Donald Wegel

Design of a 3-stage ADR for the soft x-ray spectrometer instrument on the ASTRO-H mission

The Japanese Astro-H mission will include the Soft X-ray Spectrometer (SXS) instrument, whose 36-pixel detector array of ultra-sensitive x-ray microcalorimeters requires cooling to 50 mK. This will be accomplished using a 3-stage adiabatic demagnetization refrigerator (ADR). The design is dictated by the need to operate with full redundancy with both a superfluid helium dewar at 1.3 K or below, and with a 4.5 K Joule-Thomson (JT) cooler. The ADR is configured as a 2-stage unit that is located in a well in the helium tank, and a third stage that is mounted to the top of the helium tank. The third stage is directly connected through two heat switches to the JT cooler and the helium tank, and manages heat flow between the two. When liquid helium is present, the 2-stage ADR operates in a single-shot manner using the superfluid helium as a heat sink. The third stage may be used independently to reduce the time-average heat load on the liquid to extend its lifetime. When the liquid is depleted, the 2nd and 3rd stages operate as a continuous ADR to maintain the helium tank at as low a temperature as possible - expected to be 1.2 K - and the 1st stage cools from that temperature as a single-stage, single-shot ADR. The ADRs design and operating modes are discussed, along with test results of the prototype 3-stage ADR.

OPTIMIZATION OF A TWO‐STAGE ADR FOR THE SOFT X‐RAY SPECTROMETER (SXS) INSTRUMENT ON THE ASTRO‐H MISSION

NASA/Goddard Space Flight Center has begun developing the Soft X‐ray Spectrometer (SXS) instrument that will be flown on the Japanese Astro‐H mission. The SXS’s 36‐pixel detector array will be cooled to 50 mK 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 stages are operated independently, and this opens up a very large parameter space for optimizing the design. This paper discusses the optimization process and most relevant trades considered in the design of the SXS ADR, and its expected performance.