A miniature dilution refrigerator for sub-kelvin detector arrays

We describe a miniature dilution refrigerator (MDR). operated in continuous mode and suitable for many detector applications at temperatures down to 50mK. It distinguishes itself from other refrigerators in that it is self-contained and benefits from an internal cycle of the 3He gas. As a result, no external gas handling system is required so size, weight and complexity of the system is dramatically decreased. The system has no fine capillaries, moving parts or cooled O-rings. It is therefore mechanically very reliable, has no risk of blockages and is unlikely to develop cryogenic leaks. One direct application is balloon-borne or ground-based observations of the CMB using large detector arrays, When these experiments are operated remotely on platforms or at sites with limited infrastructure and maintenance support, a compact and reliable dilution refrigerator becomes essential. We describe a complete system incorporating an MDR which we have built and integrated with a pulse-tube refrigerator to achieve a cooling power of several micro Watt at 100mK. This system is being developed for a CMB polarization experiment (CLOVER) which requires three independent cryostats to cool large TES detector arrays.