Global plasma oscillations in electron internal transport barriers in TCV

Global plasma oscillations have been observed in the Tokamak a Configuration Variable (TCV; R/a = 0.88 m/0.25 m, B_t < 1.54 T) in reversed magnetic shear discharges (fully non-inductive driven) with Electron Internal Transport Barriers (eITBs). These slow oscillations involve the electron temperature, density and the plasma current. The triggering mechanism is the periodic destabilization and stabilization of a magnetic island near the foot of the barrier. This regime is similar to the O-regime first observed on Tore Supra, but the link to the MHD and the global nature are specific of TCV. The regime observed on TCV relies on the large pressure gradients in a region of low magnetic shear, typical in eITB plasmas. Depending on the proximity to the marginal infernal stability limit, the phenomenology of the mode can be more ideal (minor or major disruptions) or resistive. Regardless of the character, the result is detrimental to the confinement and leads to a lowering of the electron pressure and thereby a decrease of the bootstrap current. These fully-non-inductive discharges have large bootstrap fractions, therefore the total plasma current is affected. Due to the proximity to the marginal stability limit, the modes can be stabilized by the changes induced by the MHD in the pressure gradient and local q profile. This results in a recovery of good confinement properties with the re-attainment of the internal transport barrier, and the cycle can begin again. The oscillations can be controlled by modifying the current or the pressure profiles, either with Ohmic current density perturbation or by means of RF heating and localized current drive. (author)