The structure of nonequilibrium electrode microwave discharge in nitrogen

The structure of an electrode microwave discharge in nitrogen at a pressure of 1 torr and incident power of up to 500 W is studied by the optical and probe methods, when the size of the plasma formation is much less than the size of the discharge chamber. It is shown that the discharge has a spheric al structure and a bright glowing film near the electrode surface. This coi ncides with the structure obtained in a discharge in hydrogen. The observed differences, in particular, a more symmetric shape of the discharge relati ve to the electrode end, may be due to the different mechanisms of ionizati on and excitation of discharges in hydrogen and nitrogen. In nitrogen, the role of excited particles is significant, which results in a possibility of existence of a discharge in lower fields. The measurements with a double p robe have shown that the voltage-current characteristic is shifted along th e voltage axis and passes through zero current at nonzero voltage. This may be interpreted as the presence of constant fields in the plasma. This shif t slightly depends on the radial position of the probe in the discharge, de creases drastically on the discharge boundary, and is absent outside of the glowing spherical region of the plasma. The emergence of such fields may b e due to the nonlinear interaction of the microwave field with the plasma. The constant field may prove to be a determining factor in the formation of a spherical structure. Spherical structures are known to form when a de vo ltage is applied to the electrodes in the plasma. The measurements of the p otential difference between the antenna and the probe placed in the plasma have shown that this potential difference is of the order of kT(e) and weak ly grows with an increase of incident power. This explains the absence of i ntensive ionic bombardment and electrode sputtering in the microwave discha rge plasma.