Enhanced power absorption in planar microwave discharges

Abstract

The wave propagation and absorption in planar microwave discharges without magnetic fields are calculated with nonuniformity in plasma density taken into account. When the wave is incident in the direction oblique to the density gradient, the field amplitude is strongly peaked at a position where the wave frequency coincides with the plasma frequency. The absorption coefficient of the wave for the oblique incidence is calculated to be much larger than for the parallel incidence. A comparative experiment of the plasma production is performed to find that the power coupling efficiency is larger for the oblique incidence antenna than for the parallel incidence antenna, especially in low collisionality regimes. The comparison between the calculation and the experimental result reveals that the mechanism of the efficient production of overdense plasmas in planar microwave discharges is due to the enhanced electric field near the plasma resonance and subsequent acceleration of electrons in the case of oblique incidence.