Darmstadt 2008 – wissenschaftliches Programm
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A: Fachverband Atomphysik
A 26: Posters: BECs, ultracold gases and plasmas
A 26.3: Poster
Donnerstag, 13. März 2008, 16:30–18:30, Poster C3
Observations and Simulation of the evaporation process in an optical dipole trap — •Christoph Käfer1, Thomas Boll1, Riad Bourouis1, 2, Jürgen Eurisch1, and Hanspeter Helm1 — 1Department of Molecular and Optical Physics, Stefan-Meier-Str. 19, 79104 Freiburg — 2Institut für Angewandte Physik, Universität Bonn
We investigate the temporal development of atom number and atom temperature in an optical dipole trap which is established by a focused CO2 laser. At full power the trap frequencies in radial and longitudinal direction are vr= 2.4 kHz and vz= 160 Hz. When forcing the evaporation process by lowering the laser power, these trap frequencies and the elastic scattering rate decrease and spilling of atoms prior to rethermalization is playing an ever more important role as the evaporation process proceeds. Also the effect of gravity becomes a crucial factor at the end of the evaporation process. To our knowledge no published models account for all these parameters, and they cannot quantitatively explain the observations. We present a numerical model which can quantitatively account for all these parameters. In our model, the atoms are distributed according to a chopped Maxwell-Boltzmann distribution, where atoms with a kinetic energies above the potential depth are removed from the sample. After calculating the redistributed kinetic energy of two randomly chosen particles after scattering, the energy distribution of the entire sample can be updated and chopped again. In this fashion natural evaporation and forced evaporation can be traced out numerically. We compare predictions of this recursive model to our experimental result.