Testing Method to Determine the Thermal Parasitics of the NICMOS Cryogenic Cooler

The NICMOS Cryogenic Cooler (NCC) is a turbomachine-based reverse-Brayton cycle cryocooler combined with a single-phase cryogenic heat transport loop. The NCC will be installed on the Hubble Space Telescope (HST) on a future servicing mission, and will provide refrigeration to the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) detectors that were previously cooled by a dewar of sublimating solid nitrogen. The NCC is designed to remove a total of 7 W of heat and maintain the NICMOS detectors at 77 K. The 7 W heat load is comprised of the dissipation from the NICMOS detectors and the parasitics into the heat transport loop.

During testing of the NCC it was crucial to quantify parasitic heat loads in the heat transport loop since this governs the minimum temperature that can be achieved at the NICMOS detectors. The determination of parasitics was aided by the installation of a well-instrumented section of tubing, referred to as a shunt, that connected the inlet and outlet heat transport lines of the NCC. This shunt and its instrumentation were thermally isolated from the NCC environment so as not to introduce any additional parasitics. Using this shunt, we were able to determine the mass flow rate of the neon cooling gas and the distribution of parasitics throughout the heat transport loop of the NCC.

The value of the method developed herein is twofold: (1) it provided a means to reliably quantify the cooling capability of the two-loop NCC; and (2) it provided a means to quantitatively evaluate modifications to the NCC to lower NICMOS detector temperature for better detector performance.