Coherence imaging spectroscopy with a polarization-sensitive sensor to visualize the plasma flows in fusion devices

The W7-X stellarator was recently upgraded with a new feedback control system for the radiated power. The radiated power, observed using bolometer cameras, is controlled using injection of impurities from the gas valve system. For designing a controller, system identification techniques were employed. These identify the response of the radiation to impurity seeding for specific plasma parameters. Using the obtained frequency response data, a controller was designed which provided accurate control of the radiated power. The system was able to reach and reliably maintain different setpoints using N2 and Ne for the injection, both for attached and detached conditions. It enabled stable long pulse detached operation and multiple steady state radiated power levels during discharges. This allowed heat load mitigation during long discharges (> 300 s), as well as studying the plasma in different radiative regimes efficiently.