Behavior of fully ionized seed plasma excited by microwave

Nonequilibrium plasmas with cesium metal vapor ionization in helium and argon gases at moderate pressures are excited with microwave power. The structures and behavior of the seeded plasmas are experimentally examined, particularly under the condition of full seed (cesium atoms) ionization. By cesium seeding, the minimum power sustaining the plasma is reduced markedly, and both a broad plasma observed in pure helium and unsteady filament-like plasmas in pure argon change to the steady and broad plasma locating close to the inner surface of a discharge tube. It is revealed from the electron temperature measurements that the plasma can he in the regime of full seed ionization for suitable microwave powers, where the electron density is kept almost constant. The thickness of the fully ionized seed (FIS) plasma decreases with increasing the mole fraction of cesium vapor, and is almost independent of noble gas pressure. The thickness almost coincides with the skin depth determined from the electrical conductivity almost uniform in the FIS plasma. These facts suggest that the FIS plasma will be easily produced and maintained as long as the microwave power is consumed to the electron heating.