Transport barrier and current profile studies on the JET Tokamak: Dissertation

One of the crucial problems in fusion research is the understanding of heat and particle transport in plasmas relevant for energy production. The neo-classical theory of tokamak transport is well-established, but it cannot explain experimental results. Instead, the micro-turbulence driven anomalous transport has been found to be dominant in present tokamak experiments. There are several mechanisms that can locally suppress micro-turbulence and reduce significantly the anomalous transport. These regions of reduced transport are called transport barriers. The presence of Internal Transport Barriers (ITBs) is one of the bases in 'Advanced Tokamak Scenarios'. One of the principal goals in the 'Advanced Tokamak Scenarios' is to improve the fusion power density and confinement with internal transport barriers by controlling the current density profile and maximising the bootstrap current - and ultimately rendering the tokamak compatible with continuous operation. This thesis reports on studies and modelling of internal transport barriers and current density profiles in the Joint European Torus (JET) tokamak with a fluid transport code. Explanations for the following open questions are sought: what are the mechanisms that govern the formation and dynamics of the ITBs in JET and secondly, how can the current density profile be modified and further, how does it affect ITBs and plasma performance? On the basis of the empirical study at the ITB transition, the wE B flow shear and magnetic shear appear as strong candidates in determining the onset time, the radial location and the dynamics of the ITBs in JET. This ITB threshold condition, employed in the semi-empirical Bohm/GyroBohm transport model, has been found to be in good agreement with experimental results in predictive transport simulations. On the other hand, the simulation results from the predictive transport modelling with a theory-based quasi-linear fluid transport model strongly emphasise the importance of the density gradient in the ITB formation. According to the current density modelling studies, lower hybrid and electron cyclotron current drive are the most versatile current drive methods in terms of the produced q-profile in the preheating phase in JET. With lower hybrid preheating, a core current hole has been found and a physics-based explanation, confirmed by the transport modelling, is given. The predictive transport simulations indicate that application of lower hybrid current drive during the high performance phase can enhance the fusion performance significantly by increasing the ITB radius.