Particle transport in pellet fueled jet plasmas

Pellet fueling experiments have been carried out on the Joint European Torus (JET) tokamak with a multi-pellet injector. The pellets are injected at speeds approaching 1400 m/s and penetrate deep into the JET plasma. Highly peaked electron density profiles are achieved when penetration of the pellets approaches or goes beyond the magnetic axis, and these peaked profiles persist for more than two seconds in ohmic discharges and over one second in ICRF heated discharges. In this dissertation, analysis of electron particle transport in multi-pellet fueled JET limiter plasmas under a variety of heating conditions is described.

The analysis is carried out with a one and one-half dimensional radial particle transport code to model the experimented density evolution with various particle transport coefficients. These analyses are carried out in plasmas with ohmic heating, ICRF heating, and neutral beam heating, in limiter configurations. Peaked density profile cases are generally characterized by diffusion coefficients with a central (r/a < 0.5) diffusivity ~ 0.1 m²/s that increases rapidly to ~ 0.3m²/s at r/a = 0.6 smd then increases out to the plasma edge as (r/a)². These discharges can be satisfactorily modeled without any anomalous convective (pinch) flux. Auxiliary heated peaked density profile discharges with MHD activity that is measurable at the edge show rapid redistribution of central density which can be explained by temperature-dependent changes in the transport coefficients. Particle balance calculations of particle diffusivity in the plasma core region show a reduction in diffusivity with density peakedness. Correlation with locally determined η_e shows a general increase in diffusivity as η_e increases above ~ 1.5. In all cases, neoclassical particle diffusion coefficients are substantially smaller than the determined values.

Fluctuation induced transport is frequently suggested as the mechanism for the anomalous electron particle and heat fluxes in tokamaks. As part of our analysis, we compare the calculated anomalous electron particle and heat fluxes with the theoretical fluxes from the neoclassical quasilinear formulation of fluctuation induced transport using electron and ion drift waves as the dominant fluctuation mechanism. The Onsager symmetry found in this fluctuation induced transport theory is then used to examine the correlation between the anomalous electron particle and heat flux.