Experimental Studies of Nonlinear Plasma Effects with Slot Antennas in Ionized Helium

摘要

Pulsed RF breakdown for a slot antenna was investigated. In particular, the formative time lag for breakdown, tau was measured as a function of gas pressure, power level, pulse repetition rate, pulse width, and imposed preionization level. Measurements revealed that breakdown initially occurs only after a large number of pulses according to an accumulative electron density buildup scheme which is statistical in nature. The relationship between tau and power and pressure was similar to what was measured in waveguides with the breakdown field strength nearly equal to the maximum field in the waveguide feeding the antenna. Tau increased markedly with the time between pulses, T, particularly at the lower pressures. After a few milliseconds tau increased linearly with T consistent with an exponential decay of the plasma sheath and diffusion being the only electron removal process during the formative time lag. Various imposed preionization levels were furnished by striking a pulsed DC discharge to the ground plane of the antenna and selecting various times, t, during the afterglow for the high power RF illumination. The density across the central portion of the DC discharge vessel N1(o) was measured by microwave diagnostic techniques. Tau increased by decreasing N1(o) and increasing t; the relationship between tau and t was found to be linear indicating an exponential decay of the local electron density the afterglow, which was in accordance with theoretical computations. As expected, tau did not depend on the high power RF pulse width. (Author)