Distinguishing internal and external sources of measured “3D” magnetic fields in the DIII-D tokamak*

The magnetic field on a closed surface can be uniquely decomposed into contributions from currents internal and external to the surface [A.H. Boozer, Nucl. Fusion 55, 025001  (2015)]. This general principle implies that measurements just outside the surface of a magnetically confined plasma can distinguish the plasma’s contribution to the magnetic field from the contribution by external currents. Spatially resolved, normal and 1- or 2-axis tangential measurements are required, but there is no need for a specific model of either the plasma or the external currents. In a tokamak, this technique enables direct measurement of the field of a growing plasma instability, distinct from the current that it induces in the resistive vessel wall, and direct measurement of the stable plasma response to an external magnetic perturbation, distinct from the external perturbation itself. The separation of internally and externally sourced fields also provides a natural framework for determining the electromagnetic torque on the plasma. The DIII-D “3D” magnetic diagnostic system [J.D. King, et al., Rev. Sci. Instrum. 85, 083503 (2014)] is well suited to such measurements, and applications of the technique to DIII-D data will be shown.