| Ion-beam-plasma system exists widely in diverse areas of plasma physics, e.g., in low temperature plasma-surface interactions, in space plasma environment, and in the excitation of collective waves and instabilities. The investigation of such system has potential applications in plasma material processing as well as in basic plasma physics research. Among them, the sheath structure near a wall or an electrode immersed in the beam-plasma system is one of the important topic. In previous studies on the excitation ion modes and sheath structures using an ion-beam-plasma system, the ion beam energy and flux (or current) ratio were not controlled independently, which makes the explanation and comparison with theoretical model difficult. The influence of an ion beam on an ion sheath was once studied theoretically and experimentally on the upstream side. However, the difference between the sheath structures on the up-and downstream sides of a wall (or electrode) and its relation with the ion beam has not been investigated.In order to study the characteristics of the ion-beam-background-plasma system, the experiments were carried out in a modified double plasma device (DPD), in which two separation grids (instead of one as in a conventional DPD) with a bias difference were used between source and target chambers to generate the ion beam that streams from the source into the target chamber. The ion distribution function (IDF) in the experimental region (target chamber) was measured with a retarding field energy analyzer (RFEA) and its dependence on discharge parameters was investigated. By appropriately adjusting the relevant discharge parameters, it is shown that the ion beam energy and flux ratio can be controlled independently. The effects of the ion beam on the up-and downstream sheaths near a metal plate and a mesh were studied in detail. The main conclusions are as follows:1. The ion distribution function exhibits a double-peak structure containing a back-ground and a streaming-ion (ion beam) groups. The beam energy can be controlled nearly independently by either the voltage drop between the two separation grids or the discharge voltage, while these two parameters have little effect on the beam flux ratio. The beam flux ratio can be controlled mainly by the hot-filaments heating current (which has minor effect on the beam energy), with a supplemental adjustment of the voltage drop between the two separation grids or the discharge voltage. However, the neutral gas pressure causes simultaneous changes of both the beam energy and the flux ratio and is inappropriate for the independent con-trol. This study provides a method for controlling the beam energy and the beam flux ratio independently that are applicable for the investigations of the sheath structure and the wave excitations in the beam-plasma system.2. Measured sheath potential profiles near the negatively biased metal plate immersed in the beam-plasma system exhibit asymmetric structure, showing thicker sheath in the downstream side. The presence of the ion beam causes the shrink of the sheaths on both sides. The sheath thickness decreases with the increase of beam energy and density. Furthermore, when the plate is replaced with a mesh, the sheaths near the mesh are substantially thinner than that near the plate because of the partial transmission of the mesh to the ions. In addition, the increase of neutral gas pressure leads to the reduction of the beam energy and density, resulting in the increase of the sheath thickness.The remaining part of this thesis deals with the construction of a Steady Magnetic Mirror (SMM) device and the excitation of electrostatic ion cyclotron wave (EICW). EICW is an ion mode in a magnetized plasma propagating in the direction almost normal to the magnetic field. Nearly all previous experiments on the EICW were carried out in Q-machines, in which the plasma is a slender column of only a few cm in diameter and the means of excitation is current drive along the magnetic field, thus, the wave propagation is very limited. Our newly build SMM device has much larger diameter (～80 cm) with moderate magnetic field (-1200 Gauss in the center of the magnetic mirror) and is in steady state operation, which is ideal for the studies of the EICW excitation and the magnetized plasma sheath as well as other basic researches. Using the coils of the SMM, the discharge properties and the preliminary experiments on EICW were carried out in a small test vacuum chamber. The main results are as follows:1. In hot filaments discharges, the probe I-V characteristics show two electron temper-atures in the plane composed of magnetic field and filament (plane of ionization), corresponding to the primary electrons and background ones. However, in the re-gions between two such planes, only single electron temperature is found in the Ⅰ-Ⅴ trace, which is the background electron diffused cross the magnetic field from the plane of ionization. Thus, the plasma is nonuniform across the magnetic field.2. A wave, of which the direction of the propagation is perpendicular to the magnetic field, is excited by applying an external signal to a small mesh whose surface is parallel to the magnetic field. The frequency of the excited wave increases with the magnetic field strength, and the measured dispersion relation is in good agreement with EICW, verifying the feasibility of the externally driven EICW in a large scale. |
PDF Full Text Request