Reflectometry-based plasma position feedback control demonstration at ASDEX Upgrade

In fusion experiments, real-time feedback control of the plasma position plays a vital role for machine protection and disruption avoidance. This control task is presently performed using magnetic measurements that, in future long pulse tokamak devices of the ITER class, may be affected by drifting integrators or radiation induced voltages in the magnetic pickup coils. These effects could have an impact on the magnetic equilibrium reconstruction, causing potential losses of position control and, consequently, leading to premature discharge termination or plasma-facing component damage. Frequency modulated continuous wave O-mode reflectometry, a non-magnetic dependent technique used to measure the density profile, was proposed to backup or complement the standard magnetic-based control in such devices. This new control scheme has just been successfully demonstrated for the first time on the ASDEX Upgrade (AUG) tokamak. The location of the plasma boundary, used in the control of the plasma column position, was tracked in real-time (RT) using dedicated algorithms and a new approach that combines the reflectometry edge profile and a scaled line integrated density measurement from interferometry. Although feasibility studies on the viability of this method had been previously conducted at AUG, the capabilities required to produce this on-line demonstration were only incorporated in the diagnostic after a recent upgrade of its data acquisition and processing hardware. The results herein presented show the first successful demonstration of the reflectometry plasma position application as proposed for ITER.