Search

The Soft X-ray Spectrometer (SXS) instrument (Mitsuda et al., 2010) [1] on Astro-H (Takahashi et al., 2010) [2] will use a 3-stage ADR (Shirron et al., 2012) to cool the microcalorimeter array to 50 mK. In the primary operating mode, two stages of the ADR cool the detectors using superfluid helium at ⩽1.20 K as the heat sink (Fujimoto et al., 2010). In the secondary mode, which is activated when the liquid helium is depleted, the ADR uses a 4.5 K Joule–Thomson cooler as its heat sink. In this mode, all three stages operate together to continuously cool the (empty) helium tank and single-shot cool the detectors. The flight instrument – dewar, ADR, detectors and electronics – were integrated in 2014 and have since undergone extensive performance testing. This paper presents a thermodynamic analysis of the ADR’s operation, including cooling capacity, heat rejection to the heat sinks, and various measures of efficiency.

The Japanese Astro-H mission will include the Soft X-ray Spectrometer (SXS) instrument provided by NASA/GSFC. The SXS will perform imaging spectroscopy in the soft X-ray band using a 6 × 6 array of silicon microcalorimeters operated at 50 mK. The detectors will be cooled by a 3-stage adiabatic demagnetization refrigerator (ADR). The configuration allows the ADR to operate with both a 1.3 K superfluid helium bath and a 4.5 K cryocooler as its heat sink. Initially, when liquid helium is present, the two coldest stages of the ADR will operate in a single-shot mode to cool the detectors from 1.3 K. During this phase of the mission, the 3rd stage may be used to reduce the net heat load on the liquid helium and extend its lifetime. When the liquid is depleted, the 2nd and 3rd stages will operate in a continuous mode to maintain the helium tank at about 1.3 K, allowing continued operation of the 1st stage (in a single-shot mode) and hence the SXS instrument. This paper describes the design and operating modes of the ADR, as well as details of critical components.

In its instrument suite, the Japanese Astro-H mission will include the Soft X-ray Spectrometer (SXS), whose 36-pixel detector array of ultra-sensitive X-ray microcalorimeters will be cooled to 50 mK. This will be accomplished using a two-stage adiabatic demagnetization refrigerator (ADR). A complicating factor for its design is that the ADR will be integrated into a superfluid helium dewar at 1.3 K that will be coupled to a 1.8 K Joule–Thomson (JT) stage through a heat switch. When liquid helium is present, the coupling will be weak, and the JT stage will act primarily as a shield to reduce parasitic heat loads. When the liquid is depleted, the heat switch will couple more strongly so that the ADR can continue to operate using the JT stage as its heat sink. A two-stage ADR is the most mass efficient option and it has the operational flexibility to work well with a stored cryogen and a cryocooler. The ADR’s design and operating modes are discussed, with emphasis on how they reflect the capabilities and limitations of the hybrid cryogenic system.

We have obtained the superfluid fraction of 4 He and mixtures confined in channels 0.0185 μm ×1.08 μm ×∞ . We compare this with film data L×∞×∞, and data for channels 0.0483 μm ×3.0 μm ×∞ . The behavior of the data in the smallest channels is quite different from what one might expect for a planar film of a given thickness. The transition is shifted to a lower temperature than expected; and, the overall behavior of ρs does not follow the trend observed with other confined films. This suggests that the lateral dimension plays a significant role in the behavior. We believe our observations are consistent with the recent proposal of Sobnack and Kusmartsev about a new mechanism for vortex unbinding in 2D films.

We have measured the heat capacity of helium mixtures confined to lithographically created cylindrical boxes whose height, 1.08 μm , equals their diameter. The specific heat at constant concentration x, C px , is renormalized due to the 3 He impurity. Thus, in order to observe critical behavior, a conversion must be made to a specific heat at constant φ=μ 3 −μ 4 , C pφ . The confined system's specific heat, near it's maximum, has values which rise above the bulk system's specific heat at the same temperature. This is unexpected and is true for both Cpx and Cpφ. This enhancement, not observed with pure 4 He , becomes more dramatic as x and the correlation length increase. The shift of the maximum with x is much larger for the boxes than for 2D confined mixtures. These observations might be related to a “band structure” effect associated with the 18.5 nm channels which connect the cylinders.

We report measurements of the specific heat near the superfluid transition of 4 He confined to 1 μm 3 cylindrical boxes patterned in SiO2. This system crosses from a 3D behavior to a 0D behavior near the transition. This has a marked effect on the specific heat as seen by a pronounced rounding of the maximum and a shift to a temperature much lower than the transition of the bulk system (and systems with 2D or 1D crossover). We plot the data according to correlation-length scaling theory and compare this to a planar system with the same smallest confinement. Compared to our previous studies of planar systems, the 0D cell has 3× the surface to volume ratio as well as ∼750× as much edge length. We examine the regions where surface and edge effect contributions can be separated. We find that the data do not reach the expected value for the surface region. There is also evidence for a region where the term associated with edge contributions dominates.

We report new data for 4 He confined between two silicon wafers spaced 9869 A apart. This spacing complements a series of previous measurements which now span a factor of 20 between the smallest and largest confinements. These new data allow us to further check scaling predictions. We find, as reported with previous data, that the present data scale well except near the heat capacity maximum, and below into the superfluid region.

We have previously described a resonance involving the adiabatic motion of 4He from a superleak into a reservoir. We consider here the case of 3He–4He mixtures and show some preliminary results for ρs for confinement to a film with thickness L=0.0483 μm . We relate this to the behavior expected from correlation-length scaling.