Pinch Mitigation in the UCLA tokamak

The UCLA tokamak (R = 5 m, B = 0.25 T, A = 5, a = 1 m, I$_{p}$ = 60 kA) produces long-pulse ($<$ 5s), clean plasmas that exceed the Greenwald density limit and approach the \textit{$\beta $} limit. Ohmic discharges exhibit a significant negative radial electric field and associated poloidal rotation, which are thought primarily responsible for an observed particle pinch. Calculations indicate that this ``electric pinch'' is an order of magnitude stronger than the omnipresent Ware pinch. This particle accumulation produces a rather peaked density profile. Due to strong MHD activity, the density build-up slows and a large disruption follows, returning the core density back to initial values without terminating the discharge. A series of experiments aimed at explaining and controlling the pinch are presented. Direct mitigation of the pinch has been demonstrated using low power ICRF, a positively biased electrode or a perturbative magnetic field at the edge of the plasma.