Measurement and control of heat flow in tokamaks

One of the challenges raised by magnetic confinement fusion concerns the design and operation of wall components capable of withstanding high heat fluxes, up to 10 MW.m^-2 in steady state and more during transient events. The ITER project, based on the simultaneous operation of a large metal wall with a burning core plasma, is a decisive step in assessing the performance of the components (reliability and durability). The main theme of this Accreditation to Supervise Research is the operation of components in the face of plasma in the Tore Supra, JET and COMPASS tokamaks. To guarantee operation and protect components during plasma experiments, it is necessary to assess two physical quantities: the temperature of the component on the one hand, and the heat flux density deposited by the plasma on the other. The surface temperature of the components is determined experimentally using infrared (IR) thermography. Because of the potential uncertainties associated with the surface state (emissivity of the material or presence of co-deposited layers), it is possible to supplement the IR thermography system with intrusive measurements (temperature sensors buried in the component). Knowledge of the temperature is then used to determine the heat flux density absorbed by the component. Depending on the geometry and characteristics of the component, the instrumentation and the plasma scenario, several methods and numerical tools have been developed. The operational limitations associated with heat flux and the risks of component damage are discussed. Understanding the mechanisms behind the heat flux is fundamental to ensuring the protection of components and, where necessary, to considering the control of the flux on targets. Three issues associated with the physics of heat flux transport and deposition are described and experimentally characterised: (i) heat flux decay ('Toroidal Pumped Limiter' configuration with steady state plasma and high injected power), (ii) perpendicular heat flux on a leading edge (misaligned needle in Tore Supra, grooved limiter in COMPASS and misaligned tungsten lamella in JET) and (iii) heat flux modification generated by heating antennas (ICRH and LHCD antennas in Tore Supra). The control of heat flow is illustrated in a practical way by two experiments with 'feedback' on the plasma quantities measured in order to achieve higher levels of performance. The first is carried out at JET and is based on a scenario with improved 'hybrid' confinement and injection of nitrogen impurities to cool the on-board plasma. The second is being carried out at Tore Supra and concerns a 'high power - long time' scenario. In conclusion, we will summarise the main results and identify areas for further research within the framework of the WEST project and in preparation for the operation of the ITER project