The macroscopic coherent tunneling through the barriers of a periodic potential is used as an atomoptical filter to separate the condensate and the thermal components of a $^{87}$Rb mixed cloud. We condense in the combined potential of a laser standing-wave superimposed on the axis of a cigar-shape magnetic trap and induce condensate dipole oscillation in the presence of a static thermal component. The oscillation is damped due to interaction with the thermal fraction and we investigate the role played by the periodic potential in the damping process.
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