Error Field Measurements Using the Torque on a Magnetic Island

The toroidal position of a static $n=1$ magnetic island is determined by a balance between torques acting on the island. These include electromagnetic torques due to non-axisymmetric magnetic fields from external sources, as well as a possible viscous torque associated with plasma rotation. The amplitude and toroidal phase of an unknown $n=1$ error field can be inferred from analysis of the island position as an applied $n=1$ field is varied. In principle, the measurement can be accomplished in a single discharge. The results of error field measurements based on island torque balance in DIII-D will be compared to the standard method using mode-onset thresholds.