Radial electric field studies in the plasma edge of ASDEX upgrade

In magnetically confined fusion plasmas, edge transport barriers (ETBs) are formed during the transition from a highly turbulent state (low confinement regime, L-mode) to a high energy confinement regime (H-mode) with reduced turbulence and transport. The performance of an H-mode fusion plasma is highly dependent on the strength of the ETB which extends typically over the outermost 5% of the confined plasma. The formation of the ETB is strongly connected to the existence of a sheared plasma flow perpendicular to the magnetic field caused by a local radial electric field Er. The gradients in Er and the accompanying E x B velocity shear play a fundamental role in edge turbulence suppression, transport barrier formation and the transition to H-mode. Thus, the interplay between macroscopic flows and transport at the plasma edge is of crucial importance to understanding plasma confinement and stability. The work presented in this thesis is based on charge exchange recombination spectroscopy (CXRS) measurements performed at the plasma edge of the ASDEX Upgrade (AUG) tokamak. During this thesis new high-resolution CXRS diagnostics were installed at the outboard and inboard miplane of AUG, which provide measurements of the temperature, density and flows of the observed species. From these measurements the radial electric field can be directly determined via the radial force balance equation. The new CXRS measurements, combined with the other edge diagnostics available at AUG, allow for an unprecedented, high-accuracy localization (2-3 mm) of the Er profile. The radial electric field has been derived from charge exchange spectra measured on different impurity species including He2+, B5+, C6+ and Ne10+. The resulting Er profiles are found to be identical within the uncertainties regardless of the impurity species used, thus demonstrating the validity of the diagnostic technique. Inside the ETB the Er profile forms a deep, negative (i.e. directed towards the plasma center) well, which is localized near the last closed magnetic surface. The maximum Er shear and the steepest gradients in the ion profiles lie inside the position of the minimum of the Er well indicating that the negative Er shear region is the important region for turbulence reduction. The Er profile has been compared to the main ion pressure gradient term, which is found to be the dominant contribution at the plasma edge, supporting the idea that the Er well is created by the main ion species. From these measurements the perpendicular main ion flow velocity is evaluated and is found to be close to 0 in the ETB. This result is evidenced by direct measurements of the main ion species in helium plasmas. The fact that the pressure gradient term of the main ions matches Er in the ETB is consistent with the main ion poloidal flow being at neoclassical levels. Quantitative comparisons between neoclassical predictions and experimental measurements of both impurity and main ion poloidal rotation show that the sign and the magnitude are in remarkably good agreement. The Er profile has been measured in different confinement regimes including L- and H-mode. The depth of the Er well is correlated with the ion pressure at the pedestal top, in keeping with the main ion pressure gradient term being the dominant contribution. The findings obtained in this work shed more light on the physics governing the radial electric field and the ETB and provide experimental evidence that the Er well is created by the gradients of the main ion species. The results discussed in this thesis underline that the ion channel plays a key role in the interplay between E x B shearing, turbulence and transport reduction.