Electron fishbones: theory and experimental evidence

Abstract

We discuss the processes underlying the excitation of fishbone-like internal\nkink instabilities driven by supra-thermal electrons generated experimentally by\ndifferent means: Electron Cyclotron Resonance Heating (ECRH) and by Lower Hybrid\n(LH) power injection. The peculiarity and interest of exciting these electron fishbones\nby ECRH only or by LH only is also analyzed. Not only the mode stability is explained,\nbut also the transition between steady state nonlinear oscillations to bursting (almost\nregular) pulsations, as observed in FTU, is interpreted in terms of the LH power\ninput. These results are directly relevant to the investigation of trapped alpha particle\ninteractions with low-frequency MHD modes in burning plasmas: in fact, alpha\nparticles in reactor relevant conditions are characterized by small dimensionless orbits,\nsimilarly to electrons; the trapped particle bounce averaged dynamics, meanwhile,\ndepends on energy and not mass.