New insights on divertor parallel flows,E x B drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak a Configuration Variable

In situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV outer divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuation behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and E x B drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, and the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.

Tokamak Edge Plasma Turbulence Interaction with Magnetic X-Point in 3D Global Simulations

Davide Galassi, Eric Serre

Turbulence in the edge plasma of a tokamak is a key actor in the determination of the confinement properties. The divertor configuration seems to be beneficial for confinement, suggesting an effect on turbulence of the particular magnetic geometry introduced by the X-point. Simulations with the 3D fluid turbulence code TOKAM3X are performed here to evaluate the impact of a diverted configuration on turbulence in the edge plasma, in an isothermal framework. The presence of the X-point is found, locally, to affect both the shape of turbulent structures and the amplitude of fluctuations, in qualitative agreement with recent experimental observations. In particular, a quiescent region is found in the divertor scrape-off layer (SOL), close to the separatrix. Globally, a mild transport barrier spontaneously forms in the closed flux surfaces region near the separatrix, differently from simulations in limiter configuration. The effect of turbulence-driven Reynolds stress on the formation of the barrier is found to be weak by dedicated simulations, while turbulence damping around the X-point seems to globally reduce turbulent transport on the whole flux surface. The magnetic shear is thus pointed out as a possible element that contributes to the formation of edge transport barriers.

2019

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

Hugo De Oliveira

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.

EPFL2021

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

Hugo De Oliveira

EPFL2021