The Characteristics of Closed-Loop Inductively Coupled Low Mercury Pressure Discharge

Abstract

The characteristics of inductively coupled ferrite-free low mercury pressure discharges generated in closed-loop 815-mm-long quartz tubes 16.6 mm in diameter are studied. Discharges were excited at a frequency of 13.56 MHz with plasma power ranging from 10–270 W in a mixture of mercury vapor (~0.01 Torr) and argon (0.5, 1.0 and 2.0 Torr) by means of an induction coil placed over the discharge tube wall external perimeter. With a growth of plasma power, the lower the argon pressure, the faster the growth rates of the RF voltage across the induction coil and the current through it. The plasma impedance as a function of plasma power calculated within the framework of the inductively-coupled discharge transformer model has a minimum, which shifts toward higher plasma power values with increasing the argon pressure. The lower the argon pressure the higher the RF electric field averaged over the plasma cross section, and the lower the plasma power at which the dependence of discharge current on plasma power reaches its maximum.