Magnetically enhanced electromagnetic wave penetration in weakly ionized plasmas

The homogeneity and size of radio-frequency and microwave-driven plasmas are often limited by insufficient penetration of the electromagnetic radiation. To investigate increasing the skin depth of the radiation, we consider the propagation of electromagnetic plane waves in a weakly ionized plasma immersed in a steady magnetic field where the dominant collision processes are electron-neutral and ion-neutral collisions. Retaining both the electron and ion dynamics, we have adapted the theory for cold collisionless plasmas to include the effects of these collisions and obtained the dispersion relation at arbitrary frequency omega for plane waves propagating at arbitrary angles with respect to the magnetic field. We discuss in particular the special cases of magnetic field-enhanced wave penetration for parallel and perpendicular propagation, examining the experimental parameters which lead to electromagnetic wave propagation beyond the collisional skin depth. Our theory predicts that the most favourable scaling occurs for waves propagating nearly parallel to Band for omega << Omega(e) where Omega(e) is the electron cyclotron frequency. The scaling is less favourable for propagation perpendicular to B, but the skin depth does increase for this case as well, provided that omega << omega(LH) where omega(LH) is the lower hybrid resonance frequency. Still, to achieve optimal wave penetration one must design tine plasma configuration and antenna geometry so that one generates primarily the appropriate angles of propagation.