A series of EDGE2D-EIRENE cases was run in different wall materials of JET L-mode plasma with the input power close to the H-mode power threshold in ITER-like wall (ILW), with tungsten (W) covering the divertor, and beryllium (Be)—main chamber surfaces. Cases with all carbon (C) and all W walls were run at the same input power as in ILW cases. Contrary to expectations based on the idea that deeper penetration of neutrals re-emitted from surfaces with higher atomic mass into the plasma core raises core density, EDGE2D-EIRENE cases had the highest density in the C wall case. This is explained via the effect of the 'power deficit' in the divertor, as recycling neutrals deposit a fraction of their power into walls, thereby cooling the divertor plasma. This effect is stronger in C compared to W divertor surfaces. The plasma cooling in the divertor increases plasma density and neutral recycling, with more neutrals penetrating into the core. Metal wall (ILW and W wall) EDGE2D-EIRENE cases were found to have larger radial electric field (Er) both inside and outside of the separatrix, in line with the mechanism of the E × B shear turbulence suppression and experimental observations of lower H-mode power threshold in metal wall machines. The originally expected trend for higher core densities in metal wall machines, based on the effect of deeper penetration of neutrals re-emitted from higher atomic mass surfaces, was recovered in much lower density EDGE2D-EIRENE cases, where the effect of the power deficit in the divertor was less important. Subtle effects of interplay among mechanisms involving neutral circulation in the divertor and in the main chamber wall are considered to explain differences between ILW and W wall cases.
Impact of divertor configuration on recycling neutral fluxes for ITER-like wall in JET H-mode plasmas