Direct Measurement of the Maintenance Voltage of a Thermal Induction Plasma

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Abstract
The axial distribution of voltage induced near the inner wall of a 3.8-cm-diam tube confining an rf argon induction plasma at atmospheric pressure was measured at a flow rate of 115 cm3/sec with a water-cooled ring probe. Also measured as functions of probe position were the excitation frequency, voltage, and current on the inductor coil. The plasma reduced the probe voltage to about 60% of its value in the absence of the plasma. After correction for the effects of cooling and displacement of the plasma by the probe, the total induced voltage around the circumference of the plasma at the midsection was found to be 64 V. The corresponding value for electric-field intensity was 6 V/cm. This value falls between published values obtained by indirect methods and is in fair agreement with prediction on the basis of a one-dimensional theory.