Generation of energetic electrons during X2 ECRH start up

Abstract In the initial stages of plasma initiation in the Wendelstein7-X stellarator with electron-cyclotron-resonance heating, a downshifted electron cyclotron emission signal is observed. Due to the absence of a corresponding upshifted signal, this signal is believed to come from energetic electrons (∼10 keV). We propose a mechanism for the generation of these fast electron based on the overlap of cyclotron resonances on flux surfaces close to the centre of the plasma. Multiple passages through these resonances lead to the stochastisation of particle trajectories, which ultimately results in the formation of a quasi-steady-state electron distribution function that is predominantly flat in energy space across regions of resonance overlap. The electron cyclotron emission of such a distribution function agrees with the experimental observations.