Global Plasma Oscillations in Electron Internal Transport Barriers in TCV

Global plasma oscillations have been observed in the Tokamak à Configuration Variable (TCV; R/a=0.88m/ 0.25m, BT<1.54T) in reversed magnetic shear discharges (fully non-inductive driven) with electron Internal Transport Barriers (eITBs). These slow oscillations involve the electron temperature and density and the plasma current. The triggering mechanism is the periodic destabilisation and stabilisation of a magnetic island near the foot of the barrier. The regime observed on TCV is due to the large pressure gradients in a region of low magnetic shear, typical in eITB plasmas. Depending on the proximity to the marginal ideal infernal stability limit, the phenomenology of the mode can exhibit more ideal-like (minor or major disruptions) or resistive-like behaviour. Regardless of the character, the result is a reduction of the confinement, a lowering of the electron pressure and thus a decrease of the bootstrap current. These fully non-inductive discharges have large bootstrap fraction; therefore, the total plasma current is affected. Due to the proximity to the marginal stability limit, the modes can be stabilised by the changes in the pressure gradient and local q profile induced by the MHD. This results in a recovery of good confinement properties with the re-establishment of the internal transport barrier, and the cycle can begin again. The oscillations can be controlled by modifying the current density or the pressure profiles, either with Ohmic current density perturbations or by means of localised RF heating and current deposition. Results from transport simulations are used to show how the mechanism involved in the development of global oscillations through the periodic stabilisation-destabilisation of resistive MHD can be linked to the observed modifications of the pressure profile.