooler. The estimated load would equate to a continuous 6 mW. For an extreme 50% duty cycle, the cooling power required to sink this load would be about 12 mW, but for only 50% of the cycle period. Adding the 6 mW contribution from parasitic sources identified in the previous paragraph, we estimated a total 6 K load between 12 and 18 mW–– also within the ACTDP specification.
Increasing attention is being payed to the development of components for cryogenic integration of new systems [5]. This includes coldfinger interfaces (e.g. [6– 8]), heat switches [9] as well as cryocooler microphonics, EMI and temperature control [10–14]. However, it remains easy for a project to postpone system development during pre-formulation when energy is focused on the development of new technologies. It is difficult to go beyond conceptual design and begin the detailed design of ‘‘existing technologies’’ even though these highly integrated systems may be primary determinants of technology requirements. The cryostat will provide the necessary structural support and thermal isolation for all microcalorimeter, CADR and cryocooler components contained within the outer shell. In addition to the strut supports (Fig. 3) and the folded-cylinder support (Fig. 5) we will examine other support schemes [15]. It is uncertain at this point how important the thermal efficiency of the support structure will be relative to manufacturing and integration ease. In any case one could characterize the result as a ‘‘multistage cryostat’’ with specific temperature stages and shields actively cooled by either the cryocooler or the CADR. As the system design matures, main shell and intermediate temperatures will be set by trades involving achievable cryocooler stage temperatures, CADR efficiencies, blocking filters, and the entire read-out chain of striplines, series SQUID arrays and leads to the warm electronics. Interfaces among the detector, its read-outs and the CADR are the most intimate within the XMS. For this reason, all three are being combined into a modular ‘‘6 K can’’ that can be integrated and tested separate from the remainder of the system. The volume of the CADR drives the volume of the 6 K can and ultimately the size of the cryostat. Work will begin in the next phase of ADR development to reduce the size and mass of specific stages. Optimizing stages and heat switches for faster cycling can accomplish this while also increasing the cooling power over a responding increase in parasitics. Increased cycling rate reduces the peak heat flow to the cooler, bringing it closer to the full time average and reducing the requirements on the cryocooler. A mechanical cryocooler will provide the 6 K heat sink for the CADR and will actively cool several thermal shields within the cryostat. It will also thermally anchor internal detector signal and CADR current leads. It will be critical to verify the adequacy of cooling capacity requirements, based on those loads, before the ACTDP critical design review (CDR) scheduled for the summer of 2004. XMS calibration will be following the Astro-E/ E2 model, placing an additional constraint that may
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P.L. Whitehouse et al. / Cryogenics 44 (2004) 543–549 Pulse Tube Cold-head 260 - 290 K
Cold-head thermal isolator
>
90
11 0
K
Radiation shields
K 00 K - 1 40 K 35 18
Differential-CTE clamp with Kevlar supports "Folded cylinder" primary support structure
" 6 Kelvin Can "
6K
" 1 Kelvin Can "
ADR components (5 stages with heat switches) SQUID series arrays (4-6)
1K
CuNi flex striplines (1 K - 6 K)
Nb flex striplines (50 mK - 1 K) 50 mK
Detector leads (via Striplines) " Detector Can " 50 mK superconducting magnetic shield Blocking filter and mount (at main shell and select shields)
MLI space Optical Path
Gate valve (not shown)
Fig. 5. Cross-sectional view of the XMS cryostat with the ‘‘folded-cylinder’’ internal structural support option.
adversely affect certain cryocoolers. X-ray monochromotors, with their extensive GSE, will be attached to a horizontal cryostat. Single stage pulse tubes are known to carry an additional internal 1 G load from convection in orientations with the warm end below the cold [16]. The horizontal orientations of 0 and 180 are transition points where the effect is hot yet too great. No data is yet available for 2-stage pulse tubes and tests on 3- and 4stage pulse tubes has yet to be taken. The effect on each stage is uncertain and therefore of great interest. Blocking filters in the aperture of the cryostat prevent heating of the detector stage by non-X-ray radiation. Transmission of these filters determines the low energy limit to the bandpass (0.25 keV). The goal for Constellation-X is to use aluminized polyimide, as in XRS, but to try to go thinner in order to increase the X-ray throughput at low energies. What we actually fly is uncertain, as is the number of filters, their temperature staging and how they’re mounted, but the filter supplier may be able to reduce the thickness to half that of those for XRS. We may use some very fine, but strong meshes to support the larger filters. Contamination will be a concern during the 7-year operational life of the instrument. Low power defrost methods are under
consideration that would impart low thermal stress to the filter. Although all 1024 pixels of the microcalorimeter could be read on independent channels, reducing the number of channels through use of a SQUID MUX significantly reducing the heat load on the ADR, and the complexity of the front-end assembly. A further reduction in complexity will be achieved, at least below 6 K, through the use of flexible striplines of Kapton ribbon with superconducting or non-superconducting film conductors.
