Error Field Measurement Techniques for ITER Using Plasma Response

The plasma response to external magnetic field asymmetries is a potential tool for detection and correction of the intrinsic error field, but MARS-F modeling and DIII-D data show that the ideal MHD response to error fields is very small in low beta, ohmic plasmas. This indicates that simple proportional feedback control based on a linear plasma response may not be appropriate for error field correction during ITER's initial operational phase. However, modeling and experimental data suggest that the nonlinear response of a tearing mode may be useful for error field measurement and correction at low beta. We discuss several possible approaches, including the onset threshold of an induced tearing mode, open-loop manipulation of a saturated island, and feedback control of the tearing mode. We also assess the possible use of plasma rotation as an error field diagnostic in plasmas with some neutral beam injection.