Electron fishbones: theory and experimental evidence

Abstract

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

Linear study of the precessional fishbone instability

“…Many different works have studied this problem, retaining more effects than we shall use. The more general effects can include: gyrokinetic ions 14 , vertical plasma elongation 15 , or a different kind of energetic particles, such as barely trapped/circulating electrons 16,17 .…”

Nonlinear Dynamics and Complex Behaviors in Magnetized Plasmas of Fusion Interest

“…A large variety of Alfvenic oscillations are observed experimentally and there exists an entire "zoology" for their classification based on empirical observations [21]. Here, we want to stress that all these fluctuations can be classified, according to one single unified theoretical framework, as either AUven Eigenmodes (AE) or Energetic Particle Modes (EPM) [6,8,13,22,23,24,25]. In fact, perturbations of the s.A. wave spectrum generally consist of singular (inertial) and regular (ideal MHD) structures.…”

“…In fact, perturbations of the s.A. wave spectrum generally consist of singular (inertial) and regular (ideal MHD) structures. For this reason, via asymptotic analyses it is always possible to derive a general "fishbone-like" dispersion relation in the form [6,8,13,22,23,24,25] iA{co) = 5Wf+5Wk .…”

“…characterized by much larger dimensionless orbits due to limited plasma current [25]. The single and general dispersion relation, Eq.…”

“…Meanwhile, in the high-frequency regime coa <C \co\ <C COA [25,26], with (OBAE = q(Oti{T/^ + Te/TiYl^ being the asymptotic expression of the BAE [29,30] accumulation point in the fluid limit and (Ott = {2Ti/mi)^^^/{qRo). The general kinetic expression of A (A is generally complex) for cOhi<\co\-^ COA is given in Ref [26,36].…”