Fast Langmuir probe measurements of the bifurcation to the X-point radiator regime in the WEST tokamak

Advanced concepts and design are necessary to efficiently address power exhaust in highperformance tokamaks. Power dissipation via induced local radiation known as XPR (X-Point Radiator), [1], in a suitably designed magnetic configuration, is one realistically promising solution. Further, the XPR refime is effective also in reducing or even suppressing plasma instabilities at the plasma edge, [2], thus aiding the safeguard of plasma facing materials and component. Transition to the XPR regime was observed in WEST pulse #57980 by an array of pop-up probes having a 1 MHz data acquisition rate. The temperature dropped from ~15-20 eV to ~2 eV concomitantly with a correspondingly fast density increase at the divertor, both on a µstime scale. Similar effects were measured by an array of flush-mounted probes at another toroidal location, although with a slow acquisition rate (250 Hz). Simultaneously, measurements by reciprocating probes at three poloidal locations (?=-20, +20, and +90°) showed very little change in the scrape-off, suggesting that the XPR is associated with strong poloidal gradients of the plasma parameters between the X-point and the divertor. The bifurcation to the XPR regime observed in WEST pulse #57980 showed the desired characteristics of effective power radiation near the divertor (below the X-point), but without disrupting the upstream SOL plasma. Acknowledgement This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 - EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. [1] M. Bernert et al., Nucl. Fusion 61, 024001 (2020) [2] T. Lunt et al., PHYSICAL REVIEW LETTERS 130, 145102 (2023)