4. Summary/conclusion Constellation-X has recently entered its Formulation Phase (Phase A). The technology development that so characterized the pre-Formulation Phase will taper off as the efforts reach fruition. Critical milestones will be reached in the coming year for each of the technologies being considered here. The development of a cryostat design must catch up to generate definitive requirements, particularly where they drive requirements of the other technologies.
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This admonition from Tomlinson et al. [5] seems most appropriate here: ‘‘Cryogenic integration technology has been often a neglected system design activity. . . .Given the varied requirements for cryogenic cooling systems, it can be seen that an integrated research and development plan is needed to efficiently invest technology funds for maximum return’’. Acknowledgements Special thanks to Theo Muench and Rob Chalmers for TSS/SINDA modeling of the FPM thermal environment, and Michael Jackson for 3D design work on cryostat concepts.
[6]
[7]
[8]
[9]
[10]
[11]
References [1] Stahle CK et al. Toward a 2-eV microcalorimeter X-ray spectrometer of Constellation-X. Proc SPIE 1999;82:3765. [2] Irwin KD. SQUID multiplexers for transition-edge sensors. Physica C 2002;368:203. [3] Shirron P, Canavan E, DiPirro M, Francis J, Jackson M, Tuttle J, et al. Development of a cryogen-free continuous ADR for the Constellation-X mission. These proceedings. [4] Ross Jr RG. Estimation of thermal conduction loads for structural supports of cryogenic payloads/systems? These proceedings. [5] Tomlinson BJ, Flake B, Roberts T. Air Force Research Laboratory space cryogenic technology research initiative. In: Cryocool-
[12]
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ers, vol. 12. USA: Kluwer Academic/Plenum Publishers; 2003. p. 9–16. Bugby D, Marland B, Stouffer C, Kroliczek E. Advanced components for cryogenic integration. In: Cryocoolers, vol. 12. USA: Kluwer Academic/Plenum Publishers; 2003. p. 693–708. Sparr L, Boyle R, Nguyen L, Frisch H, Banks S, James E, et al. Design and test of potential cryocooler cold finger interfaces. Adv Cryogen Eng 1994;39:1253–62. Sugimoto M, Sekimoto Y, Yokogawa S, Okuda T, Kamba T, Ogawa H, et al. Thermal link for cartridge-type cryostat. Cryogenics 2003;43:435–9. Marland B, Budby D, Stouffer C. Development and testing of an advanced cryogenic thermal switch and cryogenic thermal switch test bed. These proceedings. Bangma MR, Rijpma AP, de Vries E, Reincke HA, Holland HJ, ter Brake HJM, et al. Interference characterisation of a commercial Joule–Thomson cooler to be used in a SQUID-based foetal heart monitor. Cryogenics 2001;41:657–63. Bhatia RS, Ade PAR, Bradshaw TW, Crook MR, Griffin MJ, Orlowska AH. The effects of cryocooler microphonics, EMI and temperature variations on bolometric detectors. Cryogenics 2002;41:851–63. Veprik AM, Babitsky VI, Pundak N, Riabzev SV. Suppression of cryocooler induced microphonics in infrared imagers. Adv Cryogen Eng 2002;47:1133–40. Ravex A, Feger D, Duband L. A simple and efficient thermal link assembly for cryocoolers. Cryogenics 1999;39:997–1001. Clappier R, Kline-Schoder R. Precision temperature control of stirling-cycle cryocoolers. Adv Cryogen Eng 1994;39:1177–84. Kittel P. Comparison of dewar supports for space applications. Cryogenics 1993;33(4). Ross Jr RG. Effect of gravity orientation on the thermal performance of Stirling-type pulse tube cryocoolers. These proceedings.
