Particle pinch mitigated by radial currents in the electric tokamak

Abstract

A dominant particle pinch is observed in Ohmic plasmas of the electric tokamak (ET) associated with enhanced poloidal rotation. The density increases dramatically, with strong profile peaking. In pinch dominated particle transport, the pinch velocity profile is determined from the Thomson density profile data. The pinching rate is controlled with soft gas puffing. Hard puffing produces inverted density profiles that do not pinch due to the MHD instabilities. The build-up time of the density is typically 1 s. Due to density accumulation in the absence of significant core fuelling, the characteristic Troyon limit (βN = βaB/I ∼ 3,%, m, T, MA) is reached even in Ohmic plasmas. Density ramps are terminated by internal disruptions due to beta collapse without any significant radiative energy loss. The loop voltage remains low (0.4 V) during the ramp. Prior to, and during the ramp, we observe no reduction in the electrostatic fluctuations in the present experiments. Electrode biasing, using the J × B force, shows that the density accumulation can be reduced and even stopped through slowing the poloidal rotation (reducing the magnitude of the background negative Er). This observation is consistent with the presence of a ‘viscous’ pinch driven by the dominance of the radial electric field through ion mobility. Other neoclassical pinch mechanisms (i.e. Ware and thermoelectric) contribute to the density accumulation and are shown to be secondary effects as revealed by the radial current modulation effects. The easily achieved thermal Mach numbers are ±0.15 for poloidal and ±0.2 for toroidal rotation using the present biaser. There is no significant spontaneous toroidal rotation. The spontaneous poloidal rotation seen in Ohmic plasmas has the thermal Mach number Mp ∼ 0.15 across the measured profile where r/a > 0.5. This rotation is sufficient to account for the observed radial pinch velocity.