He flow rate measurements on the engineering model for the Astro-H Soft X-ray Spectrometer dewar

The sixth X-ray Japanese astronomy satellite, namely Astro-H, will be launched in 2015. The Soft X-ray Spectrometer onboard the Astro-H is a 6 x 6 X-ray microcalorimeter array and provides us with both a high energy resolution of <7 eV at 0.5-10 key and a 3' x 3' modest imaging capability for the first time. To cool the detector down to the operation temperature of 50 mK, five cryocoolers, a 30-L superfluid helium cryostat, and a 3-stage adiabatic demagnetization refrigerator are utilized. A very small heat load up to similar to 0.9 mW on the helium tank is allowable to realize the helium lifetime of >3 years, which consequently requires that the vapor flow rate out of the helium tank should be very small <42 mu g/s. We adopted a porous plug phase separator in combination with a film flow suppression system composed of an orifice, a heat exchanger and knife edge devices to retain the liquid helium under zero gravity and safely vent the small amount of the helium vapor. We measured helium mass flow rates from the helium tank equipped in the engineering model dewar. We tilted the dewar at an angle of 75 degrees so that one side of the porous plug located at the top of the helium tank attaches the liquid helium and the porous plug separates the liquid and vapor helium by thermomechanical effect. Helium mass flow rates were measured at helium tank temperatures of 1.3, 1.5 and 1.9 K. We confirmed that resultant mass flow rates are in good agreement within the systematic error or low compared to component test results and achieve all the requirements. The film flow suppression also worked normally. Therefore, we concluded that the SXS helium vent system satisfactorily performs integrated into the dewar. (C) 2014 Elsevier Ltd. All rights reserved.