Computer experiments on electromagnetic environment of plasma sheath at conducting surface

Abstract

We focused on the electromagnetic environment of the sheath region created at the antenna surface by performing EM-PIC (Particle-In-Cell) simulations. In the previous theoretical studies electromagnetic wave modes called sheath waves are found, which propagate along the plasma–metal interface on the assumption that the sheath region is vacuum. To examine the property of the sheath waves, we adopted a two-dimensional model which includes a metal surface of antenna immersed in magnetized background plasma. Due to the difference of thermal velocity between electrons and ions in the isothermal plasma, an ion sheath where electron density is low is basically created at the antenna surface. In the simulations, we could basically confirm the sheath waves propagating along the metal–plasma interface at the frequencies lower than those of normal wave modes for uniform plasma with no external magnetic field. In magnetized plasma, we also could observe the sheath waves below the Upper Hybrid Resonance frequency. However, in the dispersion relation obtained by taking the Fourier transformation of electromagnetic components, the intensity of the sheath waves is rather weak and difficult to be recognized. The difference between the theoretical analysis and the present PIC simulations can be accounted for by the difference of electron density profiles in the sheath region. In the simulations, we found that the electron density gradually changes at the transition region from the background plasma to the sheath while, in the theory, a step function between a vacuum sheath and the background plasma is assumed. In the simulation of a large sheath case where low density region at the antenna surface can be assumed as a vacuum region, sheath waves are clearly observed as predicted in the linear dispersion relation. We could also confirm unidirectional waves propagating below the UHR frequency.