Collisionless motion and evaporative cooling of atoms in magnetic traps

We analyze the collisionless motion of atoms in magnetic traps in relation to evaporative cooling. For the example of a long Ioffe quadrupole trap we investigate both the regular and stochastic regimes of motion. We emphasize a strong influence of the regime of collisionless motion on the process of evaporative cooling. For evaporation of atoms across an axial potential barrier the stochastic motion of atoms at energies above the barrier enables three-dimensional evaporation, i.e., particles acquiring in elastic collisions a total enery higher than the barrier height ${\mathit{E}}_0$ escape from the trap. The regular motion leads to axial evaporation: only atoms which due to collisions acquire an axial energy higher than ${\mathit{E}}_0$ leave the trap. The rate of axial evaporation is smaller by a factor ∼η=${\mathit{E}}_0$/T≫1, where T is the gas temperature. This has important consequences for evaporative cooling, which we discuss in relation to trapped atomic hydrogen. ©1996 The American Physical Society.