Double-kink fishbone instability caused by circulating energetic ions

The destabilization of double kink modes by the circulating energetic ions in tokamaks with the plasma current having an off-axis maximum is studied. It is shown that the high-frequency fishbone instability [Energetic Particle Mode (EPM)] and the low-frequency (diamagnetic) fishbones are possible for such an equilibrium, their poloidal and toroidal mode numbers being not necessarily equal to unity. A new kind of the EPM instability, ''doublet fishbones,'' is predicted. This instability is characterized by two frequencies; it can occur in a plasma with a non-monotonic radial profile of the energetic ions when the particle orbit width is less than the width of the region where the mode is localized. It is found that the diamagnetic fishbone branch exists even when the orbit width exceeds the mode width; in this case, however, the instability growth rate is relatively small

LOCALIZED FAST MAGNETOACOUSTIC EIGENMODES IN TOKAMAK PLASMAS

ABSTRACT. An equation is derived for fast magnetoacoustic eigenmodes (FMEs) in a tokamak fusion plasma with an elongated cross-section. It is shown that edge localized fast magnetoacoustic waves (FMWs) with both positive and negative poloidal wavenumbers are possible, provided that the productna 2 exceeds a certain critical magnitude Ncr, wheren is the plasma density at a point near the plasma edge and a is the plasma radius. Fulfilment of this condition in various devices is analysed, which is important in explaining the fine structure of the spectrum of suprathermal ICE from tokamak plasmas. It is found that a small population of high energy ions with a non-equilibrium distribution function can significantly affect the FMEs

Low frequency fishbone mode induced by circulating particles in spherical tori

It is found that high beta in low-aspect-ratio tori tends to stabilize the fishbone instability in a plasma with energetic circulating ions. The stabilization results from enhancement of the toroidal drift motion by large Shafranov shift, which makes it difficult to reconcile the condition of considerable energy exchange between the ions and the internal kink perturbation with the condition of the resonant wave-particle interaction

Non-conventional Fishbone Instabilities

New instabilities of fishbone type are predicted. First, a trapped-particle-induced m = n = 1 instability with the mode structure having nothing to do with the conventional rigid kink displacement. This instability takes place when the magnetic field is weak, so that the precession frequency of the energetic ions is not small as compared to the frequency of the corresponding Alfven continuum at r = 0 and the magnetic shear is small inside the q = 1 radius [the case relevant to spherical tori]. Second, an Energetic Particle Mode fishbone instability driven by circulating particles. Third, a double-kink-mode instability driven by the circulating energetic ions. In particular, the latter can have two frequencies simultaneously: we refer to it as ''doublet'' fishbones. This instability can occur when the radial profile of the energetic ions has an off-axis maximum inside the region of the mode localization

Effect of Trapped Energetic Ions on MHD Activity in Spherical Tori

It is shown that the increase of beta (the ratio of plasma pressure to the magnetic field pressure) may change the character of the influence of trapped energetic ions on MHD stability in spherical tori. Namely, the energetic ions, which stabilize MHD modes (such as the ideal-kink mode, collisionless tearing mode, and semi-collisional tearing mode) at low beta, have a destabilizing influence at high beta unless the radial distribution of the energetic ions is very peaked

Destabilization of fast magnetoacoustic waves by circulating energetic ions in toroidal plasmas

An instability of fast magnetoacoustic waves (FMW) driven by circulating energetic ions in axisymmetric toroidal plasmas and characterized by the frequencies below the ion gyrofrequency is considered. An important role of the l=0 resonance (l is the number of a cyclotron harmonic) in the wave-particle interaction is revealed: It is shown that this resonance considerably extends an unstable region in the space of the pitch-angles of the energetic ions and the wave frequencies. The analysis is carried out for a ''slow'' instability, which has the growth rate less than the bounce frequency of the energetic ions. Specific examples relevant to the National Spherical Torus Experiment (NSTX) [J. Spitzer et al., Fusion Technol. 30 (1996) 1337], where instabilities of this kind were observed, are considered