Tungsten wall compatibility in long pulse operation: Overview of WEST achievement and lessons learnt for ITER.

In view of preparing ITER and future fusion power plant operation, plasma wall interaction for long pulse discharges is a crucial issue. In this respect, the results obtained on the WEST superconducting tokamak, equipped with a tungsten first wall and an actively cooled ITER-grade tungsten divertor, are particularly relevant. A continuous effort coupling experimental analyses and integrated modeling has allowed the development of reproducible scenarios achieving very long pulse durations. These discharges are characterized by electron temperatures in the range 2-4keV and good L-mode confinement (H_98y,2~1.0) with pure electron heating. This effort has culminated with discharge #61299, maintained for 1337 seconds, with an injected LHCD-energy of 2.6 GJ. Compatibility of long pulse scenarios with the W-wall has been achieved in controlled divertor detached plasma conditions. X-point radiator regime is routinely obtained in WEST with Nitrogen seeding, showing dense and cold divertor plasma conditions, with improved core plasma confinement. A robust control scheme has been developed using the interferometry diagnostic as a sensor to perform XPR discharges up to 30s. Interestingly one observes an asymmetric behavior between the two divertor targets: while on the inner strike point the temperature gradually decreases from 20eV to few eV, on the outer strike point the transition is abrupt with a bifurcation from 20 to 2-3 eV in few µs, followed by a global rearrangement of the radiation front moving from HFS to LFS after the bifurcation. The mechanisms responsible for such bifurcation and ionization front dynamics are investigated with SOLEDGE3X-EIRENE code with and without drifts. The redistribution of plasma density related to the ExB drift is crucial to explain the dynamics of the transition to XPR regime in WEST, so as the imbalance between the two targets.