The X-ray microcalorimeter spectrometer (XMS): a reference cryogenic instrument for Constellation-X Paul L. Whitehouse a b
a,*
, Peter J. Shirron a, Richard L. Kelley
b
NASAGoddard Space Flight Center, Code 552, Greenbelt, MD 20771, USA NASAGoddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
Abstract The first two of four observatories in the constellation will be launched together in 2013 and followed a year later by the launch of the remaining pair. The four will independently orbit the Sun–Earth Lagrange point L2. An instrument compliment resides in the focal plane module (FPM) of each observatory 10 m from the optics module and consists of three hard X-ray telescope (HXT) detectors, a reflection grating spectrometer (RGS) focal plane CCD camera (RFC) and an X-ray microcalorimeter spectrometer (XMS). Instrument awards are scheduled for early 2006. The reference detector system for XMS is a 32 · 32 array of microcalorimetric superconducting transition edge sensors (TES) with SQUID based multiplexed readout and amplification. A multi-stage continuous ADR will provide the stable 50 mK desired for the TES array and a stable 1 K for the SQUID amplifiers while also lifting thermal parasitic and inefficiency loads to a 6 K cryocooler interface. The 6 K cryocooler is expected to emerge from the jointproject advanced cryocooler technology development program (ACTDP) in which Constellation-X is an active partner. Project preformulation activities are marked by extensive technology development necessitating early, but realistic, thermal and cooling load requirements for ADR and ACTDP-cryocooler design points. Such requirements are driven by the encompassing XMS cryostat and ultimately by the thermal environment imposed by the FPM. It is further desired that the XMS instrument be able to operate horizontally in the laboratory, with a warm vacuum shell, during an extensive calibration regime. It is that highly integrated reference instrument (microcalorimeter, ADR, cryocooler and cryostat) that will be examined here. Published by Elsevier Ltd. Keywords: Microcalorimeter; ADR; Cryocooler; Cryostat; Space cryogenics
1. Introduction 1.1. General description Constellation-X is being planned to inherit the X-ray sky from Chandra, XMM-Newton and Astro-E. Two pair of observatories are scheduled for separate launches in 2013 and 2014. Each pair will be mounted side by side within the single fairing of an Atlas V launch vehicle. Each observatory will execute a number of phasing loops in separate orbits about Earth and then take advantage of a lunar assist to send it on a cruise trajectory to the Sun–Earth Lagrange point L2. Once on station in their Lissajous orbits about L2, the observatories will observe in concert to achieve high throughput, but will not fly in formation.
*
Corresponding author. E-mail address: [email protected] (P.L. Whitehouse).
0011-2275/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.cryogenics.2004.03.011
Each observatory (Fig. 1) is divided into a spacecraft bus and a telescope module (TM). The full mission bandpass is covered by two telescope systems within the TM of each observatory: • Spectroscopy X-ray telescope (SXT) with bandpass from 0.25 to 10 keV. • Hard X-ray telescope (HXT) with bandpass from 6 to 40 keV. The telescope module is further subdivided into three modules: • The optics module (OM) consists of the SXT flight mirror assembly (including mirrors, reflection grating assembly, and collimators), the HXT mirrors, star tracker, associated kinematic mounts, and supporting structure. • The focal plane module (FPM) includes the detectors and associated coolers, electronics, focus mechanisms, support structure, and sunshade.
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Spacecraft Bus 1.6 m dia. Spectroscopy X-ray Telescope (SXT) Flight Mirror Assembly (FMA) and Reflection Grating Assembly (RGA)
High Gain Antenna
Spacecraft Bus Component Compartments
Telescope Module
Optical Bench Instrument Electronics Bay and external radiators
Solar Panel
Optics Module
Hard X-ray Telescope (HXT) Mirrors (3) Hard X-ray Telescope (HXT) Detectors (3)
Focal Plane Module
X-ray Microcalorimeter Spectrometer (XMS)
RGS Focalplane Camera (RFC)
Sunshade
Fig. 1. Exploded view of a single Constellation-X observatory presenting the component nomenclature.
• The optical bench (OB) is a five-sided optical metering structure between the FPM and OM which provides the 10 m focal length between the mirrors and detectors, and also includes thermal control, X-ray baffling, and electrical harnesses. The HXT consists of three mirror assemblies located in the OM, each with a dedicated detector at its focus in the FPM. The SXT consists of a single flight mirror assembly (FMA) in the OM shared by the CCD camera a reflection grating spectrometer (RGS) and the microcalorimeter detector of an X-ray microcalorimeter spectrometer (XMS). Coincident with the beginning of its pre-formulation phase in 1996, Constellation-X embarked on a comprehensive technology development program. In fact, the preponderance of pre-formulation activities center around technology development––all in preparation for instrument awards in early 2006. Technologies needed for the detector package of XMS are among these first development efforts. We begin with an overview of those technologies and then those of the cooling system, to be added to Constellation-X funding beginning in fiscal year 2004. The remaining considerations will be given to the cryostat and the instrument system––to the integrated dance between evolving requirements and developing technology.
