Description
The WEST tokamak is well equipped to study the impact of high particle fluences and heat fluxes on plasma-facing components (PFCs), because of its capability of running long plasma discharges [1]. In particular, WEST features an ITER-grade tungsten divertor, turning the machine into a technological test bed for future large fusion devices [2]. Dedicated high-fluence campaigns have been led to study the erosion of tungsten PFCs under ITER-relevant particle fluences [3]. By the end of 2025, such a campaign has been performed, using a nitrogen-seeded X-point radiator (XPR) configuration [4], totalling about 4 h 10 min of plasma and 42 GJ of injected energy over 16 days of experiments, with plasma discharges lasting more than 60 s and a record duration of 73 s in XPR regime. In WEST, such long-pulse operation is only possible thanks to the Lower-Hybrid current drive (LHCD) system, using a total of 3.8 MW for these XPR long pulses, heating up the electrons and sustaining the major part of the plasma current. This XPR scenario enables the operation at cold plasma edge and improved core confinement, through a combination of ion dilution effects and reduced tungsten sources, in particular from the divertor. An evolution of the divertor surface has been observed via infrared measurements, but, contrary to the previous high-fluence campaign in attached regime [3], no UFO was observed, potentially suggesting a less severe erosion of the divertor PFCs. These repetitive experiments represent a unique occasion for better understanding the LHCD properties and trying to find routes to improve the plasma duration or performances within the LHCD operational domain [5]. This contribution reports on the analysis of the LH wave coupling and of the LHCD efficiency during this high fluence campaign, to identify in which conditions some of these plasma discharges disrupted before their expected safe termination. In particular, LH waves generate suprathermal electrons, which emit hard X-rays (HXRs) via Bremsstrahlung. LHCD efficiency is therefore inferred from HXR and loop voltage measurements, and the potential dependence on machine conditions is studied using parameters such as the effective charge, the radiated power or edge and core temperature and density. These LHCD analyses are supported by preliminary bounce-averaged quasilinear drift-kinetic Fokker-Planck simulations, which computes the non-Maxwellian electron distribution function and the associated Bremsstrahlung, taking into account effects of the atomic physics of tungsten [6]. These simulations can be constrained by experimental data like the total plasma current and the HXR profiles.
[1] T. Fonghetti et al, Nucl. Fusion 65 056018 (2025)
[2] J. Bucalossi et al, Nucl. Fusion 64 112022 (2024)
[3] J. Gaspar et al, Nucl. Materials Energy 41 101745 (2024)
[4] N. Rivals et al, Nucl. Materials Energy 40 101723 (2024)
[5] J. Morales et al, Nucl. Fusion 65 106003 (2025)
[6] Y. Savoye-Peysson et al, Nucl. Fusion 63 126041 (2023)