Dynamics of neutral gas depletion investigated by time- and space-resolved measurements of xenon atom ground state density

The dynamics of neutral gas depletion in high-density plasmas is investigated by time- and space-resolved measurements of the xenon ground state density. Two-photon absorbed laser induced fluorescence experiments were carried out in a helicon reactor operating at 10 mTorr in xenon gas. When the plasma is magnetized, a plasma column is formed from the bottom of the chamber up to the pumping region. In this situation it is found that two phenomena, with different time scales, are responsible for the neutral gas depletion. The magnetized plasma column is ignited in a short (millisecond) time scale leading to a neutral gas depletion at the discharge centre and to an increase of neutral gas density at the reactor walls. This is explained both by neutral gas heating and by the rise of the plasma pressure at the discharge centre. Then, on a much longer (second) time scale, the overall neutral gas density in the reactor decreases due to higher pumping efficiency when the magnetized plasma column is ignited. The pumping enhancement is not observed when the plasma is not magnetized, probably because in this case the dense plasma column vanishes and the plasma is more localized near the antenna.