Theoretical and experimental assessment of aspects of tokamak confinement

Abstract

This work is a study of the importance of some of the aspects of the generally negative influence of imperfect confinement of particles and energy on a fusion plasma. In some cases, however, the imperfect confinement of impurities is necessary to keep the plasma burning. The criteria for maintaining a steady state burn condition in a reactor by linking the confinement of the helium and energy shows that steady state burn conditions can be achieved in a D,T plasma within certain limits of the ratio of helium confinement time to energy confinement time. The maximum ratio decreases sharply when impurities are included in the plasma. When profile effects are added to such a model, this ratio is not significantly affected, and for a limited one - dimensional approach the values are similar. The one - dimensional model is also compared with an exact solution of the one - dimensional diffusion equations for energy and helium. Time dependent reactor operation is also simulated, including impurity generation as a result of the ion flux leaving the plasma and bombarding the divertor material. This study shows the crucial effect of the confinement of these impurities and helium ash in the plasma. When confined for too long in the plasma, the burn is quenched. A new definition of the maximum impurity concentration, allowed in the plasma during steady state operation based on the zero - dimensional model, leads to strict limits on impurity concentrations allowed for the operation of a reactor in steady state conditions. Results are presented of an experimental study of the ion loss from the plasma and subsequent bombardment of the walls and limiters in JET. The measurements were made for the different stages of the plasma discharge, as a function of the radial distance from the plasma volume. Probes implanted with depth markers were inserted into the edge plasma of JET and analyzed after plasma exposure. Experimental data presented on the measurement of the creation of abnormally high energy particles in the edge plasma during the application of additional heating by the ICRH (Ion Cyclotron Resonance Heating) method confirms the existence of fast particles in the edge plasma with energies ( rv 1 ke V) that contribute dominantly to the higher concentration of impurities in the plasma of ASDEX. The large number of neutral particles that are created are lost from the plasma volume. These particles have several effects on the plasma, such as sputtering of the wall material with subsequent dilution of the plasma fuel and radiative losses. A simple model to describe the neutral flux from the plasma volume during ohmic discharges is presented, and the effects of sputtering of the wall material is simulated for the JET configuration as well as for ITER. A direct effect of the neutrals bombarding the walls is their contribution to the fusion process inside the walls, and this process is discussed. The contribution of this process during discharges on JET and ITER are considered and results presented.