Pedestal particle transport in high opacity regimes on DIII-D and C-Mod

Future fusion reactors will operate at high neutral opacity, which will strongly limit the influence of neutrals to penetrate inside the separatrix, whereas in current day devices part of the pedestal density steep gradient can be attributed to direct ionization of neutrals. We will show experimentally that operation at high neutral opacity is compatible with a steep density pedestal structure. Experiments on DIII-D and C-Mod scanning opacity ($n \times a$, where $n$ is the electron density and $a$ is the minor radius) span a range from $1.5 \times 10^{19}$ to $7 \times 10^{19} \; m^{-2}$. The highest reached opacity values in C-Mod are only a factor 2 smaller than those expected on ITER and at these opacity values we did not find a degradation of the pedestal density structure. We find that the neutral concentration as measured by the from the filterscopes in DIII-D and modeled with SOLPS-ITER decreases inside the pedestal structure with increasing opacity and increasing pedestal density. Transport in DIII-D based on the ratio of $D_{\Chi_i}/D_{\Chi_e}$ is dominated by ETG and MTM turbulence in between ELMs, neither of which contribute much to particle transport. In C-Mod edge transport is characterized by a quasi-coherent enhanced $D_{\alpha}$ mode and no ELMs are present.