Table of contents

An inconsistency in the toroidal standard model for drift waves is found which may be responsible for the counterintuitive antiballooning behaviour of the complete numerical solution of the 1D version of the standard equations as well as of simplified analytically obtained results.

We consider the scattering of electromagnetic waves from drift turbulent vortices in a plasma. The polarization and wavevector of the incident wave are assumed arbitrary, and the vortices are described by exact solutions of the Hasegawa-Mima equation. An exact expression for the scattering cross section is derived, within the framework of single-scattering perturbation theory. An equivalent electric charge for the drift vortex is derived and the case of an incident ordinary wave is considered in detail. The possible implications of this theoretical model for the interpretation of laser scattering experiments in tokamaks are briefly discussed.

The use of aneutronic fusion fuels, such as D-³He, is shown to require much higher values of plasma beta and much longer confinement times, in a system of smaller size, than for the conventional D-T fuel. It has been suggested that direct conversion might help to offset these disadvantages. We discuss generic features of the conversion process. We conclude that direct conversion at high efficiency, required for aneutronic fuels, appears to be feasible only in open field line configurations, which do not have adequate plasma confinement qualities.

The nonlinear evolution of the long-wavelength external m=1 (kink) mode of a general, straight screw pinch is investigated analytically by means of bifurcation theory. The investigation covers both the ordinary screw pinch ordering of parameters, B_z approximately B_phi , as well as the tokamak limit, B_z>>B_phi . Conducting wall effects are included in both cases. In the case of the ordinary screw pinch, general expressions, valid up to second order in the axial wavenumber, are derived for the coefficients D₁ and D₃ in the mode evolution equation eta _n+D₁ eta +D₃ eta ³=0. Generally, the nonlinear stability near a linear stability boundary is found to depend on the current profile, but not on the axial field profile. Furthermore, peaked current profiles are found to be more stable than broad profiles, but for complete nonlinear stability a conducting wall is in general required. In the tokamak case, a general expression for D₃, covering all previously calculated values of this coefficient (for particular profiles), is derived. Near the first marginal point q_a=1/b² (b is the wall radius normalized to the plasma radius), the shape of the current profile is found to be of decisive importance for the nonlinear evolution of the m=1 mode. In general, sufficiently peaked profiles lead to nonlinear stability, whereas broad profiles lead to explosive growth of the mode. Near the second marginal point q_a=1, D₃ is shown to have a singularity either of order (1-q_a)^-1, or of order (1-q_a)^-2, depending on whether J_a not=0 or J_a=0, respectively, where J_a is the current density at the plasma edge.

The influence of plasma parameter variations on the interpretation of spectroscopic flux measurements in terms of particle fluxes is investigated by analytical and numerical methods. In addition to the temperature and density dependence of the local photon efficiencies, transport processes and spatial shifts of the source distribution result in variations of the effective photon yields which are used to interpret measurements. Parameter studies for typical variations of edge conditions of the ASDEX Upgrade tokamak reveal changes by a factor of 3 for the ratio of the particle influx and corresponding photon flux. If fast temporal changes of the plasma density occur, the photon emission basically follows the density fluctuations for frequencies higher than the inverse ionization time, while the fluctuating part of the photon flux is damped for lower frequencies.

In the ASDEX Upgrade, X-point marfe formation and its behaviour up to the density limit is investigated in gas-fuelled ohmically-heated single-null discharges over a wide range of parameters: I_p=0.6-1.2 MA, B_TF=1.35-2.4 T, a plasma elongation of 1.6 and Z_eff<2. The standard-ion Del B drift is directed towards the X-point. At medium electron densities inevitably a marfe develops in the vicinity of the active X-point. The marfe formation is consistent with a model of thermal instabilities in the radiating edge plasma. Moreover, stable steady-state operation is demonstrated with marfes which can extend significantly into the bulk plasma. The density limit is always connected with quick marfe expansion and movement followed by mode-locking leading to a major disruption. The limit scales linearly with I_p and is in good agreement with the Greenwald density limit scaling. The resulting experimental Hugill limit is n_eRq₉₅/B_TF=2.8*10²⁰) m^-2 T^-1. Reversal of the ion Del B drift direction away from the target has a detrimental effect on the density limit.

A one-dimensional model is presented for the study of RF heating of tokamak plasmas in a fully self-consistent way. It involves the simultaneous description of the temporal evolution of the heated species' velocity distribution function and the electric field responsible for the acceleration of the particles. The non-Maxwellian character of the heated species is explicitly accounted for in the wave equation. As examples RF heating of both very energetic beams and minorities are discussed.

We consider an axially magnetized cold plasma column contained in a dielectric tube, surrounded by a vacuum and shielded by a metallic cylinder. Closed-form analytical expressions for the wave dispersion relation, wave power, absorbed power and wave attenuation coefficient of the right-hand circularly polarized dipolar electromagnetic waves are given. In view of plasma production by such waves their phase diagrams are discussed. The external DC magnetic field is found to facilitate the column sustainment, especially at low values of the product plasma radius*wave frequency, and can remove occasionally occurring wave reflections from the column end. In the presence of a metallic shield one can observe fast wave propagation at higher powers. The axial structure of plasma columns sustained by the fundamental plasma mode HE_II has been obtained under a free-fall/diffusion gas-discharge regime. In the limit of an infinite metallic cylinder radius the results for the wave phase and attenuation coefficients coincide with those derived numerically by Margot and Moisan (1991).

The method of oblique-incidence electron cyclotron wave transmission is studied in a lower hybrid driven tokamak plasma. Cyclotron wave transmission measures the asymmetry in the suprathermal electron parallel-momentum distribution which may be integrated to obtain a measure of the suprathermal electron current. Questions of resolution, sensitivity, and technique are addressed experimentally. The results indicate that cyclotron wave transmission is well suited for studies of electron velocity-space phenomena and may be used to detect small perturbations in the suprathermal current.