Table of contents

In the edge plasma of the ASDEX tokamak, electrostatic fluctuations were observed with Langmuir probes and in H_alpha light with high poloidal and temporal resolution. These fluctuations contribute a significant fraction to the 'anomalous' radial particle transport in the scrape-off layer (SOL). The basic properties and the dependence of the fluctuation parameters on the discharge conditions are documented. A model for an instability mechanism specific to the SOL is introduced and the experimentally observed fluctuation parameters are compared with the predictions of the linearized version of this model. For plasma temperatures above approximately 10 eV in the SOL the observed parameter dependences of the fluctuations are well reproduced by the model. By mixing length arguments the radial transport and the resulting density and pressure gradients in the SOL are estimated from the model. Their dependence on plasma temperature and density qualitatively agrees with the behaviour observed in ohmic discharges on ASDEX

A computational model is developed that allows one to estimate in a real 3-D geometry the electrodynamic characteristics of complicated waveguide antennas ('grills') generating plasma waves in the lower hybrid frequency band. The antenna coupling efficiency and the shape of the wavenumber spectrum are found as a solution of a self-consistent problem taking into account a complete set of waveguide eigenmodes and finite waveguide dimensions in both directions and an arbitrary orientation to the plasma magnetic field. Electric and magnetic fields inside the waveguides are equated to the outside fields represented as a Fourier sum over wavenumbers N_y, N_z in the plane parallel to the plasma surface. The fields at the plasma edge are determined by the 2*2 plasma surface impedance matrix found as a numerical solution of the wave equation in the cold plasma approximation using the finite element (Galerkin) method. The solution is found on a 1-D mesh, i=1, 2, ..., N, in the form E(xi,N_y,N_z) exp(i(N_yy+N_zz- omega t)) so that the fourth order differential equation with appropriate boundary conditions is reduced to a set of 4N+2 algebraic equations. The developed model is applied to modelling a quite new antenna design for generating the fast H waves, which can be used for plasma heating and current drive in tokamaks of ITER scale. The antenna parameters are optimized to obtain the best coupling efficiency. The structure of waves excited in the plasma at various angles between the antenna and the plasma magnetic field is considered. The nature of the waves with N_z<1 excited by the grill is discussed

The level of density fluctuations is shown to decrease during ergodic divertor operation in Tore Supra. This decrease of the turbulence is correlated with the onset of a temperature pedestal and a local improvement of the confinement. This pedestal is located close to the electric shear layer, i.e. within a narrow region between the plasma core and the ergodic layer. The onset of such a pedestal explains why the central electron temperature is not changed when the ergodic divertor is switched on, in spite of an ergodic zone where the temperature is low

The characteristics of tokamak demonstration reactors (DEMOs) that would extrapolate from both the existing and the emerging physics and materials databases are calculated. The emerging advanced physics design base would allow more compact DEMO designs at lower plasma currents than would be possible with the existing physics design base upon which the International Thermonuclear Experimental Reactor (ITER) is being designed. The primary structural material for ITER, 316SS, has a relatively low first-wall heat flux limit, which probably would preclude full utilization of the emerging advanced physics database in the DEMOs. Furthermore, the use of 316SS would lead to DEMOs that required deep repository burial of radioactive waste upon decommissioning. An advanced structural material (e.g. V-4Ti-4Cr) would allow fuller utilization of the advanced physics design base and would lead to DEMOs that possibly would satisfy near surface burial criteria

An application is discussed of a new method based on laser induced ionization that is an alternative to the laser induced fluorescence diagnostics. Here ionization of the hydrogen upper atomic states gives the local hydrogen densities in a plasma. On the basis of a rate equation analysis for a diffusion dominated plasma a probe saturation flux density of a few MW/cm² was introduced and appropriate values were used for the plasma diagnostics. A multipulse neodymium laser was designed for plasma probing on the FT-I tokamak to measure the neutral density profiles along the plasma diameter. Local neutral densities as low as 1*10⁸ cm^-3, which is close to the sensitivity limit of the employed device, have been detected in a single tokamak discharge. The experimental results obtained on the FT-1 tokamak are presented and discussed

An analytic simple fluid theory (SFT) is derived to predict when a tokamak divertor should retain impurities. Only the simplest, one dimensional (1-D), case of leakage for ions from the point of ionization, along B to points far upstream, is considered. The SFT builds directly on earlier 1-D treatments of divertor retention of impurities. It is found essential to introduce cross-field leakage into the SFT, associated with the two dimensionality of the actual divertor situation, otherwise the upstream regions suffer catastrophic, and unphysical, impurity accumulation. Without this correction, divertor leakage is predicted to occur even under rather cold (collisional) divertor operation. With this correction, appreciable leakage is predicted to occur only for divertor temperatures that are so high as to be unlikely to occur, at least for the case where impurity neutrals sputtered from the target plate are ionized on their first pass through the divertor plasma fan-the 'shallow injection' case. Thus, previous analytic prescriptions for divertor retention are too pessimistic. For 'deep injection' cases, as can occur with recycling gases such as neon, or with wall sputtered sources, a prescription is found for the plasma temperature above which impurity leakage occurs. The most critical factor governing retention is the location at which the impurity neutrals are first ionized. The predictions of the SFT are compared with results using the Monte Carlo impurity transport code DIVIMP (Divertor Impurity). Close agreement is found for plasma conditions that are strongly collisional, but for weaker collisionality the SFT is found to overestimate leakage

A simple model is described for analysis of data on fast particle and current diffusion during lower hybrid heating. The method allows arbitrary variation of density and spatial diffusion coefficient with radius and is simple enough to allow a rapid scan of different magnitudes and radial dependences of the diffusion coefficient to be carried out. Some preliminary results on data from the Tokamak de Varennes are included to illustrate the power of the method