Pulsed microwave plasma production in a conducting tube with a radius below cutoff

High-power (60-100 kW), short-pulse (0.05-1.0 mu s) microwaves of 3 GHz wit h a repetition frequency of 10-500 Hz are used to produce a plasma in a pre ssure range of 10(-2)-10 Torr in a circular conducting tube with a cross se ction below the cutoff value for the fundamental waveguide mode (TE11). The waves are launched perpendicular to the cusped magnetic field, treated by permanent magnets surrounding the tube. Results indicate that, the plasma d ensity is higher at the tube entrance and at a location inside the tube nea r the exit. From regions of higher density, plasma diffuses towards the tub e center with the radial diffusion suppressed by the magnetic bottle. The d ensity uniformity is better below 0.1 Torr. The repetition frequency has on ly a small influence on the discharge properties, although the peak current and the decay time constant increase with pulse duration. The electron tem perature is 6-10 eV even in the power-off phase and the current build-up is delayed to a few to tens of microseconds from the end of the pulse. These findings indicate distinct differences from conventional afterglows. (C) 20 00 Elsevier Science Ltd. All rights reserved.