PIC-MCC Simulation Of Low Pressure Capacitively Coupled Hydrogen Plasmas

Hydrogen plasmas are widely used in material technology, nuclear fusion and high-energy physics. Especially in the nuclear fusion field, hydrogen plasmas used as the main reaction materials in the neutral beam heating reactor have been intensively studied in relevant theoretical, experimental and simulation aspects. Especially, negative hydrogen ion sources are particularly indispensable for the neutral beam heating in the future fusion reactors because of the high neutralization efficiency of negative hydrogen ions in the MeV energy range in which the neutralization efficiency of positive hydrogen ions is almost zero. One of the most important problems we have to solve is how we can produce the negative hydrogen ions efficiently via the volume production. In this paper, we use the PIC-MCC model to search for the way of producing the negative hydrogen ions more efficiently.In Chapter one, we briefly introduce some basic characters of the plasma and the relevant backgrounds of the paper. In Chapter two, the one-space-dimension and three-velocity-dimension electrostatic PIC-MCC (Particle-in-Cell with Monte Carlo collision) method used in this paper is described detailedly. In Chapter three, the PIC-MCC model is used to investigate the low pressure capacitively coupled hydrogen discharges driven by combined radio frequency (rf) and pulse sources. The rf source is used for providing the necessary electrons to maintain the plasma, while the high and short pulse source is used for modulating the electron energy and enhancing the plasma density and the production rate of highly vibrationally excited hydrogen molecules. The characters of the plasma driven by single rf source and dual-source are described respectively.In the single rf source discharge, the simulation displays the electron oscillations and the formation process of the plasma sheath. In the same time, it shows that the electron energy distribution functions (EEDF) is approximately two temperature distribution, which is the typical character of the stochastic heating mechanism in low pressure capacitively coupled plasmas.In the dual-source discharge, we display the spatiotemporal evolutions of the electron energy and density respectively to show the effect of the pulse source in the discharge. The simulation results show that the short and high pulse source can modulate the energy of electron effectively:in the early and late pulse-on time, the electron energy increases rapidly to a very high value, which is much higher than that in the single rf source discharge; during the plateau time, the electron energy drops rapidly to a low value, but it is still higher than that in the single rf source discharge; in the early pulse-off time, a few peaks of attenuated electron energy appear periodically; as time goes, these peaks become weak, and disappear gradually. Similar phenomena can be found in the production rate of highly vibrationally excited hydrogen molecules. In the paper, we discuss these phenomena detailedly. The phenomena are mainly caused by the strong electric field which is induced by the high and short pulse source. The rapid formations and disappearances of the strong electric field greatly influence the electron energy and density distributions in the discharge space, and then cause the above phenomena. The simulated results demonstrate that in the dual-source capacitively coupled discharges, the short and high pulse source can modulate the electron energy and increase the plasma density and the production rate of highly vibrationally excited hydrogen molecules effectively.