Physics-based modeling of RF-assisted start-up in tokamaks: effect of magnetic field on impact-ionization

A tokamak relevant physics-based model has been developed for the energization of electrons undergoing elastic and impact ionization collisions with atoms and molecules of a gas. In the presence of a background electrostatic potential, the energization is due to electron cyclotron (EC) heating by a spatially localized Gaussian beam. In contrast to Monte-Carlo based techniques in which impact ionization is predominantly based on the motion of an electron parallel to the magnetic field, we follow the complete orbit in a magnetic field. Electrons that are predominantly energized by EC transverse to the magnetic field while being slow in their toroidal motion, have the same probability for impact ionization as the toroidally fast ones. The dominant energization appears in the gyro motion and does not affect the cross-section for impact ionization. From our simulations we can estimate the time needed to ionize a neutral gas for plasma startup in a tokamak. The ionization time depends on power in the EC beam, and on the loop voltage and neutral gas pressure. We will present results from our simulations, in particular, for ITER relevant parameters. We will also discuss an analytical formalism that gives a measure of the startup time and agrees with the simulations.