Mode change of 1 MHz discharge in O-2 gas at low pressure

Radial distributions of plasma density change significantly at pressures above and below 0.2 Torr. At 0.5 Torr, the discharge is almost hollow with high density around the edge, and its color is almost white. At 0.1 Torr, the density distribution becomes high in the central region both axially and radially, and the discharge color becomes yellowish white. Two prominent lines of optical emission show different spatiotemporal profiles. The O-2(+) 559.8 nm line is bright in the central region, while the OI 777.2 nm line is bright just in front of the instantaneous cathode.