Structure of inductively coupled plasma with metal vapor ionization

Structures of nonequilibrium inductively coupled plasmas with cesium metal vapor ionization in argon gas are revealed experimentally and are compared with ones from two-dimensional numerical simulations. The main object of the present paper is to clarify the behavior of the plasma in which cesium atoms as seed atoms are completely ionized, and the ionization of the argon gas as a mother gas is negligible, that is, the fully ionized seed (FIS) plasma, produced by the inductive radio-frequency electric fields. The plasmas are generated in a cylindrical quartz discharge tube around which a four-turn induction coil is wound, under the conditions of currents of <10 A, excitation frequencies ~10 MHz, argon gas pressures 30-50 torr, and cesium mole fractions on the order of 10^-5. By cesium seeding, the quite uniform plasma in the azimuthal direction of the discharge tube is realized even at small coil currents. Under suitable operating conditions, the electron temperatures are in the range of 5000-7000 K near the tube wall, whereas the temperature around the axis is relatively low (~3000 K). Then, the plasma consists of three layers, that is, the weakly ionized argon plasma, the FIS plasma in which the electron density is maintained almost uniform, and the partially ionized seed plasma. The thickness of the FIS plasma is determined by the distribution of the inductive electric field. The experimental results can be explained well by the numerical simulation based on two-dimensional vector potential and two-temperature plasma models