The purpose of the NICMOS Cooling System (NCS) is to enable
continued operation of NICMOS by cooling the detectors to a scientifically useful temperature. This is achieved by a closed-loop circuit which runs cryogenic gas through a coil inside the NICMOS dewar. The NCS provides excellent temperature control, the detectors are maintained at 77.15ħħ0.10K. There is margin in the compressor speed to compensate for variations in the parasitic heat load due to orbital and seasonal changes in spacecraft attitude as well as in the operation of the HST science instruments in the aft shroud.
The NCS consists of three major subsystems: 1) a cryocooler which
provides the mechanical cooling, 2) a Capillary Pumped Loop (CPL) which transports the heat dissipated by the cryocooler to an external radiator, and 3) a circulator loop which transports heat from the inside of the NICMOS dewar to the cryocooler via a heat exchanger. Additional elements of the NCS are the Power Conversion Electronics (PCE) which provides the up to 400 W of power needed by the cooler, and the Electronic Support Module (ESM) which contains a microprocessor to control the heat flow. Figure E.1
shows a schematic of the system.
The cryocooler, manufactured by Creare, Inc., is a reverse-Brayton
cycle turbine design. The compression and subsequent expansion of the Neon gas results in a net cooling which is used to remove heat from the NICMOS dewar via the heat exchanger to the circulator loop (see Figure E.1
). The Creare design has several major advantages for application on HST. First, the closed loop system operates at very high speed—about 7000 revolution per second—which limits the probability of mechanical coupling to the HST structure, thus minimizing the risk of spacecraft jitter. Second, the system is capable of providing large cooling power. Finally, the Creare design is compact enough to fit into the previously unused space between the NICMOS enclosure and the HST aft end bulkhead.
On average, the cryocooler dissipates about 375 W of power which
needs to be removed from the aft shroud. This is achieved via a Ammonia-filled Capillary Pumped Loop (CPL) that thermally connects the housing of the compressor pump with an external radiator. The continuous heat flow through the (passive) CPL lines is maintained by a set of heaters that are controlled by the ESM micro controller.
The heat from inside NICMOS is transported to the cryocooler via a set
of flex lines. The flex lines connect to the inside of the dewar via bayonet fittings at an interface plate outside of the dewar which was accessible to the astronauts during SM3B. The circulator loop was then filled with Neon gas from high pressure storage bottles. The bottles provide enough gas to purge, pressurize, and, if necessary, re-pressurize the circulator loop. The Neon gas, driven by a high speed circulator pump and cooled via the heat exchanger to the cryocooler loop, circulates through the cooling coil at the aft end of the NICMOS dewar (see Figure E.2
), thus cooling the entire instrument.
The ESM uses the telemetry readings from the inlet and outlet
temperature sensors at the NICMOS/NCS interface in an active control law to regulate the cooling power of the NCS, and thus provide stability of the operating temperature of the NICMOS detectors. Because of the sensitive dependence of a number of detector characteristics on temperature (see Chapter 7
), the temperature stability is a crucial requirement on NCS performance. To date, the stability has been extremely good with T=77.15 with a variation of ~0.10K
All components of the NCS are combined into a common enclosure, the
NICMOS Cryo Cooler (NCC). Figure E.2
shows a line drawing of the NCC, without any Multi-Layer Insulation (MLI). The system was successfully tested in space during the HST Orbital Systems Test (HOST) shuttle mission in 1998. Remaining uncertainties about the NCS performance stemmed from the natural lack of tests with the actual NICMOS dewar. This led to a longer than expected cooling time following the NCC installation, but has not affected the temperature stability.