Impact of the separatrix plasma on core L mode performances in the WEST full W environment

WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source and turbulent transport up to the core in L mode, high recycling plasmas, an unavoidable phase of all scenarios.

WEST data, natively in IMAS, automatically generated 804 D L mode plasma plateaus (stable power and I_p >0.3s) used for time averaging. The best core performances correlate with the lowest density at the separatrix, n_sep, similarly to AUG and JET in H mode. Reflectometry in the midplane provides nsep while T_sep is inferred by the 2 point model using Langmuir Probe data on targets. n_sep scales as T_sep^1/2 hence the separatrix resistivity does not correlate with core performances.

As expected in presence of efficient neutral recycling fluxes, n_sep does not correlate with the controlled line integrated density but rather with the D recycled particle flux at the divertor from visible spectroscopy. Coherently, lower n_sep correlates with larger density gradient around the separatrix as well as larger global density peaking from interferometry. The later correlates as well with lower collisionality in the core, questioning the causality chain from SOL to core.