Impact of the plasma current on access conditions and stabilization of XPR in the WEST tokamak.

The X-Point Radiator (XPR) regime enables power dissipation near the separatrix by enhancing radiative cooling in the vicinity of the X-point, thereby reducing divertor heat loads. In WEST, nitrogen seeding has been observed to trigger XPR formation [Rivals et al., NME 2024], though its access and stability depend on scenario parameters, including the plasma current. From magnetic field line geometry considerations, increasing the plasma current reduces the parallel connection length, which limits impurity radiative cooling time and hinders detachment and XPR formation.

This work draws on the extensive XPR database available in WEST, where the plasma current spans a wide range (250–500 kA), to assess its role in XPR access conditions and stability via SOLEDGE3X-EIRENE simulations. Modeling is performed with and without drifts, to evaluate their effect on XPR onset and localization. Additionally, this study analyzes the impact of nitrogen content in the divertor on the XPR transition, through a scan of the nitrogen seeding rate.