Some considerations on the microwave electrodeless discharge : Plasma diagnostics in argon, helium or nitrogen atmospheres

Abstract

Plasma diagnostics of several microwave plasmas are determined by making electrical (with double floating probes) and optical measurements in pure Ar, He or N 2 plasmas, and also in Ar plasmas containing various metals, i.e. Cs, Tl or Zn; plasma parameters, such as, electric field ( E), electron ( j e) and ion ( j i) current densities, electron density ( n e), electron temperature ( T e) electron conductivity (σ e), ion density ( n i), electron mean free path (λ e) electron (μ e) and ion (μ i) mobilities and electron [( v e) drift] and ion [( v i) drift] volocities are either directly measured or calculated. The reversal temperature ( T r) of excited (0.96 eV lower level) thallium atoms is measured, and the steady-state conditions of the plasma are analyzed by the energy balance equation. The experimental measurements indicate that the electric field strength E decreases as the space charge decreases (ionization extent) increases. Although the plasma appears to be under steady-state conditions, it is not under local thermodynamic equilibrium conditions, i.e. T e > T r. In addition, the measurements indicate that there is a deficiency of electrons in the plasma ( n e < n i), probably due to electron affinity processes; and the plasma has a small positive space charge.