MEASUREMENTS OF NON-AXISYMMETRICAL HALO CURRENTS WITH AND WITHOUT KILLER PELLETS DURING DISRUPTIONS IN THE DIII-D TOKAMAK

Non-axisymmetric halo currents are always observed during disruptive i nstabilities in DIII-D. These halo currents appear to have a helical s tructure which rotates toroidally in the electron current drift direct ion with frequencies ranging between 200 and 400 Hz prior to and durin g the initial plasma current quench phase of the disruption. Sometimes the halo current rotation locks at random toroidal phase angles durin g the plasma current quench. The total halo current rarely exceeds 30% of the pre-disruptive plasma current (I-po) and peak-to-average toroi dal peaking factors (TPF) are usually less than 3 during most disrupti ons. Neon 'killer' pellets have proven very effective in reducing both the total halo current amplitude, often by as much as 50%, and the TP F from similar to 3 to similar to 1.2.