Modelling of the positive column of a medium-pressure Cs–Xe dc discharge affected by a millimetre wave pulse

Abstract

A time-dependent zero-dimensional kinetic model of the positive column (PC) of a medium-pressure Cs–Xe dc discharge was used to gain a better insight into the physical basics of plasma techniques for imaging and control of millimetre wave (MMW) beams. The model allowed one to study the effect of MMWs on the kinetic and electrical characteristics of the spatially homogeneous PC of a Cs–Xe dc discharge. We computed the PC plasma parameters for 30 Torr and 45 Torr xenon and discharge current densities of about 0.1 A cm−2. First, the dependences of the PC parameters on caesium density were calculated in the case of no MMWs incident on the PC plasma. Then, the temporal evolution of the parameters of the PC plasma affected by a long watt-scale Ka-band MMW pulse was modelled for caesium densities of about 3 × 1012 and 5 × 1012 cm−3. The calculations showed that the electron temperature in the PC plasma attained quasisteady-state values for about 1 µs after the beginning of the MMW pulse. The electron temperature rises by 0.2–0.3 eV as the MMW intensity increases from 0 to 1 W cm−2. The rise time of the electron density decreased with an increase in the MMW intensity W from about 1 ms for W = 0.15 W cm−2 to tens of microseconds for W > 1.5 W cm−2. The steady-state values of the electron density increased in proportion to W, if W < 0.5 W cm−2. They were approximately constant for 0.5 < W < 2 W cm−2 due to the nearly full ionization of caesium atoms in the PC plasma. Efficient xenon excitation and ionization for W > 3 W cm−2 could be a cause of the microwave breakdown of the homogeneous PC plasma. The results of the modelling are in good agreement with the published experimental data.