detector. Semiconductor thermistor microcalorimeters (specifically, neutron transmutation doped [NTD] Ge) are being developed as an alternate implementation [1] and paramagnetic based microcalorimeters have recently come under consideration. Each pixel casts a variable resistance in a superconducting quantum interference device (SQUID) based multiplexed readout circuit (MUX) [2] which is coupled to series SQUIDarrays for amplification and final read-out by external electronics. A series of test arrays of increasing size are planned on the road to the final reference 32 · 32 pixel detector. Later demonstration arrays will be married with the continuous ADR, discussed below and elsewhere in these proceedings, to create an XMS demonstration sub-unit that will double as a cold test platform for testing components and systems. 2.2. Continuous adiabatic demagnetization refrigerator Cooling the detector package to a continuous, stable 50 mK will be a 4-stage adiabatic demagnetization refrigerator (CADR) [3]. An additional continuous and stable intermediate stage will anchor the series SQUIDarrays near 1 K. The warmer stages of the CADR are sequentially linked through heat switches and cycled to transfer heat to a relatively warm (6 K) cryocooler interface. The ‘‘1 K’’ stage has an added benefit of guarding lower temperature stages from temperature extremes in the cycles.
2. XMS instrument technologies 2.3. Cryocooler 2.1. Microcalorimeter Superconducting transition-edge sensor (TES) microcalorimeters are in development for the XMS baseline
A key component of the XMS is a mechanical cryocooler that provides several stages of active cooling inside the instrument cryostat. Its primary purpose is to
P.L. Whitehouse et al. / Cryogenics 44 (2004) 543–549
545
amounted to an inside-out packaging exercise, utilizing a strut support system, intended to form initial thermal load estimates for cryocooler and ADR development requirements.
provide a 6 K heat-sink for the CADR. A secondary purpose is to actively cool a stage at the 90–100 K ‘‘warm’’ end of high temperature superconducting ADR power leads. A tertiary requirement is for heat-sink stages intermediate to those just mentioned. A cryocooler to meet the needs of Constellation-X is part of a cooperative effort within NASA to develop several 6 K cooling technologies through the Advanced Cryocooler Technology Development Program (ACTDP).
3.1. Objectives and requirements While the scientific heritage will be accepted with some pride, we are reluctant to accept certain aspects of the application heritage. We dream of creating a more benevelant integration and testing environment with a much shorter close-out, cool-down, warm-up and opening turn-around time. It is desired, that after cryocooler integration, all instrument components and leads be easily accessible from one end of the cryostat. The XMS will be calibrated based on the Astro-E/E2 model with X-ray monochrometers attached to the cryostat in a horizontal configuration. It will be very
3. XMS system A block diagram of the TES based XMS is shown in Fig. 2. A much different cryostat would be needed for an NTD based XMS since its JFETs require a cold-bias heat sink near 110 K and will not be discussed here. The initial realization of the baseline concept (Fig. 3) x-rays gate valve
main shell Legend
filters fil te rs thermal sink detector package
Microcalorimeter/CADR 6 K insert
thermal strap
50 mK Microcalorimeter array
fluid lines readout amplifier (SQUIDs)
Anticoincidence detector
heat switch superconducting cable
CADR Stage 1
50 mK 2nd stage amplifier (SQUID arrays)
45 mK CADR Stage 2
command
0.275 K CADR Stage 5
0.25 K CADR Stage 3
1K
Microcalorimeter Controller/Amplifier
1K
data
Microcalorimeter Digital Electronics
1K CADR Stage 4
0.9 K 6.5 K
CADR Control Electronics
LVPS
6K 18 K Cryocooler Control Electronics
35 - 40 K 90 - 100 K 125 K (on orbit)
cold head
300 K (on ground)
loop heat pipe to radiator
28 V from s/c bus
science data condenser
compressor
Cryocooler
command from and engineering data to s/c 1553 bus
Fig. 2. Block diagram of the X-ray microcalorimeter spectrometer (XMS).
