Second harmonic ECRH breakdown: a theoretical insight and comparison to experimental results from L-2M

The breakdown by the 2nd electron cyclotron harmonic microwaves is studied theoretically, with a particular regard to the geometry and experimental set-up of the L-2M stellarator. The collisionless time-averaged electron dynamics in a microwave field is re-examined. There is a widespread opinion that breakdown in stellarators and tokamaks is governed by deeply trapped electrons that undergo nonlinear regime of energy oscillation. Our analysis has shown that there exist four groups of electrons differing by the scale of energy oscillation, and that all these groups participate in the breakdown, although with different roles. We have determined the boundaries of all groups in the 2D parametric space, their relative population and the rate of energy gain. This allowed us to find out which of the groups is actually responsible for the electron avalanche onset near the magnetic axis and how the breakdown expands then across the magnetic surfaces. Theoretical estimates show good agreement with the L-2M experimental results.