Start-up of tokamak plasmas using electron cyclotron waves – theory and modelling.

Electron cyclotron wave power is being used for initiating neutral gas ionization during the start-up phase of ITER. The EC field is launched as a spatially localized Gaussian beam propagating through the gas. Stray electrons, energized by their cyclotron resonance interaction with the beam, impact ionize gas atoms and molecules creating additional electrons. When a sufficient number of electrons are present, an avalanche process is initiated giving rise to a partially ionized plasma. The lead up to the avalanche phase is modelled by the nonlinear interaction of seed electrons with the beam in the presence of a static, homogeneous magnetic field. The analytical cross-sections for impact ionization and elastic electron-neutral collisions are included in the simulations as we follow the dynamics of each electron. The time required to trigger an avalanche and also the electron density growth rate, depend on the beam power, its spatial width and polarization, the direction of propagation, and the pressure of the neutral gas. We will present detailed results that follow from our studies.