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.

Geodesic acoustic modes (GAMs) are studied as plasma eigenmodes when an electrostatic potential nearly constant around a magnetic surface is applied to collisionless toroidal plasmas. Besides the standard GAM, a branch of low frequency mode and an infinite series of ion sound wavelike modes are identified. Eigenfrequencies of these modes are obtained analytically and numerically from a linear gyrokinetic model. The finite gyroradius effect is found to enhance the collisionless damping of the standard GAM, while this enhancement is not monotonic as the safety factor varies. Moreover, additional damping due to higher-harmonic resonances becomes important when the safety factor increases. The mode structure of the GAM is also discussed.

The existence of unstable ion temperature gradient driven Alfvén eigenmodes (AITG) is demonstrated in tokamak plasmas, which are ideally stable with respect to magnetohydrodynamics (MHD). Conditions for the destabilization of such modes are quantitatively discussed on the basis of numerical solutions of a set of one-dimensional integral equations along the ballooning coordinate (quasi-neutrality and parallel Ampère’s law). Furthermore, theoretical analyses of the eigenmode dispersion relation, which is given in a compact analytical form in the small ion orbit width limit (compared to the radial wavelength), provide a basis for explaining the general properties of the modes. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be not too far away from ideal MHD marginal stability.

The toroidal symmetry of the geodesic acoustic mode (GAM) zonal flows is identified with toroidally distributed three step Langmuir probes at the edge of the HuanLiuqi-2A (commonly referred to as HL-2A) tokamak plasmas for the first time. High coherence of both the GAM and the ambient turbulence for the toroidally displaced measurements along a magnetic field line is observed, in contrast with the high coherence of the GAM but low coherence of the ambient turbulence when the toroidally displaced measurements are not along the same field line. The radial and poloidal features of the flows are also simultaneously determined. The nonlinear three wave coupling between the high frequency turbulent fluctuations and the flows is demonstrated to be a plausible formation mechanism of the flows.

Experiments on HL-2A, DIII-D and EAST show that turbulence just inside the last closed flux surface (LCFS) acts to reinforce existing sheared ExB flows in this region. This flow drive gets stronger as heating power is increased in L-mode, and leads to the development of a strong oscillating shear flow which can transition into the H-mode regime when the rate of energy transfer from the turbulence to the shear flow exceeds a threshold. These effects become compressed in time during an L-H transition, but the key role of turbulent flow drive during the transition is still observed. The results compare favorably with a reduced predator-prey type model.

In this letter, we demonstrate the existence of unstable ion temperature gradient driven Alfvén eigenmodes in tokamak plasmas, which are ideally stable with respect to magnetohydrodynamics (MHD). Conditions for the destabilization of such modes are quantitatively discussed on the basis of theoretical analyses of the mode dispersion relation, which is given in a compact analytical form. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be sufficiently close to ideal MHD marginal stability.

The dynamic features of the low-intermediate-high-(L-I-H) confinement transitions on HL-2A tokamak are presented. Here we report the discovery of two types of limit cycles (dubbed type-Y and type-J), which show opposite temporal ordering between the radial electric field and turbulence intensity. In type-Y, which appears first after an L-I transition, the turbulence grows first, followed by the localized electric field. In contrast, the electric field leads type-J. The turbulence-induced zonal flow and pressure-gradient-induced drift play essential roles in the two types of limit cycles, respectively. The condition of transition between types-Y and -J is studied in terms of the normalized radial electric field. An I-H transition is demonstrated to occur only from type-J.

A systematic study of the electromagnetic effects on the toroidal ion temperature gradient mode is presented using the local and nonlocal theories with the full kinetic terms. For the nonlocal study, a numerical code is developed to solve the electromagnetic gyrokinetic equation in the ballooning space. The electromagnetic coupling to the shear Alfvén mode is shown to give a stabilization of the toroidal temperature gradient mode at almost the same plasma pressure as that at which the kinetically modified magnetohydrodynamic (MHD) ballooning mode becomes destabilized. The transitional β value is shown to be lower in the full kinetic description than in the fluid theory. Possible correlations of these stability results with experimental observations are discussed.