The Spatio-Temporal Evolution of Laser-Induced Initial Plasma in Rf ICP Source for Cfetr Nbi

Abstract

The basis for the negative hydrogen ion-based China Fusion Engineering Test Reactor (CFETR) neutral beam injection (NBI) is the radio frequency (RF) inductively coupled plasma (ICP) source, which operates at 2 MHz under the gas pressure of 0.3 Pa. However, it is difficult for the RF ion source to ignite the plasma at such low gas pressure. Thus, the formation of the initial RF plasma is very important for improving the stability of the ion source. In our work, it is demonstrated that pulsed laser ablation with a tungsten (W) target can cause the formation of initial plasmas, as measured by ICCD camera. Meanwhile, a two-dimensional self-consistent fluid-plasma model is used to study the spatio-temporal evolution of the laser-induced plasma in RF ICP source. In the experiment, the gas pressure is kept at 0.3 Pa, and the laser power density is kept at 2 GW/cm2. The results show that the spatio-temporal evolution of the plasma remains almost unchanged after 10 μs and has similar profile for various powers. The results of our model fit well with the experimental data. In the simulation, the plasma expansion progress is much slower at a higher pressure due to the increased collision processes. Moreover, when the initial electron production rate given in this model increases, more time is required to reach a steady state plasma. It is expected that the results can provide more insight into the plasma expansion process of laser-induced ignition by RF ICP source.

Keywords: RF ICP source, fluid plasma model, laser-induced ignition, spatio-temporal evolution of the plasma