The Physics of Electron Cyclotron Assisted Initiation of Fusion Plasmas*

Electron Cyclotron (EC) wave injection can assist plasma initiation by providing additional heating during the burn-through phase, and also by pre-ionizing the neutral gas (pre-plasma) prior to the application of loop voltage (V_loop). The first mechanism is well understood and can be modelled and extrapolated to ITER, but that is not the case for the pre-ionization mechanism. New experiments have been conducted on DIII-D to improve the physics understanding of the EC plasma initiation process by clearly separating the effect of these two mechanisms. This has been achieved by applying an EC pulse before the V_loop is applied. For this study, pre-plasma formation was characterized in terms of the maximum density and temperature attained prior to V_loop application. Parametric scans were performed to study the influence of the EC injected power, EC injection angle, and pre-fill gas pressure on the pre-plasma creation process. The experiments showed that the pre-plasma assists the subsequent V_loop induced plasma initiation process by providing a high degree of ionization before V_loop is applied. It increases the likelihood of successful burn-through at relatively low values of toroidal electric field (~0.4 V/m). This particular aspect of the data is being modelled with the DYON code.