3D magnetic geometric effects during 3D field application and comparison to measurements in DIII-D

Density pumpout during the application of 3D fields in tokamaks may be caused by changes to the plasma equilibrium shaping that destabilize microinstabilities, thereby increasing transport.\footnote{T.M. Bird, PoP 21 (2014) 100702.} Local geometric quantities of the magnetic field that are relevant for microinstabilities (curvature and local shear) are calculated using VMEC equilibria in typical RMP discharges on DIII-D. Measurements of phase-differenced soft X-ray emission in the pedestal region show a clear helical structure that is compared with a model of localized impurity transport based on the 3D geometry. Broadband density fluctuations measured by beam emission spectroscopy also show changes in magnitude with I-coil phase, in support of the theory that microstability changes with the magnetic geometry. A scan of 3D equilibria over a large range of DIII-D geometric parameter space has been preformed in order to map out the operating space of the microstability mechanism.