Speaker
Description
Among the potential reactor-relevant divertor regimes of operation, the “X-Point Radiator” (XPR) regime [1] which features a radiating MARFE at the X-Point and strongly reduced target heat loads, is a strong contender. This regime is investigated extensively in the WEST tokamak on the actively cooled ITER-grade tungsten divertor [2], over a wide range of operational parameters (density, power, magnetic configurations), and characterized in a database of over a hundred discharges. XPR’s in WEST are demonstrated to be real time controllable with a simple interferometry signal, and highly repeatable.
Upon sufficient seeding, the divertor plasma eventually condenses within microseconds into a stable, dense and cold (Te < ~5 eV at both targets) regime, followed in the millisecond timescale by the formation of a characteristic radiating MARFE at the X-Point. Divertor heat loads and tungsten erosion are strongly reduced. Nonetheless, a finite ion flux is maintained on both divertor targets, and its deposition pattern changes. Main scrape-off layer profiles are weakly impacted by the XPR appearance, but SOL flows do reorganize, as a consequence to the change in target density and temperature. Core confinement is improved through a combination of ion dilution effects and reduced tungsten contamination.
Statistical analyses are performed on the WEST XPR database, which now includes long and repeated pulses from the new XPR high-fluence campaign. They show the influence of operational parameters (plasma current, density, input power) on divertor particle and heat flux deposition and radiated power patterns. Nitrogen visible spectroscopy signals informing on divertor impurity concentration and sources are also linked to core plasma performance metrics including central ion and electron temperatures and effective charge.
ACKNOWLEDGEMENTS
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.
References:
[1] M. Bernert et al. Nucl. Fusion, 61 (2020) 024001
[2] J. Bucalossi et al., Nucl. Fusion, 64 (2024) 112022