MHD as trigger of electron temperature oscillations in ECCD discharges in TCV**This paper is an extended version of two EPS papers [1, 2

Abstract

Global plasma oscillations, involving electron temperature, plasma density and current, have been observed during electron cyclotron current driven ECCD discharges on TCV (Tokamak à Configuration Variable; R/a = 0.88 m/0.25 m, k ≤ 2.8, BT < 1.54 T). This is confirmed by multiple diagnostics operating on different physical principles: electron cyclotron emission radiometer, soft x-ray, far-infrared reflectometer, bolometer and magnetic probes. The oscillations develop in the presence of electron internal transport barriers and reversed magnetic shear. They are reminiscent of the central electron temperature oscillations (the so-called O-regime) seen in low-loop voltage or fully non-inductive lower-hybrid current driven plasmas with reversed central magnetic shear on Tore Supra (Giruzzi et al 2003 Phys. Rev. Lett. 91 935001). The oscillations have m/n = 0/0 periodicity and are not the manifestation of a magnetohydrodynamic (MHD) instability in itself, but they are invariably linked to the presence of MHD modes on TCV. In fact, no plasma discharge has been obtained so far displaying oscillations in the absence of MHD activity. The interaction of the oscillations with MHD can play a strong role in the coupled dynamics of heat and current transport, as the modes can significantly perturb the q profile. More generally the presence of MHD modes can aid in the correct identification of rational q-surfaces [4]. The interplay between MHD and the plasma oscillations, with the first being the cause for the development of the second, is the main focus of this paper. It is also shown that the global oscillations can be removed by adding an Ohmic current perturbation and therefore by modifying the current density profile.