Table of contents

The frequency spectral broadening of lower hybrid (LH) waves injected into tokamak plasmas is extensively analysed with reference mostly to experimental data from the ASDEX tokamak. The link between the magnitude of the pump spectral width and the degradation of the LH current drive efficiency (up to a factor of 2), pointed out in previous works, is explained. The experimental behaviour of LH power absorption is also well reproduced, even in situations when the access of the launched LH waves to the core plasma should be largely forbidden. Experiments are described that are aimed at determining whether parametric decay instabilities (PDIs) or scattering of LH waves by density fluctuations in the plasma edge are causes of the broadening of the LH pump frequency spectrum. Fluctuations emerge as the largely dominant process, while no signature of PDI processes is observed. Careful measurements of the density fluctuations in the ASDEX scrape-off layer plasma allow the analytical description given by Andrews and Perkins (1983) to be assumed as the appropriate model for LH scattering. Indeed, it supplies the correct magnitude for the frequency spectral width of the LH pump, and explains quantitatively, together with a ray tracing code, why the CD efficiency decreases with the broadening of the pump spectrum. It can also account for the observed LH power absorption coefficient

Externally applied, static, resonant low m=1, n=1 magnetic field perturbations have a strong effect on the behaviour of the sawtooth crash in the DIII-D tokamak. An external coil was used to produce magnetic field perturbations large enough to slow the core plasma rotation and greatly alter the radial electric field profiles but not large enough to induce a locked mode. In these cases the global plasma parameters and the density and temperature profiles remained unchanged; however, the sawtooth oscillation, in particular the sawtooth crash, changed character from resistive in nature to ideal as the static error field increased. The m=1, n=1 resistive MHD internal kink mode normally responsible for the sawtooth crash in DIII-D appears to be stabilized by resonance detuning caused by the combination of the applied static error field and the slower plasma toroidal rotation. The stabilization of the resistive internal kink allowed the plasma to evolve until an ideal mode caused a sawtooth crash. This effect may explain the differences in sawtooth behaviour reported between different tokamaks

The bounce average Fokker-Planck equation describing fast particles in a tokamak with toroidal magnetic field ripple has been solved numerically by a Monte Carlo approach. The essential element is that the ripple effect is treated as a diffusion of banana trapped particles. The diffusion coefficient is precalculated from a semi-analytical expression for a given plasma geometry. The kinetic equation is solved for the processes of fast particle slowing down, pitch angle scattering, ripple diffusion and acceleration by ion cyclotron resonance heating. This approach was found to be particularly useful for the study of fast particle behaviour in non-stationary conditions, including sawtooth effects. A ripple loss code has been developed on the basis of this principle. The code has been benchmarked against a full orbit following Monte Carlo code. Applications of the code to the experimental results from the JET experiments with 16 and 32 toroidal field coils are given

Computer simulations with special versions of the one dimensional BALDUR predictive transport code are carried out to investigate the particle confinement of helium and hydrogen, the energy confinement and the burn control in the high density scenario of the ITER (CDA) physics phase. The code uses empirical transport coefficients for ELMy H mode plasmas, an improved model of the scrape-off layer (SOL), an impurity radiation model for helium and iron, and fast burn control by neutral beam injection feedback. A self-sustained thermonuclear burn is achieved for hundreds of seconds. The necessary radiation corrected energy confinement time τ_E is found to be 4.2 s, which is attainable according to the ITER H mode scaling. In the ignited ITER, a significant dilution of the DT fuel by helium takes place. Steady state helium fractions of up to 8% are obtained, which are found to be compatible with self-sustained burn. The SOL model yields self-consistent electron densities and temperatures at the separatrix (n_e = 5.8 × 10¹⁹ m^-3, T_e = 80 eV)

Axisymmetric hydromagnetic equilibria are obtained, for the case of strongly anisotropic pressure, by an expansion in the inverse aspect ratio. The calculation is carried out by expanding the inverse Grad-Shafranov equation to second order. The calculation shows that non-circular distortion of the magnetic surfaces is driven to lower order than has previously been found. Many shaping harmonics are found to be present in this order

In JT-60, it has been found that the measurements of divertor radiation loss are subject to large errors (sometimes exceeding 100%) due to the charge exchange neutral particles generated in the ripple loss process. This phenomenon occurs only in the neutral beam heated discharges performed under the condition that the direction of the ion ∇B drift is towards the divertor and the neutral beam heating contains the perpendicular beam component. This influence is strong as the safety factor is large because both the ripple loss fraction and the neutral particle pressure near the divertor, as a target of charge exchange events, increase with the safety factor. The quantitative relationship between the bolometric measurements and the charge exchange loss is discussed, and the charge exchange loss, with about 10% or less of the ripple loss, is found to affect the bolometric measurements of the radiation loss. A method of avoiding this influence on the bolometric measurements is presented on the basis of the experimental results

Fast ions with energies significantly larger than the bulk ion temperature are used to heat most tokamak plasmas. Fast ion populations created by fusion reactions, by neutral beam injection and by radiofrequency (RF) heating are usually concentrated in the centre of the plasma. The velocity distribution of these fast ion populations is determined primarily by Coulomb scattering; during wave heating, perpendicular acceleration by the RF waves is also important. Transport of fast ions is typically much slower than thermal transport, except during MHD events. Intense fast ion populations drive collective instabilities. Implications for the behaviour of alpha particles in future devices are discussed