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P.L. Whitehouse et al. / Cryogenics 44 (2004) 543–549
Transfer line & Inertance tube to compressors under platform
Pulse Tube Cold-head
" 6 Kelvin Can "
" 1 Kelvin Can " S-Link ADR/Cryoooler interface Focus Mechanism (1 of 3) " Detector Can " 50 mK superconducting magnetic shield
Instrument Platform (section)
platform aperture
Optical Path
Fig. 3. Conceptual XMS instrument.
desirable to operate the cryostat, ‘‘as is’’, with a room temperature shell. From the Spitzer Space Telescope example we are likely to have sensitive coatings on the cryostat exterior, but hope to require no more than a cover or non-vacuum shell to protect those coatings. We are working on the assumption that we will require the main shell to provide a vacuum enclosure for ground and flight operations. It is a first hope for cryogen-free systems that a vacuum shell would not have to be flown, but contamination concerns and realities may not allow us to fly a system with only thermal shielding or even simple localized vacuum protection. 3.2. Thermal analysis XMS temperatures are controlled within the cryostat; however, the instrument relies on TM thermal control for its external conductive and radiative environment. The instrument section has a relatively open view of space to accommodate passive radiation temperature control of the cryostat main shell and CCD detectors.
A thermal model of the instrument platform was developed in SINDA, fed by the radiation couplings generated by TSS (Fig. 4), to determine the passive cryostat shell temperature before embarking on any optimization efforts. Boundary conditions include white painted cryostat shell, sun loaded sunshade, room temperature platform mounts for external struts and views divided between space and white MLI blankets. These basic conditions yielded a shell temperature of 110 K. There were thoughts at the time of using the shell as an intermediate stage radiator for an integrated cryocooler to dissipate 6–8 W. That power would raise the shell temperature to 125 and 132 K respectively. Some dissipation of this sort will be inevitable for the case of a mounted cold-head from a split pulse tube or stirling cryocooler configuration as in the current baseline. Recent flights of WMAP and SIRTF indicated that a colder shell should be possible with careful selection of outer shell surface preparations. However, the lowest possible shell temperature may not be optimum for a cryostat with an integrated cold-head.
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3.3. Cryostat design considerations
Fig. 4. Graphical representation of TSS/SINDA model for the instrument platform of the focal plane module (FPM) without pulse tube cold head mounted on cryostat. Smaller graphic elements represent conductive and radiative nodes.
Assuming some dissipative loading of the shell, a boundary temperature of 125 K was used to make beginning estimates of the internal heat map of the cryostat including the 6 and 18 K cryocooler heat loads. Strut sizing for a supported mass of 15 kg at 6 K was estimated using the methods of Ross [4]. Effective MLI conductivities were obtained from the Ball ATC model for couplings above 18 K. Radiation loads were estimated from design surface areas and average emissivity for goldized surfaces below 18 K. Estimated detectorlead loads were obtained from the microcalorimeter development team. ADR lead loads were derived from thermal conductivity data for Nordic Superconducting BSSCO/Ag–Au(18%) high temperature superconducting tape. The ADR 6 K load to the cryocooler was extrapolated from 4.2 K sink test data available at the time. The 18 K load to the cryocooler was a straightforward 55 mW, a factor of roughly three lower than the ACTDP requirement. The contribution of the ADR to the 6 K load requires some qualification. The ADR is an entropy mover and will need to eventually dump a certain amount of energy to a cold-load interface with the cryocooler. The cycle period and profile determines the instantaneous load to the cryocooler. The estimated load would equate to a continuous 6 mW. For an extreme 50% duty cycle, the cooling power required to sink this load would be about 12 mW, but for only 50% of the cycle period. Adding the 6 mW contribution from parasitic sources identified in the previous paragraph, we estimated a total 6 K load between 12 and 18 mW–– also within the ACTDP specification.
