Sorption Cryocoolers for Planck
Building on the successful BETSCE program, JPL is presently working on the development of a hydrogen sorption cryocooler for the Planck mission of the European Space Agency. The objective of the Planck mission is to produce very high resolution mapping of temperature anisotropy in the cosmic microwave background (CMB) radiation. The Planck spacecraft, shown in concept above, is scheduled to be launched around 2007 into a deep-space L2 Lagrangian orbit in order to reduce stray infrared radiation from earth and to permit passive cooling of the telescope and optical system to 50 to 60 K. In addition to the 50 K passive cooling, the two key instruments are using an active cooling system of three cryocoolers to achieve the low temperatures required to measure the CMB. The Low Frequency Instrument (LFI) will have an array of tuned radio receivers based on High Electron Mobility Transistors (HEMT) to detect radiation in the range 30-100 GHz. These receivers will be operated at a temperature of about 20 K. The High Frequency Instrument (HFI) will use bolometers operated at 0.1 K for frequencies from 100 GHz to 900 GHz. Redundant hydrogen sorption cryocoolers are being developed to cool the LFI detectors to 18 - 20 K and to precool the RAL 4 K helium J-T that cools the 0.1 K dilution refrigerators in the HFI cooling system .
Planck Sorption Cooler Features
Shown above is the compressor assembly of the 20 K sorption cooler being built by JPL for the Planck instruments. The compressor assembly contains multiple individual compressor beds, each filled with powdered LaNi4.78Sn0.22 alloy for the reversible absorption and desorption of hydrogen gas. During operation of the cooler the compressed refrigerant H2, desorbed from a heated compressor element (~465 K), is precooled in a heat exchanger and expanded through the J-T expander orifice to create a gas/liquid refrigerant mixture at T< 18 K. The liquid hydrogen evaporates as it absorbs heat from the instruments and is warmed as it returns through the heat exchanger to be absorbed in a cool (~270 K) compressor element. By mounting and heatsinking the compressor assembly to the Planck spacecraft platform, no cryocooler heat is dissipated on the cooled telescope.
Closed-cycle operation is achieved as six compressor beds are switched through each step in the complete heating-cooling cycle, which takes a little over an hour. At any point in time one sorbent bed will be heating to pressurize, one will be hot to desorb and supply pressurize H2 gas, one will be cooling to depressurize, and three beds will be cold to absorb the returning gas. Switching is accomplished by turning on and off electrical heaters embedded in the compressor elements. Gas-gap thermal switches are incorporated into the compressor element design to provide thermal isolation of the bed while in the heating and desorbing phases of operation and to make the thermal connection to the ~270 K heat sink during the cooling and absorbing phases of operation.
Stable cold end temperatures are to be achieved through maintaining constant pressure at the J-T expander and the liquid reservoirs. To help accomplish this behavior, a reservoir tank is added to stabilize the high pressure at the compressor outlet and an extra sorbent bed is maintained at the ~270 K sink temperature to stabilize the low pressure by simulating an approximately 200 liter plenum volume. A gas manifold with passive check valves, one inlet and one outlet for each compressor element, complete the compressor assembly and is used to direct the gas flow.
The cold stage includes a contamination filter, a porous plug flow restrictor as the J-T expansion device, and three liquid hydrogen reservoirs. The first reservoir of each cooler is mated to the 4 K helium J-T cooler to provide around 0.1 W of precooling at ~ 18 K. The second liquid reservoir is used to provide the rest of the refrigeration required to cool the LFI, shield the HFI, and to intercept parasitics to both instruments, which totals approximately 1.3 W at 20 K. The third reservoir is controlled at about 22 K to wick and evaporate any excess liquid refrigerant.
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Latest update: June 14, 2012