Ionization waves in plasma columns detected by microwaves

Abstract

Ionization wave frequency spectra and their spatial distribution along argon and neon plasma positive columns are detected by microwaves in the 8–18 GHz frequency band. The plasmas are created at medium pressure by d.c. discharges in narrow tubes. Detection takes place in a rectangular waveguide on the microwave fields reflected and transmitted by plasma columns which are placed transversely across the waveguide in different positions and at several values of the discharge current. The frequency of the microwaves is adjusted for a parametric resonance condition with respect to Langmuir's electron frequency. The properties and dispersion relations of these waves are derived from a linear theory developed by K. W. Gentle, in which, for discharge operation below the Pupp limit, metastables are also included in the ionization and species conservation equations. An estimate of the total collision frequency is derived from a simplified two-fluid model valid essentially near the Pupp equilibrium limit.