Effects of DC Preionization Voltage and Radial Location on Pulsed Inductive Plasma Formation

The effects of dc preionization (PI) voltage and radial location on plasma formation repeatability are presented for argon prefills of 20-200 mtorr with a discharge energy of 79.5 J at 15 kV. Current profiles of a ringing theta-pinch are compared with circuit simulation in SPICE to estimate plasma resistance and inductance and quantify plasma formation uncertainty. Plasma thickness is calculated using axial imaging and experimental geometry and used to determine the mutual inductance coupling of the plasma and the theta-pinch coil. At all pressures tested, plasma formation failed to occur in the absence of dc PI. At pressures less than 100 mtorr, PI voltage has a significant impact on plasma formation, repeatability, and energy coupling into the plasma. At 20 mtorr, 0.20 W of dc power is sufficient to stabilize plasma formation at the first zero-crossing of the current. With 1.5 W, an additional 39% of inductive energy is coupled into the plasma. Increasing pressure also increased plasma repeatability and resulted in a convergence of plasma circuit parameters.