Dynamic decoupling and multi-mode magnetic feedback for error field correction in RFX-mod

Magnetic field errors can have a significant impact on the confinement properties of magnetized fusion plasmas. In the RFX-mod reversed-field pinch (Sonato et al 2003 Fusion Eng. Des. 33 161) a significant error field is produced during the current ramp by the eddy currents induced in the 3D wall structures, such as the gaps and some large portholes, by the temporal variation of the vertical magnetic field. A set of 192 magnetic sensors and 192 active coils allowed accurate identification of the error field spatiotemporal pattern and its correction. The correction scheme combines pre-programmed current waveforms and multi-mode magnetic feedback. The pre-programmed currents were computed with the dynamic decoupling algorithm developed in Soppelsa et al (2008 Fusion Eng. Des. 83 224). This accounts for the mutual interaction between different feedback coils and magnetic sensors, which is affected by the frequency-dependent response of the 3D wall structures to external magnetic fields. At the same time, multi-mode magnetic feedback is applied to the main error field harmonics. During the current ramp, multiple tearing modes are normally phase-locked and produce a toroidally localized deformation of the plasma column that tends to grow where the error fields are larger. With error field correction, this deformation does not grow at preferred positions, thus avoiding the plasma-wall interaction being too localized there. In general, the decoupling approach used in this work may find applications in other machines.