Effects of ECH/ECCD on Tearing Modes in TCV and Link to Rotation Profile

Full text: This paper will focus on the MHD modes observed on the TCV tokamak, their sensitivity to local ECH and ECCD deposition and their role with respect to the measured toroidal rotation profile. The first part is directly related to the classical tearing mode parameter D', which is a key value for predicting NTMs in ITER. In TCV L-mode plasmas, tearing modes are usually not unstable but they can be destabilized with localized co-ECCD at very specific radial positions. It is shown that counter-CD at the same position is stabilizing. These plasmas are used to determine the sensitivity of the tearing mode triggering to the current profile modifications. This is then compared to the expected q profile variation required to stabilize these modes, as obtained from NTM modeling. Calculations using PEST-III and a cylindrical calculation of D', using the q profile, have been performed. The observed sensitivity is indeed very high and small changes can lead to significant positive values of D', and thus destabilize the tearing modes. These simulations have shown that the value of q, its 1st and 2nd derivatives but also the position of the ECCD perturbation with respect to the rational surface all have a significant influence on D'. The role of ECH and/or of MHD modes on the toroidal rotation profiles are analyzed, in particular the role of MHD on the rotation inversion observed in TCV L-modes. The rotation inversion is always observed when q95 is close to 3 and it is known that these discharges are more prone to MHD. In many cases the inversion occurs with the onset of a stationary tearing mode. The inversion is also observed at lower plasma current and density when the plasma is forced from a limited to a diverted configuration. It is found that during the transition a significant modification of the current profile occurs which can lead to MHD activity. Several scenario optimization tools -modifying the transition from a limiter to a diverted plasma such as to avoid the onset of tearing modes, or the addition of ECH to stabilize or destabilize a mode -are used to decouple the effects of high current and of MHD modes on the stationary toroidal rotation profiles. Several effects occur simultaneously and detailed experiments are required to study the link between MHD and rotation. (author)