Density effects on sawtooth oscillations for high-theta 120 kA discharges in ZT-40M

The effects of using pulse discharge cleaning and neutral gas refueling to increase the plasma density for 120 kA high-theta discharges on ZT-40M have been studied. Experimentally, increasing the plasma density quenches the sawtooth oscillations. Theoretical analysis indicates that the likely reason for this observation is that increasing the plasma density lowers the on-axis streaming parameter which in turn flattens the effective resistivity profiles leading to smaller (or nonexistent) sawtooth excursions.