Transport With Pellet Fueling In The Texas Experimental Tokamak

In the Texas Experimental Tokamak, a discharge regime characterized by persistent peaked profiles was observed to be induced by pellet fuelling, and its transport properties were studied. The hallmark of the regime is the suppression of sawteeth, and the regime was attained by injecting hydrogen pellets to promptly double the plasma density. In each of several pairs of experiments, a pellet fuelled discharge was compared with an edge fuelled discharge with similar averaged electron density, plasma current and toroidal magnetic field in order to characterize the transport change and to look for causal changes in the plasma turbulence. The impurity, radiation and working gas particle profiles were more peaked for the pellet fuelled case. The values of rjt derived from measured ion temperature and density profiles for high density edge fuelled and pellet fuelled discharges indicate that ion pressure gradient driven turbulence should be reduced in the pellet fuelled case. The macroscopic effects were accompanied by microscopic changes. Measurements of turbulent density fluctuations in high density edge fuelled discharges give strong evidence that a component of the turbulence propagates in the ion diamagnetic direction and that this particular mode is reduced in pellet fuelled discharges. The effects of the reduction of an ion mode turbulence were sought in the energy confinement of the discharges, but it was found that for these experiments (tailored for the turbulence diagnostics) the energy flowing in the ion channel was not large enough to affect the energy confinement. Simulations were used to interpret some of the results. Discharge simulations which include the pellet injection can reproduce the sawtooth suppression. This result and known properties of discharges in which sawteeth are suppressed suggest that some of the profile effects (including peaking of the working gas particles) induced by pellet injection are due to sawtooth suppression. The particle peaking may cause the observed reduction in the turbulence which follows pellet injection.