Particle transport in pellet fuelled JET plasmas

Abstract

Multipellet fuelling experiments have been performed on the Joint European Torus (JET) and the resulting density profile evolution has been analysed. A local particle transport analysis is carried out using a 1½–D radial particle transport code to model the experimentally determined evolution of the electron density profile. Plasmas in the limiter configuration, with Ohmic heating, ion cyclotron resonance frequency (ICRF) heating and neutral beam injection are analysed. Ohmic and ICRF heated discharges with peaked density profiles generated by deep pellet penetration are usually characterized by a low core particle diffusivity and can be satisfactorily modelled without any anomalous convective (pinch) term. The evolution of the density profile in auxiliary heated discharges with enhanced levels of magnetohydrodynamic fluctuations shows a rapid decay of the peaked central density that can be modelled with a temperature dependent increase in the diffusivity. Particle balance calculations of effective particle diffusivity in the core region yield values as low as 0.04 m2/s in discharges with highly peaked density profiles. The determined particle diffusivity in all cases is 10–100 times larger than the neoclassical diffusivity. The dependence of the particle diffusivity on density profile peakedness correlates well with the effective thermal diffusivity from power balance calculations.