PREIONIZATION OF THE POSITIVE-COLUMN OF A HIGH-FREQUENCY CAPACITIVE DISCHARGE BY ELECTRONS FROM THE ELECTRODE SHEATH IN A MOLECULAR GAS-FLOW

A high-frequency capacitive discharge under conditions typical of prel iminary ionization is studied. Unlike in external preionization, the e lectrons in the discharge gap are produced by the ionization of gas in the positive column by electrons from the electrode sheath during the corresponding phase of the column field. For this purpose, it is nece ssary to decrease the electric field and temperature in the sheath, wh ich prevents the screening of the sheath by a weak or even reverse fie ld in the dark Faraday space and, thereby, prevents electron drift int o the positive column. The time and corresponding velocity are also re quired for the drifting electrons to hp the discharge gap and ionize t he gas. These conditions can be fulfilled in the subnormal regime of t he HF discharge with a frequency of 1.76 MHz in a high-velocity gas ho w when there is a phase shift between the sheath and positive-column f ields.