Non-adiabatic particle dynamics in a time-dependent magnetic trap

Magnetic trapping has become a standard technique for confining and evaporatively cooling ultra-cold samples of neutral atoms. One particular variety of magnetic traps, the time-orbiting-potential (TOP) trap, uses a rotating bias field which results in a harmonic potential for the trapped atoms if its rotational frequency (typically around 10 kHz) is much larger than the oscillation frequency of the effective harmonic potential. Furthermore, the Larmor precession of the atomic spin about the instantaneous magnetic field is usually neglected. Theoretical studies show, however, that when these two approximations are lifted, the TOP-trap exhibits non-adiabatic features. In particular, dropping the first approximation leads to a micromotion of the trapped particles at the frequency of the bias field, whereas when the Larmor precession of the spin is included, it is found that the equilibrium position of the atoms depends on the sense of rotation of the bias field. In spite of the wide-spread use of TOP-traps, so far no comprehensive experimental studies of such effects exist. In this work, we study these non-adiabatic effects by observing Bose-Einstein condensates in a triaxial TOP-trap for a variety of trap parameters. The results of our experiments are in good agreement both with analytical models and numerical simulations.