Non-Axisymmetric Equilibrium Measurements in DIII-D,

Stationary non-axisymmetric states may arise spontaneously in a tokamak plasma, or they may be driven by external magnetic perturbations (intrinsic or applied). Examples include distortion of the plasma shape by field errors, non-rotating magnetic islands, resonant response of wall-stabilized kink modes at high beta, and stochastic boundary layers generated by shorter-wavelength perturbations. Despite the small amplitudes of typical magnetic perturbations, the effects on plasma performance can be significant. DIII-D$\rq$s new capability for balanced beam injection allows experiments at low plasma rotation where magnetic asymmetries can penetrate the plasma more easily; these conditions may also be relevant to ITER plasmas. We will discuss the impact of non- axisymmetric states on plasma performance in DIII-D, the experience with measurement of these states using the present magnetic diagnostic set, and the requirements for a full three-dimensional equilibrium reconstruction.