ON THE LOW-PRESSURE MODE TRANSITION IN ELECTRON-CYCLOTRON HEATED PLASMAS

The properties of plasmas created and sustained by electron cyclotron heating (ECH) have often been found to undergo an abrupt change as the ambient gas pressure is reduced below a value that is typically around 10(-5)-10(-4) Torr. A model is presented in which ambipolar transport of particles and energy are balanced by electron impact ionization and microwave power absorption, respectively. In the absence of magnetic-mirror confinement of high energy electrons, the equilibria predicted by the model exist only for gas pressures greater than a value determiend by the ionization rate constant, the average ion mass, and the axial length (along the steady magnetic field) of the discharge. At the critical pressure, the equilibria bifurcate into high-temperature/low-density and low-temperature/high-density branches. Magnetic-mirror confinement of high-energy electrons can lower the critical pressure and modify the bifurcation somewhat. In all cases the equilibrium electron temperature is governed primarily by gas pressure while the plasma density is proportional to the microwave power density absorbed by the plasma.