We report experiments on evaporative cooling of spin-polarized atomic hydrogen gas confined in a cryogenic magnetic trap, and present a model of the trapped gas based on a generalized truncated Boltzmann approximation for the phase-space distribution function. The model takes into account the dimension of the evaporation [three-dimensional (3D) or 1D] and the time dependence of both the depth and the shape of the confining potential. Our observations are consistent with 1D evaporative cooling. To attain maximal phase space density, we used the model assuming 1D evaporative cooling to optimize the evaporation procedure. With this work, the low dimension of evaporation was identified as the bottleneck thus far preventing us from achieving Bose-Einstein condensation in trapped atomic hydrogen.

Phys. Rev. A

Pinkse, P. W. H., Mosk, A., Weidem, M., Reynolds, M. W., Hijmans, T. W., & Walraven, J. T. M. (1998). One-dimensional evaporative cooling of magnetically trapped atomic hydrogen. Phys. Rev. A, 57, 4747–4760.