Effect of rf driving frequency on peripheral high energy electrons in a magnetically expanding plasma reactor

Two operational rf driving frequencies of 2 and 13.56 MHz are employed in a 14-cm-diameter radio frequency (rf) plasma source under an expanding magnetic field. The changes in the radial profiles of the ion saturation current and the electron temperature are observed in the magnetically expanding plasma when changing the driving frequency. Peripheral high temperature electrons are detected for the higher frequency case, which is consistent with previous studies, implying a localized electron heating in the radially outer region near the antenna and a transport along the magnetic field. However, it disappears when lowering the rf driving frequency, which would be due to an increase in a skin depth. Therefore, the present results demonstrate that the rf power would be absorbed in radially outer and entire regions of the discharge tube for the higher and lower rf driving frequency cases, respectively. As a result of the ionization induced by the peripheral high temperature electrons in the expanding magnetic field, the density in the expanding magnetic field for the 13.56 MHz case is higher than the 2 MHz case, resulting in the larger thrust as measured by a pendulum target technique.