Plasma Evolution around a Gridded Sphere*

Argon plasma was produced at 0.125 mbar by a spherical stainless-steel mesh (1 mm x 1 mm mesh) cathode of 55 mm diam. The sphere had an orifice of 17 mm diam. on its horizontal axis. Plasma structures inside and around the sphere and its orifice were observed for bias 0 to –630 V and back. Inside a spherical structure appears, one structure surrounds the grid and one appears as a hyperboloid outside the orifice. By £ –280 V the plasma appears, and all three structures are present. The most intense is the hyperboloid one, then the inner and outer structure. For more negative bias, the inner structure increases and the hyperboloidic structure creeps out through the orifice. At –630 V the hyperboloid disappears, the inner plasma is the most intense and expands, while the outer becomes a background plasma. When going back, hysteresis is observed. The hyperboloid plasma appears again and retreats into the hole. The inner plasma ball moves towards the tip of the hyperboloid plasma until they fuse, and the plasma disappears at –252 V. Optical emission spectroscopy was performed at the center of the sphere and at the orifice level at –461 V and 0.1 mbar. The temperatures calculated were similar: –5940 K at the center and 6000 K at the orifice.