Cryocooler options for NGST and other space applications

We have assessed the technology status of long-life cryocoolers for space applications below 20 K. Two-stage Joule-Thomson (J-T) cryocoolers are found to be the best candidates for a possible Next Generation Space Telescope (NGST) mid-infrared camera. A combination of a 20 K hydrogen J-T cooler using a sorption compressor precooling a 4 K helium J-T cooler using a mechanical compressor will be flight proven on Planck and provides the lowest risk option for NGST. The 4 K mechanical cooler can be replaced by a 6 K cooler that uses helium as a working gas and charcoal sorbent material in a J-T cycle to provide a vibration-free continuous-cycle cooler. We present a conceptual design and predicted performance for such a helium/charcoal sorption cooler. To provide 10 mW heat lift at 6 K such a cooler would require a total input power of approximately 93.4 W, and have a system mass of approximate to 33 kg. These coolers will also provide enabling technology for future missions, such as TPF, SPECS and IRSI.