The pedestal density structure with reduced neutral fueling*

We find that the H-mode pedestal density structure remains steep, mostly independent of the edge ionization source rate of the plasma edge in DIII-D and Alcator C-Mod high confinement plasmas, thus suggesting that a pinch is present to maintain the steep density profile. We were able to change the edge opaqueness from 1/10 to 1/2 the expected value on ITER. To calculate the magnitude of the pinch, we first confirm a reduction of neutral density by 50% in DIII-D at the separatrix and 20% in the pedestal structure using midplane filterscope measurements. The reduction of the neutral density in the pedestal with increasing opacity close to the separatrix is further confirmed by SOLPS-ITER modeling in both DIII-D and C-Mod experiments. The neutral density drops from ~10¹⁶ m^-3 in DIII-D to ~10¹⁵ m^-3 in C-Mod at the midplane just inside the separatrix, while the electron density rose from ~4x10¹⁹ m^-3 in DIII-D to 4x10²⁰ m^-3 in C-Mod. Moreover, we find good agreement between SOLPS-ITER modeling and Lyman-alpha data of the neutral density profile in C-Mod L- and H-mode plasmas. As the steepness of the density gradient is unaffected by changes in neutral fueling, but the gradient is affected by changes in heating and plasma current, this suggests plasma transport and an inward pinch play a role. Using a perturbative Helium puff as well as a modulated Deuterium gas puff technique in DIII-D H-mode plasmas, we will investigate both ion as well as the electron averaged transport in the pedestal. Based on the normalized density gradient in the pedestal, without the presence of a source, the particle pinch should be an order of magnitude larger than the diffusion coefficient (D~0.01-0.1 m²/s), resulting in a v~-0.1-1 m/s. These results suggest that predictions of weak density gradients in future machines might not be realized.