Increasing attention is being payed to the development of components for cryogenic integration of new systems [5]. This includes coldfinger interfaces (e.g. [6– 8]), heat switches [9] as well as cryocooler microphonics, EMI and temperature control [10–14]. However, it remains easy for a project to postpone system development during pre-formulation when energy is focused on the development of new technologies. It is difficult to go beyond conceptual design and begin the detailed design of ‘‘existing technologies’’ even though these highly integrated systems may be primary determinants of technology requirements. The cryostat will provide the necessary structural support and thermal isolation for all microcalorimeter, CADR and cryocooler components contained within the outer shell. In addition to the strut supports (Fig. 3) and the folded-cylinder support (Fig. 5) we will examine other support schemes [15]. It is uncertain at this point how important the thermal efficiency of the support structure will be relative to manufacturing and integration ease. In any case one could characterize the result as a ‘‘multistage cryostat’’ with specific temperature stages and shields actively cooled by either the cryocooler or the CADR. As the system design matures, main shell and intermediate temperatures will be set by trades involving achievable cryocooler stage temperatures, CADR efficiencies, blocking filters, and the entire read-out chain of striplines, series SQUID arrays and leads to the warm electronics. Interfaces among the detector, its read-outs and the CADR are the most intimate within the XMS. For this reason, all three are being combined into a modular ‘‘6 K can’’ that can be integrated and tested separate from the remainder of the system. The volume of the CADR drives the volume of the 6 K can and ultimately the size of the cryostat. Work will begin in the next phase of ADR development to reduce the size and mass of specific stages. Optimizing stages and heat switches for faster cycling can accomplish this while also increasing the cooling power over a responding increase in parasitics. Increased cycling rate reduces the peak heat flow to the cooler, bringing it closer to the full time average and reducing the requirements on the cryocooler. A mechanical cryocooler will provide the 6 K heat sink for the CADR and will actively cool several thermal shields within the cryostat. It will also thermally anchor internal detector signal and CADR current leads. It will be critical to verify the adequacy of cooling capacity requirements, based on those loads, before the ACTDP critical design review (CDR) scheduled for the summer of 2004. XMS calibration will be following the Astro-E/ E2 model, placing an additional constraint that may
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P.L. Whitehouse et al. / Cryogenics 44 (2004) 543–549 Pulse Tube Cold-head 260 - 290 K
Cold-head thermal isolator
>
90
11 0
K
Radiation shields
K 00 K - 1 40 K 35 18
Differential-CTE clamp with Kevlar supports "Folded cylinder" primary support structure
" 6 Kelvin Can "
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ADR components (5 stages with heat switches) SQUID series arrays (4-6)
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CuNi flex striplines (1 K - 6 K)
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Detector leads (via Striplines) " Detector Can " 50 mK superconducting magnetic shield Blocking filter and mount (at main shell and select shields)
MLI space Optical Path
Gate valve (not shown)
Fig. 5. Cross-sectional view of the XMS cryostat with the ‘‘folded-cylinder’’ internal structural support option.
adversely affect certain cryocoolers. X-ray monochromotors, with their extensive GSE, will be attached to a horizontal cryostat. Single stage pulse tubes are known to carry an additional internal 1 G load from convection in orientations with the warm end below the cold [16]. The horizontal orientations of 0 and 180 are transition points where the effect is hot yet too great. No data is yet available for 2-stage pulse tubes and tests on 3- and 4stage pulse tubes has yet to be taken. The effect on each stage is uncertain and therefore of great interest. Blocking filters in the aperture of the cryostat prevent heating of the detector stage by non-X-ray radiation. Transmission of these filters determines the low energy limit to the bandpass (0.25 keV). The goal for Constellation-X is to use aluminized polyimide, as in XRS, but to try to go thinner in order to increase the X-ray throughput at low energies. What we actually fly is uncertain, as is the number of filters, their temperature staging and how they’re mounted, but the filter supplier may be able to reduce the thickness to half that of those for XRS. We may use some very fine, but strong meshes to support the larger filters. Contamination will be a concern during the 7-year operational life of the instrument. Low power defrost methods are under
consideration that would impart low thermal stress to the filter. Although all 1024 pixels of the microcalorimeter could be read on independent channels, reducing the number of channels through use of a SQUID MUX significantly reducing the heat load on the ADR, and the complexity of the front-end assembly. A further reduction in complexity will be achieved, at least below 6 K, through the use of flexible striplines of Kapton ribbon with superconducting or non-superconducting film conductors.
4. Summary/conclusion Constellation-X has recently entered its Formulation Phase (Phase A). The technology development that so characterized the pre-Formulation Phase will taper off as the efforts reach fruition. Critical milestones will be reached in the coming year for each of the technologies being considered here. The development of a cryostat design must catch up to generate definitive requirements, particularly where they drive requirements of the other technologies.
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This admonition from Tomlinson et al. [5] seems most appropriate here: ‘‘Cryogenic integration technology has been often a neglected system design activity. . . .Given the varied requirements for cryogenic cooling systems, it can be seen that an integrated research and development plan is needed to efficiently invest technology funds for maximum return’’. Acknowledgements Special thanks to Theo Muench and Rob Chalmers for TSS/SINDA modeling of the FPM thermal environment, and Michael Jackson for 3D design work on cryostat concepts.
[6]
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