Jae-Min Song

The effect of collisions on the motion of magnetized ions in sheath and presheath plasma regions was investigated through the measurement of ion incident angle of a hydrogen ion at a graphite surface. The experiment was conducted in hydrogen and deuterium plasmas where the ion mean free path is 5–10 times larger than the ion gyro radius and with varying magnetic field angle ψ from 0° to 90° normal to the target surface. The hydrogen ions actively reacted with carbon, leading to the formation of conical tips with axes directed along the incident ion flow direction. The ion incident angle was measured from the etched graphite images taken by scanning electron microscopy. The measured angles were compared to those calculated using Ahedo's fluid magnetic sheath model. In addition, we adopted the nominal Bohm criterion at the electrostatic sheath edge due to the larger ion gyro radius than the sheath. The results show that the ion incident angle was inclined to the normal direction with respect to the magnetic field angle because of the effect of ion collisions on ion motion in the presheath. The collisional effect on the ion motion is drastic for an oblique magnetic field angle ψ > 85°. This study demonstrates that the collisional property of the ions is crucial to guide the ion motion in magnetic (pre)sheath and to determine the ion incidence angle at the surface, even in collisionless and weakly magnetized plasmas.

The characteristics of a two–radio-frequency (RF)–driven dual antenna inductively coupled hydrogen plasma is investigated for the development of a high efficient RF negative hydrogen ion source driver. The two-RF-driven dual antenna system consists of a 2 MHz–driven solenoidal antenna wound around a cylindrical chamber and a 13.56 MHz–driven planar antenna placed on top of it. Compared to the conventional single frequency antenna inductively coupled plasmas, the two-RF-driven dual antenna inductively coupled plasma reveals two distinctive features, i.e., an increase in the power transfer efficiency and the bi-Maxwellization of the electron energy distribution function due to the collisionless heating. These characteristics allow the two-RF-driven dual antenna inductively coupled plasma to accomplish enhanced generation of negative ions and their precursors with a high RF efficiency.