Measurement of the rf Magnetic Field Distribution in a Thermal Induction Plasma

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Abstract
The radial distribution of the rf magnetic field in the equator section of a 25-kW argon induction flame maintained at 2.6 MHz in a 155-mm-wide tube is measured with a 3-mm-diam water-cooled search coil. It is found that in the annular space between plasma flame and inductor coil the field is increased by about 10% from an unperturbed value of 15 A/cm whereas in the flame interior it drops to a value of 0.8 A/cm at the axis. The penetration depth is of the order of 1 cm from which a mean value for the plasma conductivity of 10 mho/cm is estimated. From the field distribution a variation in magnetic pressure over the plasma radius of 16 dyn/cm2 is calculated which is in order-of-magnitude agreement with values quoted in the literature. Probe errors caused by signal averaging, current obstruction, and plasma cooling are discussed and found to be of minor consequences under the test conditions employed.