Table of contents

The convective modes of an inhomogeneously drifting plasma in a shear magnetic field (a generalization of Suydam's problem) is considered. It is shown that a sufficiently great shear of E×B velocity drift suppresses the instability in the case of an arbitrary `magnetic hill'. This result can be considered again as a Rayleigh theorem analogue.

A set of transport equations is analysed, including the bifurcation of the radial electric field in toroidal helical systems. Calculations are made simulating the compact helical systems experiments. Both hard and soft transitions are found in the profile of the radial electric field. Whether the electric domain interface exists or not is found to depend on the ratio of the electron temperature to the ion temperature. The structure of the electric domain interface is also studied. The steep gradient of the radial electric field is obtained and the width of the electric domain interface is determined by the anomalous diffusivity of the electric field. The region where the electron root and ion root co-exist is obtained when changing the density or the heating power of electrons. The various types of the electrostatic potential structures are found. The condition for the turbulence suppression is examined in the parameter regime studied here.

During `type III' edge-localized modes (ELMs) on Alcator C-Mod, electron cyclotron emission (ECE) diagnostics show brief signal drops of second harmonic X-mode and signal increases of fundamental harmonic O-mode. These are explained in terms of refraction effects and are found to be useful to infer the associated ELM geometrical dimensions. A new ray tracing code, which can accommodate poloidal variations, has been developed for this investigation. The ELMs are modelled satisfactorily as a density loss from a poloidally elongated region.

Internal transport barrier (ITB) formation in the positive shear zone of reverse shear (RS) plasmas in the JT-60U tokamak is described as a series of ITB events, which are abrupt in time and wide in space variations of heat diffusivity, observed as the simultaneous rise and decay of the electron temperature T_e in two zones. A new source of heat pulse propagation (HPP) is found in RS plasmas. HPP is created by an ITB event. For the last ITB event described in the present paper, the region of strong (~20 keV s^-1) T_e rise is initially well localized (~4 cm) in space. Later, a slow diffusive broadening of the rising T_e perturbation is seen. Outward HPP is analysed in the region with ~8 cm width fully located in the positive shear space zone. Values of the electron dynamic heat diffusivity as low as ~0.1 m² s^-1 are found. A similar low value of the ion dynamic heat diffusivity (close to the neoclassical value) is obtained for ion HPP. An important consequence of HPP analysis is the absence of electron and ion `heat pinch' in the ITB region.

The formation of a core region with improved electron confinement is reported in the recent full current drive operation of Tore Supra, where the plasma current is sustained with the lower hybrid (LH) wave. Current profile evolution and thermal electron transport coefficients are assessed directly by using the data of the new fast electron bremsstrahlung tomography that provides the most accurate determination of the LH current and power deposition profiles. The spontaneous rise of the core electron temperature observed a few seconds after the application of the LH power is ascribed to a bifurcation towards a state of reduced electron transport. The role of the magnetic shear is invoked to partly stabilize the anomalous electron turbulence. The electron temperature transition occurs when the q-profile evolves towards a non-inductive state with a non-monotonic shape, i.e. when the magnetic shear in the plasma core is reduced to close to zero. The improved core confinement phase is often terminated by a sudden MHD activity when the minimum qapproaches 2.

Passing particles in toroidal geometry are described in a Hamiltonian formalism including time-dependent electric and magnetic fields. These particles are characterized by a non-vanishing toroidal velocity. The introduction of the toroidal angle as independent variable instead of the time allows one to derive a map of the poloidal plane onto itself, which is similar to the Poincaré map of magnetic field lines. In time-dependent fields the energy of the particles is not conserved leading to two coupled maps, which is characteristic for autonomous systems with three degrees of freedom. As a result, Arnold diffusion occurs and Kolmogorov-Arnold-Moser (KAM) surfaces, which in the case of energy conservation separate stochastic regions in phase space, can be bypassed leading to enhanced radial transport of particles. The mechanism of enhanced transport is resonance streaming along resonance lines, which constructs the complex Arnold web. The structure of this web depends on the drift rotational transform of drift orbits and the toroidal transit time of passing particles. Numerical examples of Arnold diffusion of test particles will be given. The theory will be applied to passing particles in a toroidal plasma and to trapped particles in stellarators and tokamaks. Some numerical examples of Arnold diffusion of circulating particles will be given.

In JT-60U ELMy H-mode discharges, the detailed behaviour of giant (type I) ELMs was measured using a heterodyne reflectometer system in order to understand the collapse mechanism of the pedestal structure and consequences on the core/edge plasma. The phase signal of the reflectometer exhibits the movement of the cutoff layer (density layer) due to the collapse of the pedestal in a density profile by an ELM. An ELM event can be classified into a precursor phase, collapse phase, recovery phase and a relaxation phase. A certain density layer measured near the shoulder of the pedestal moves about 7 cm inside the plasma in the collapse phase. The precursor oscillation in a pedestal density and the relationship between the collapse of the pedestal structure and a D_α burst are also studied.

The solution of the Vlasov equation is presented for axially symmetric low aspect ratio tokamak plasmas having magnetic surfaces with a circular cross section. The analytical expression for the parallel component of the dielectric permittivity tensor of trapped and untrapped particles is obtained. This expression is used for theoretical analyses of the low aspect ratio effect on the bounce-resonance wave dissipation. The evaluated dielectric tensor components can be used for computer calculations of the radio frequency (RF) field structure and the RF dissipated power in the low aspect ratio tokamak plasmas.