Improved Error Field Correction in High Performance Plasmas

Accurate error field correction (EFC) is highly desirable for high performance plasmas (e.g. steady-state, high-$\beta$ plasmas). Feedback-controlled ``dynamic error field correction'' (DEFC) helps us not only monitor the plasma response to non-axisymmetric error fields but also determine a better EFC waveform. In recent high-$\beta$ experiment, we confirmed that the use of ``revised'' EFC - in which the EFC waveform is pre-programmed to repeat the feedback output of a previous discharge - helped sustain the high-$\beta$ phase longer than otherwise possible. This experiment used DIII-D's C-coils, similar to ITER's external EFC coils. Additional iteration of the DEFC will allow us to asymptotically find the ``ideal'' EFC waveform, achieving higher-$\beta$ well above the $n=1$ no-wall stability limit. The combination of both internal and external coils in DIII-D, which would mimic the eddy current pattern in an ideal conducting wall, is expected to deliver substantially improved EFC.