Macroscopic plasma oscillation bursts (fishbones) resulting from high-energy populations

Abstract

Oscillation bursts (fishbones) of magnetically confined plasmas are associated with the excitation of an m0=1 mode when the ideal magnetohydrodynamic (MHD) threshold for the instability of this mode is reached. Near this threshold and in the absence of an effective ‘‘viscous’’ dissipative process, this mode remains marginally stable as a result of finite ion Larmor radius effects and has a real frequency of oscillation near the ion diamagnetic frequency. The release of the mode excitation energy related to the gradient of the plasma pressure, in the case of perpendicular neutral beam injection, is allowed by the resonant interaction of the mode with fast trapped ions that precess around the torus as a result of the curvature and the gradient of the confining magnetic field. This process plays the role of an effective viscosity. A consequence of the presented interpretation is that fishbone oscillations may also be excited in the case of parallel beam injection. In fact, for relatively low values of the beam transverse pressure, the basic mode frequency is related to the parameters of the target plasma and does not depend on the magnetic drift frequency of the beam particles that are injected nor on their velocity distribution.

The behaviour of fast ions in tokamak experiments

“…In these models, the beam beta is effectively clamped near the point of marginal stability. With reasonable choices of the free parameters in the models, the theories can match the time evolution of the magnetics and neutron signals for selected cycles in PDX [338,370,387, 3891, TFTR [338] and J E T [388]. (A systematic comparison with a large set of data has not yet been performed.)…”

“…Several semi-empirical models have been proposed t o explain the evolution of the fishbone cycle [338,370,387,3881; the simplest formulation is in Ref. [389].…”

Study of Interaction between Fast Ions and MHD Instabilities by Fusion Product Measurements in Joint European Torus

“…Trapped and circulating beam-ions resonate with the core-localised m = 1 / n = 1 mode [26]. Drops in the neutron rate dominated by beam-plasma DD reactions of up to 10% are associated with this activity in JET plasmas [27].…”

Global Hybrid Simulations of Energetic Particle Effects on the n=1 Mode in Tokamaks: Internal Kink and Fishbone Instability

“…Early work used phenomelogical predator-prey models [18,22,23] to explain the mode saturation and bursting due to resonant particle loss. More recently, Candy et al [24] simulated a full cycle of fishbone oscillations by using a simplified hybrid model where a fixed internal kink mode structure was used.…”