Table of contents

The spectrum of waves in a two-ion-species plasma at frequencies up to the ion-ion hybrid resonance frequency is investigated. A cylindrical plasma column of uniform density and magnetic field, and surrounded by a vacuum, is firstly analysed, and the results are checked when plasma and magnetic field nonuniformities are allowed for. A new m=-1 mode is found to exist at frequencies just above the ion-ion hybrid resonance frequency. This mode has much in common with a previously reported m=-1 mode above the Alfven resonance frequency in that it can have surface wave properties and its dispersion curve merges into the ion-ion hybrid resonance at a critical value of the axial wavenumber. The mode survives as an ion-ion resonance damped wave in a nonuniform density plasma or magnitude field. The m=+1 wave dispersion relation is found to pass through the ion-ion hybrid resonance via a mode crossing, with connection to a hybrid global eigenmode below the hybrid resonance, and connection to a surface wave above the hybrid resonance.

Distribution functions of the form F(H,p_z) are used to describe two-dimensional high beta plasmas with purely axial current. Both species, electrons and singly charged ions, are treated kinetically. Vacuum fields and effects due to neutral lines, finite gyro radii, particle trapping and electrostatic fields are included in the study. For z independent perturbations, a variational principle, which is simple to use in applications, is derived. Pressure profiles fulfilling the corresponding stability criterion are found in Extrap and in z pinch geometry. For a large class of displacements, the system is kinetically stable if it is MHD stable.

A phenomenologically motivated theory of nonlinear saturation of drift wave type instabilities is presented. While density gradient flattening is the physical saturation mechanism for the collisional drift wave, that for the trapped electron mode is found to be both density and electron temperature gradient flattening. The importance of inclusion of radially nonlocal structure in the nonlinear analysis is demonstrated.

Toroidicity-induced drift modes are investigated in the strongly collisional regime. Ion magnetic drift resonant effects are fully retained, also in the ion sound contribution. The lowest even mode becomes unstable due to the electron-ion collisions. When the perpendicular viscosity (ion-ion collisions) is taken into account in the perturbed ion density, the growth rate exhibits a maximum instead of growing without bound for increasing k_{perpendicular to rho i}. It is shown that, for these modes, the ion perpendicular viscosity is more important than the effect on the ion response of the friction between the ions and the electrons.

A phenomenological model of Tokamaks is constructed. The model assumes the toroidal resistivity of the plasma to be Spitzer-like and the poloidal resistivity to be anomalous. The electron perpendicular thermal diffusivity is assumed to have an anomalous value typical of ohmic and beam heated experiments. The complete set of conservation equations including pressure balance and poloidal-Ohm's law are used together with experimentally relevant sources and boundary conditions. In ohmic conditions it is shown that the particle diffusivity D_{perpendicular to} , the electron thermal diffusivity chi _{perpendicular to e}, and the magnetic diffusivity c² eta _tor are of comparable order. Under these conditions the plasma density is found to scale with plasma current like (n varies as I_p⁶5/). The poloidal-beta is found to be limited. Scalings are also derived in the presence of beam-heating for two different laws of increase of chi _{perpendicular to e} with beta _pol. These indicate that the density-current scaling is relatively insensitive to the beam power and degradation of confinement. The model also interprets some of the evolutionary characteristics such as density-clamp in DITE and L-H transition in ASDEX.

The equilibrium of an axisymmetric toroidal plasma, with almost circular cross-section, situated in an external magnetic multipole field is considered. The external field is created by a number of circular current carrying conductors symmetrically arranged outside of the plasma (Extrap configuration). The equilibrium field is expanded to first orders in the inverse aspect ratio and in a small parameter describing the non-circularity of the plasma cross-section. The current density is taken on the form j= kappa ² psi +C. With this choice, both 'peaked' ( kappa ²>0) and 'skin-type' ( kappa ²<0) profiles can be described. The results obtained indicate that, in the range of parameters where equilibria with non-elongated magnetic surfaces ('square-shaped' separatrix) can be found, in the case of four external conductors, there is a high sensitivity of the plasma location (toroidal radius) to the precise values of the currents in the external conductors.

Imaging techniques offer attractive alternatives to small-angle Thomson scattering for scale lengths of density fluctuations causing diffraction in the Raman-Nath regime. This is the case for fluctuations with wavelengths above about 3 mm for Tokamak sized plasmas, when a CO₂ laser probe beam is used. Four methods ('phase scintillation', phase contrast, wavefront shearing and interferometry) are compared, on the basis of their transfer properties. They offer new means of studying density fluctuations of magnitude up to the dimensions of the plasma, such as those associated with turbulence, magnetic islands, convective cells or driven waves. The long wavelength limitations are discussed in detail and are related to those encountered for far-field techniques. An experimental comparison of 'phase scintillation' and phase contrast is given, and the criteria relative to the depth of field of imaging instruments in this context are considered.

Broadband density fluctuations have been investigated in the TCA Tokamak by means of the phase contrast method using a CO₂ laser operating at 10.6 mu . This technique is well suited to the observation of large scale fluctuations. Correlation measurements between two detectors viewing different chords of the plasma cross-section show that more than half of the turbulence power is associated with wavenumbers smaller than 2 rad cm^-1.

A theoretical model is presented describing the spatial structure and scaling laws for the ablative corona generated by the incidence of an intense light ion beam pulse on a spherical pellet. This treatment differs, from an earlier model in the assumption that the beam velocity v_b is higher than the electron thermal velocity v_The everywhere, which allows one to obtain universal profiles for the dimensionless magnitudes describing the state of the corona. The setting of a self-regulating regime for the plasma opacity is confirmed.