Rotation and Impurity Studies in the presence of MHD activity and Internal Transport Barriers on TCV

This thesis focuses on measurements of toroidal rotation and impurity profiles in improved plasma scenarios and in the presence of MHD activity. These experiments were performed on TCV, the Tokamak à Configuration Variable in Lausanne. In TCV, plasma rotation is measured by the charge exchange recombination spectroscopy diagnostic (CXRS). The CXRS is associated with a low power diagnostic neutral beam injector (DNBI) that provides CX emission from the hot plasma core, without perturbing the plasma with additional torque. The beam is observed transversally by the CXRS diagnostic so that local ion temperature, density and intrinsic velocity measurements are obtained. During this work, a pre-existing CXRS diagnostic was improved and automated. The three systems composing the present day CXRS2013 diagnostic cover the entire TCV radial midplane with up to 80 measurement locations separated by around 7mm with a time resolution ranging from 2-30ms. The main upgrades concerned the installation of new sensitive cameras, the overhaul of the toroidal HFS system, the extended-chord configuration and the automation of the acquisition and analysis processes. These new CXRS capabilities permitted the investigation of more complex scenarios featuring low intensity and/or fast events, like the low density electron internal transport barriers (eITBs) and the sawtooth (ST) instability discussed in this work. For the first time, a comparison between rotation profiles measured over several sawtooth events and across a “canonical” sawtooth cycle has been undertaken in limited L-mode plasmas in this thesis, in order to identify the effect of the various ST phases on the rotation profile, and thus momentum transport. It is shown that the main ST effect on momentum is not simply a rotation profile flattening, as might be conjectured from their effect on electron temperature and density (consistent with a ST reconnection model), but results appear more complicated. The averaged rotation profiles obtained with the upgraded CXRS diagnostic show that ST restrict the maximum attainable |vφ,max| and that, in the plasma core, inside the q=1 surface, the rotation profiles are flattened and almost always display a small co-current contribution. It is this effect that results in the 1/Ip scaling observed in TCV limited L-mode plasmas. The co-current core contribution is identified to be related to the ST crash, whilst, during the quiescent ramp of the sawtooth period, a plasma recoil outside the mixing radius is observed. A high degree of momentum conservation, up to 80-90%, is measured, suggesting that a supplementary torque accompanying the ST crash is not required to explain the experimental observations. This study demonstrates the importance of including fast perturbating effects such as MHD modes in momentum transport models to build a complete picture, since they are likely to generate strong and rapid fluxes inside the plasma. Extensive work on transport barriers has been performed to better understand the formation and characteristics of eITBs on TCV, using, for the first time, toroidal and poloidal rotation measurements. During this work, the poloidal rotation, Er and the E × B shearing rate have been derived systematically from the asymmetry of the toroidal rotation measurements at the HFS and LFS. Since the position of the CXRS diagnostic view is at Z=0cm, two scenarios, a central barrier and a strong off-axis eITB, similar to previous targets investigated at Z=21cm, were developed to facilitate CXRS analysis during this thesis. The effect on the barrier strength and on the rotation profiles of several parameters, such as the central and total power, Ohmic current perturbations and MHD activity is investigated for both targets. The barrier strength