Pic Simulation Of The Nonlinear Oscillation Induced By A Nanosecond Pulse In Rf Capacitive Discharges

Negative ion beams have some special advantages, such as high neutralization efficiency and little surface charging, compared with positive ion beams. Therefore, these advantages entitle them to play an important role in many applications, such as neutral beam injectors, accelerators, and industrial ion beam processing. For next generation fusion devices like ITER, instead of commonly used positive ion source based neutral beam injectors (NBI) in present fusion devices, negative ion source based NBI are required based on the fact that when the ion energy is up to400KeV the neutralization efficiency of the negative hydrogen ion could keep60%whereas the positive hydrogen ion is close zero.As is well known, there are volume production and surface-conversion mechanisms for generate negative-ion-source. Negative ion sources based volume production mechanism are safe and maintenance-free, therefore they are ideal. At present the volume production of negative hydrogen ion need to be greatly increased to meet the requirement of ITER.A particle-in-cell simulation was employed to investigate the effects of the extra short pulse source on rf capacitive hydrogen discharges. It was found that the applied nanosecond pulse converted the plasma quickly from the bi-Maxwellian equilibrium formed in the rf capacitive discharge into another temporal bi-Maxwellian equilibrium. When the applied electric field collapses within a few nanoseconds, the electric field arising from the space charge serves as a restoring force to generate a swift oscillation of the electrons. The energy stored in the plasma is converted gradually into the chemical energy during the electron periodic movement. It is also found that the rise-, plateau-, and fall-times of the applied pulse affect the evolution of the electron energy distribution. The collective electron oscillation has a repetition frequency approximately equal to the electron plasma frequency, independent of pulse rise-, plateau-and fall-times. This oscillation of electrons induced by a nanosecond pulse can be used to generate highly excited vibrational states of hydrogen molecules, which are a necessary precursor for negative hydrogen ions.