Electron energy distributions in a radiofrequency plasma expanded by permanent magnets

Electron energy probability functions (EEPFs) inside the plasma source expanded by permanent magnets are measured using an rf-compensated probe and a pulsed probe technique, where the plasma is terminated by an electric double layer (DL) near the source exit. The machine has a 6.5-cm-diameter source and a 26-cm-diameter diffusion chamber; a diverging magnetic field is provided by permanent-magnet arrays. In the present experiments, the rf power and the gas pressure are maintained at 250 W and 0.35 - 2.0 mTorr, respectively. In the plasma source upstream of the DL, EEPFs are observed to be Maxwellian up to the break energies corresponding to the potential drop of the DL at the radially central part, and to the sheath voltage of the floating wall at the peripheral part. Above the break energy the EEPFs show the depleted tails; the tail electrons at the central part and near the source wall contribute to neutralizing the DL-induced supersonic ion beam by overcoming the potential drop of the DL, and charging the source wall electrically. (author)