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 (⁵²Cr) 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 250.3em0exμK, with a density of 10¹⁷0.3em0exatoms0.3em0exm^-3, resulting in an increase of the phase-space density, still limited to 7.0×10^-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 ⁵D₄ metastable states, σₑₗ=7.0×10^-160.3em0exm².