Radiation Losses From a Microwave Surface-Wave Sustained Plasma Source (Surfatron)

Surfatron is an electromagnetic (EM) field applicator yielding surface-wave (SW) discharges (SWDs) within dielectric tubes in a broad range of operating conditions. In this article, the vector wave equation for the electric field is solved numerically in 2-D to determine the influence of plasma parameters, such as electron density, normalized electron-neutral collisions frequency, plasma column length and radius, as well as the length of the surfatron body on the field strength distribution, radiation patterns, and ratio of radiated power to power absorbed in plasma. This analysis allows for estimating the efficiency of plasma generation. The model takes into account the surfatron configuration, reproducing the typical SWD plasma column behavior, namely the fact that the plasma column is “pushed out” of the surfatron interstice as absorbed power increases. The surfatron is assumed to be placed in an air-filled sphere with the surface transparent to the wave. Calculations are performed for a field frequency of 2450 MHz and a fused silica tube, considering plasma parameters typical for both atmospheric- and lower-pressure discharge gases for achieving a parametric survey. It is found that for given plasma parameters, varying the plasma column length from a few up to 30 cm significantly affects the distribution of the electric field around the surfatron and the radiation patterns. It is further shown that the radiated to absorbed power ratio increases with increasing electron density and plasma radius, and decreasing electron-neutral collision frequency. This ratio can exceed 30% for a 30-cm plasma column with a plasma radius of 3 mm and parameters typical for atmospheric and reduced gas pressures. The surfatron length at which radiation losses are minimal is at half the vacuum wavelength.