Demonstration of vertical stability control based on Faraday-effect polarimetry measurements on DIII-

Faraday-effect polarimetry measures tokamak core magnetic fields, enabling real-time determination of the magnetic axis position, and providing a non-inductive means for maintaining vertical stability control required for long-pulse or steady-state fusion reactors. Absolute control of Z-position and vertical stability using the Radial Interferometer-Polarimeter (RIP) system on DIII-D has been demonstrated for the first time. RIP is capable of microsecond resolution and was employed to absolutely determine the vertical position of the plasma magnetic axis using horizontal radial chords located at -13.5 cm, 0 cm, and 13.5 cm. A vertical stability controller was also developed to robustly stabilize diverted plasmas using Faraday-based measurements. The combination was able to stabilize vertical displacement events with growth rates up to 350 s^-1 in elongated and elliptical plasma shapes on DIII-D. Testing indicates the Faraday-based controller remains effective and is able to recover from loss of control even when the plasma vertical position is outside the desired measurement range of +/-13.5 cm. Faraday-based control performs comparably to the standard external magnetic-based approach and has been activated during plasma start-up, demonstrating the robustness of the technique to larger systematic uncertainty of the diagnostic at low density.