The spatial profiles of charged particles in a multipolar magnetically confined Ar- plasma

Abstract

The spatial profiles of charged particles in multipolar magnetically confined Ar- plasmas produced by dc discharges are investigated by varying the total pressure from 2 to 10 mTorr, the discharge current from 10 to 90 mA and the oxygen content from 4 to 20%. The measured spatial negative ion and electron density profiles, which are approximated as a parabolic distribution and a uniform one except in the vicinity of the chamber wall, respectively, allow us to divide the entire discharge into three regions: (i) an electronegative region, (ii) an electropositive region and (iii) the sheath. The ratio of the negative ion density to the electron density measured at the centre increases with increasing oxygen content and pressure, but decreases with increasing discharge current. Oxygen content- and discharge current-dependences of the electronegative region's length are similar to those of the ratio, although the length does not strongly depend on the pressure. A global model, in which the flux balance equations for charged particles and a power balance equation for electrons are taken into account, to estimate plasma parameters such as the individual charged particle density and electron temperature is proposed, and then compared with the experimental results.