Observation of neo-classical ion pinch in the electric tokamak

A dominant particle pinch is observed in discharges of the Electric Tokamak (ET). As a result, the density increases dramatically, with the profile evolving in a self-similar manner. Due to density accumulation in the absence of significant core cooling, the Troyon limit (βN = I/aB ∼ 3) is reached in Ohmic plasmas in ET. The pinching rate is controlled with soft gas puffing. Hard puffing produces inverted density profiles that do not pinch due to MHD instabilities. The density threshold for pinching is n(0) ∼ 1x1018 m-3 in Ohmic discharges and 3x1018 m-3 in ICRF heated discharges. The ramp-up time of the density is typically 1 second. The ramps are terminated by internal disruptions due to beta collapse without any significant radiation loss. This collapse takes place mostly above the density limit (n = 5x1018 m-3 with B = 0.25 T and R = 5 m, a = 1 m). The loop voltage remains low (0.4 V) during the ramp. In ICRF-heated discharges the ramps terminate at lower densities due to higher plasma temperatures. We observe no reduction in the electrostatic fluctuations. An analysis of the radial particle flux from equilibrium plasma profiles, based on Thomson data, is presented. It is found that the neoclassical 'viscous' pinch (the old anomalous particle pinch) and the Ware pinch both contribute to the spontaneous density ramps and dominate neoclassical and anomalous (neo-Alcator like) particle diffusion for r/a <0.9. We consider this pinch state H-mode like although it takes place without a core barrier, an edge transition, or fluctuation suppression. (author)