Radiation Transport Modelling in a Tokomak Plasma: Application to Performance Prediciton and Design of Future Machines

Abstract

The understanding and modelling of heat and radiation transport in tokamak plasmas is essential in order to progress in the development of thermonuclear fusion towards a practical energy source which meets all the future needs of environment, safety, and fuel inexhaustibility. This activity enables prospective studies and design to be carried out for next step tokamaks. Due to the complexity of the exact calculation, synchrotron losses are usually estimated in such studies, with expressions derived from a plasma description using simplifying assumptions on the geometry, radiation absorption, density and temperature profiles. In this thesis, a complete formulation of the transport of synchrotron radiation is performed for realistic conditions of toroidal plasma geometry with elongated cross-section, using a precise method for the calculation of the absorption coefficients, and for arbitrary shapes of density and temperature profiles. In particular, this formulation is able to describe plasmas with arbitrary aspect ratios and with temperature profiles obtained in internal transport barrier regimes, which cannot be described accurately with the present expressions. As an illustration, we show that in the case of an advanced high-temperature plasma envisaged for a steady state D-T commercial reactor, synchrotron losses represent approximately 20% of the total losses. Considering the quantitative importance of the above effects and the significant magnitude of synchrotron losses in the thermal power balance of a D-T tokamak reactor plasma, a new fit for the fast calculation of the synchrotron radiation loss is proposed. Using this improved model in the thermal balance, prospective and sensitivity studies are performed for future tokamak projects, and the key issues which limit the performance are isolated. It is shown that, the most restrictive constraint for achieving higher plasma performance is the peak heat flux on the divertor plates. In non-inductive steady-state operation, advanced tokamak regimes are required to achieve relevant thermonuclear plasma performance for next step tokamaks and for a commercial reactor. In the frame of a multi-step strategy towards a commercial reactor, a superconducting next step tokamak compatible with the European budget possibilities is optimized. Considering both the plasma physics and the magnetic system technology and for a given aspect ratio, the smallest machine meeting the physical and technological requirements is determined. In a steady state tokamak commercial reactor, we show that there is an optimal value for the confinement enhancement factor which maximizes the plasma performance, for a given and also for the highest electrical power into the network. This highest electrical power meeting the stability requirements steadily decreases with the confinement enhancement factor. This effect is crucial because both a high plasma performance and a high enough electrical power into the network are required to minimize the cost of electricity, and consequently to make fusion energy more competitive.