Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

We study the continuous accumulation of cold atoms from a magneto-optical trap (MOT) into a finite depth trap, consisting in a magnetic quadrupole trap dressed by a radiofrequency (rf) field. Chromium atoms (Cr-52) in a MOT are continuously optically pumped by the MOT lasers to metastable dark states. In the presence of a rf field, the temperature of the metastable atoms that remain magnetically trapped can be as low as 25 mu K, with a density of 10(17) atoms m(-3), resulting in an increase of the phase-space density, still limited to 7.0x10(-6) by inelastic collisions. To investigate the thermalization issues in the truncated trap, we measure the free evaporation rate in the rf-truncated magnetic trap, and deduce the average elastic cross section for atoms in the D-5(4) metastable states, sigma(el)=7.0x10(-16) m(2).