Orifice probe for plasma diagnostics: II. Multi-parameter analysis

Abstract

A multi-grid orifice probe and analysis technique are presented which permit the measurement of the following plasma parameters: the electron number density ne, the electron temperature Te, the space potential Vs, and an estimate of the metastable number density nm. Simultaneously, the analysis allows the quantitative determination of instrumental parameters such as the reflection coefficient for slow (<30 eV) electrons from the grids Re(G) and roughened (gold-black) collector Re(C), the secondary electron emission coefficient for fast (> 30 eV) electrons γe(G) from the grids, and the effective transparencies of the grids for electrons τe(G), and for metastables τm(G). Experimental ion, electron and total current characteristics are presented for a low-pressure (1·8×10−2 Torr) RF discharge in argon. The major part of the analysis method involves the use of ten `plateau values' obtained from the characteristics to determine values for most of these parameters. Te and Vs are determined separately. The values obtained are: ne=2·74×109 cm−3±18%, kTe=1·92 eV±5%, Vs=11·7±0·1 V, nm similar, equals 3·3×109 cm−3±50%, Re(G)=0·65±0·10, Re(C)=0·22±0·10, γe(G)=0·935±0·010, τe(G)=0·830±0·002, and τm(G)=0·850±0·002. The transparencies τe(G) and τm(G) are within 4% of the geometrical transparency τ(G)=0·860. Approximately equal numbers of atoms (1 in 2×105) are in metastable and ionized states.