A fast-moving Langmuir probe array for the divertor of the Tokamak à Configuration Variable

In next step tokamaks such as ITER and future fusion power plants, wall materials will be subjected to considerably higher heat and particle fluxes than in present-day machines. Maintaining plasma facing components in good condition and simultaneously assuring acceptable core performance will be an outstanding challenge. This situation is further complicated by the fact that extrapolations towards future tokamaks are difficult and the mechanisms governing transport of energy and particles are still being investigated. Proper diagnostic coverage of the boundary plasma of currently operational machines is therefore crucial. In the Tokamak à Configuration Variable (TCV), wall-embedded Langmuir probes (LPs) are one of the most powerful diagnostics to describe local properties in the boundary region of the plasma. The present thesis work includes operation, maintenance and upgrades of this LP system. In particular, the TCV wall probe coverage has been extended from 114 to 194 probes. Furthermore, a new set of amplifiers has been installed, featuring a number of improvements. In parallel, a novel fast probe array, the Reciprocating Divertor Probe Array (RDPA), has been designed, built and commissioned at TCV, providing unprecedented Langmuir probe measurements across the divertor volume. In contrast to reciprocating probes accessing the divertor region of other major tokamaks, the RDPA provides 2D measurements in a single reciprocation, enabled by a radial array of Mach probe tips moved vertically across an extended region of the divertor volume. The RDPA capabilities have allowed for a detailed particle balance study in the outer TCV divertor in attached and detached conditions and both with and without the recently installed divertor baffles. It has been observed that the contribution of the ExB drift to the poloidal ion flux is comparable and sometimes larger than that due to the ion flux parallel to the magnetic field. Except for strongly detached conditions, most of the particle flux arriving at the outer target is shown to be due to ionization along the outer divertor leg, justifying the 'closed-box approximation' frequently used in detachment models and previously inferred on TCV from spectroscopic measurements. ExB drifts also play an important role in terms of heat transport, as found from a divertor heat flux balance study in attached conditions. Together with parallel heat conduction, the ExB-convected heat flux dominates the poloidal heat flux, while parallel heat convection plays a lesser role in the example studied. A new feature of RDPA is also to provide detailed, local measurements of fluctuation characteristics across the divertor volume of TCV, thanks to acquisition frequencies of up to 2MHz. The first findings include the development of a strong poloidal gradient of the density fluctuation level, observed towards the onset of detachment, and the surprising presence of high fluctuation levels in the narrow region of downward ExB (on the high-field side of the peak electron temperature) for well attached discharges. Ongoing efforts focus on using RDPA results to validate the global 3D two-fluid code GBS in diverted TCV simulations and thus helping to improve its predictive capabilities.