CℓOVER is a multi-frequency experiment optimised to measure
the Cosmic Microwave Background (CMB) polarization, in
particular the B-mode component. CℓOVER comprises two
instruments observing respectively at 97 GHz and 150/225 GHz.
The focal plane of both instruments consists of an array of
corrugated feed-horns coupled to TES detectors cooled at 100
mK. The primary science goal of CℓOVER is to be sensitive to
gravitational waves down to r ~ 0.03 (at 3σ)in two years of operations.
We describe a miniature dilution refrigerator (MDR), operated in continuous mode and suitable for many detector applications at temperatures down to 50 mK. 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 100 mK. This system is being developed for a CMB polarization experiment (CLOVER) which requires three independent cryostats to cool large TES detector arrays.