Propagation of a de-excitation wavefront in the positive column of a pulsed Xe discharge

Abstract

When a fast rise pulse voltage is applied to a low pressure, 2-10 mm diameter Xe discharge tube, emission covers the entire positive column region with the breakdown. Within a microsecond, however, the emission intensity starts to decrease from the cathode end of the column and the dark region spreads towards the anode. The speed of this 'de-excitation wavefront' varies from 0.5*103 to 2.5*103 m s-1, depending on the applied voltage, and also on the capacitance which is in parallel with the cathode resistor. The propagation of the de-excitation wave is considered to be due to the migration of excess electrons from the cathode towards the anode, followed by the reduction of impedance, and hence the electric field, of the positive column. In a 1 mm diameter tube, however, the de-excitation wave is not observed, and the emission extinguishes simultaneously over the entire positive column 0.4 mu s after the breakdown. The difference between the two extinction modes is attributed to the discharge current density. It is also shown that, in the positive column, excited species which emit vacuum ultraviolet photons are generated mainly during the initiation stage of the breakdown period. The VUV radiation efficiency can therefore be increased by narrowing the width of the pulse voltage.