Magnetic Pinch Effect in the Thermal rf Induction Plasma

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Abstract
The pinch effect or excess magnetic pressure is calculated and measured for an rf induction plasma in argon. This excess magnetic pressure (Pm), which hitherto has been overlooked in all previous investigations concerning the induction plasma, explains the reported anomalous flow behavior of the plasma torch. Theory for calculating Pm is given using a model of the plasma as a solid conductor of constant electrical conductivity. From current density and magnetic field distribution within the plasma, the local Lorentz force is evaluated, and summation across the plasma radius gives the maximum Pm at the center of the peripheral discharge path. Published values for coupling efficiency and temperature profile are employed in conjunction with measured oscillator circuit parameters in order to make the calculation. The calculated Pm at the center of a 4.7 MHz, 2.5 kW argon induction plasma in a 30 mm tube is 64.5 dyn/cm2. At the center of the plasma, Pm was measured with a cooled probe and found to be 71.5±3 dyn/cm2. Although Pm/P≈10−4, the magnetic pressure has an appreciable effect on the fluid mechanics of the discharge. The pinch or compressive force acts inward around the circumference, but not at the ends of the cylindrical plasma. Consequently, a magnetically induced flow occurs out from both ends of the plasma.