Electron energization mechanisms in microwave-driven surface wave discharges at low pressures

Surface wave discharges driven at microwave frequencies are often used in plasma assisted technologies due the ease of their sustainment over large areas and a wide range of discharge parameters. One of the central questions of discharge sustainment is how electrons gain from the electric field enough energy to ionize neutral gas. Using an example of plasmaline-based surface wave discharge studied with a fully electromagnetic implicit energy- and charge-conserving code, we show that in overdense plasmas produced at low pressures the plasma resonance mechanism is either eliminated or strongly weakened by the oscillating plasma sheath at the dielectric supporting the surface wave propagation. Instead, electrons acquire their energy primarily through interaction with the sheath during its expansion. This conclusion agrees well with experimental observations made in previous literature.