Ion flows in the vicinity of a plasma limiting aperture

A steady-state argon plasma, produced by a helicon antenna in a 0.6-m long, 15-cm dia. chamber in a Heimholtz magnetic-field geometry, flows through a co-axial magnetic nozzle coil and a 1-cm aperture. The ion gyroradius is ∼0.4 cm. After passing through the aperture, the plasma enters a Pyrex expansion region (ER). In this experiment, the aperture plate was placed at three different positions: in the main chamber, in the magnetic nozzle region, and in the ER. Spatially resolved Laser-induced fluorescence (LIF) was used to determine the ion flow speed. The flow speed measurements on the upstream side of the aperture show clear pre-sheath ion acceleration and speeds close to ion sound speed are observed at the closest observation point to the aperture. The increase in the ion kinetic energy after passing through the aperture ranges from 8 eV to 22 eV for rf-powers of 500 to 1100 Watts. The ion beam energy is proportional to the floating potential of the aperture plate. However, biasing the aperture more negative than the floating potential does not increase the exit speed. Biasing the aperture more positive than floating potential decreases the exit speed.