Spatial structure of emission from an electrode microwave discharge in hydrogen

The structure of electrode microwave (2.45 GHz) discharges in hydrogen with electrodes of various shapes and sizes at pressures of 1-8 torr and incide nt powers of 2-150 W is studied. It is found that the discharges exhibit a common feature that is independent of the antenna-electrode design: near th e electrode surface, there is a thin bright sheath surrounded by a less bri ght, sharply bounded region, which is usually shaped like a sphere. It is s uggested that the structure observed arises because the microwave field mai ntaining the discharge is strongly nonuniform. Near the electrode, there ex ists a thin dense plasma sheath with a high electron density gradient. A st rong dependence of the electron-impact excitation coefficient on the electr ic field makes the effect even more pronounced. As the electron density dec reases due to dissociative recombination, the microwave field gradient decr eases and the discharge emission intensity lends to a near;ly constant valu e. Presumably, in the boundary region of the discharge, there exists a surf ace wave, which increases the emission intensity at the periphery of the di scharge. (C) 2000 MAIK "Nauka/lnterperiodica".