Production of pulsed microwave plasma in a tube with a radius below the cut-off value

A plasma is produced by pulsed microwaves in a circular tube with a conducting wall and a radius below the cut-off value for the fundamental circular waveguide mode (TE11). The discharge tube is surrounded with a multicusp magnetic field constructed with permanent magnets which provide a minimum-B configuration. Short pulse (0.05-1.0 mu s) microwaves of 3 GHz and 60-100 kW peak power with a repetition frequency of 10-500 Hz are launched into the narrow tube with the propagation vector perpendicular to the cusped magnetic field in a pressure range of 10(-2)-10 Torr. It has been found that, in addition to occurring at the entrance, a plasma is produced at a location of about one-third length from the tube exit. Plasmas generated at the two locations diffuse mostly along the tube axis with the radial diffusion strongly suppressed by the multicusp field. The density uniformity is better at lower pressures. The spatio-temporal evolution of electrons and ions shows a density growth after the end of a microwave pulse for a few to tens of microseconds depending on the axial position, followed by decay. The profiles have been explained by numerical simulation based upon a model that charged particles are driven by electrostatic and ponderomotive forces and diffuse along the longitudinal direction. The interpulse regime has an electron temperature of about 10 eV. This indicates that the plasma is still active between the pulses, in distinct contrast with the usual afterglows.