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

Abstract

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 $(^{52}\mathrm{Cr})$ 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 $25\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{K}$, with a density of ${10}^{17}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$, resulting in an increase of the phase-space density, still limited to $7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ 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 $^{5}D_{4}$ metastable states, ${\ensuremath{\sigma}}_{el}=7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}$.