Density transition phenomena in plasmas produced by an electron cyclotron wave

Density transition has been observed in He, Ne, N-2 and Ar plasmas produced by an electron cyclotron wave (ECW). The temporal behavior is categorized into three states: stationary high density state, stationary low density state, and autonomous oscillation state between them. Plasma response to the slowly changing external parameters such as input power or operation pressure depends on the time history of those parameters, i.e. the density behavior shows a hysteresis. During the transition oscillation, the neutral density exhibits the out-of-phase oscillation against the plasma density. It is found, by 2-dimensional directional probe measurements, that the high and low density states are characterized by opposite polarity in vorticity distribution even though the E x B plasma flow is in the same direction.