Validation of a reduced model for the H-mode density pedestal on DIII-D

Fusion performance in future tokamaks will be highly sensitive to the height of the H-mode pedestal. EPED has proved successful in predicting the pedestal pressure of Type I ELMy H-modes over a wide range of parameters [1], but it requires the pedestal density, n_e,ped, as an input. A reduced model for the density pedestal has recently been proposed [2], which combines simplified expressions for plasma transport with a two-fluid neutral penetration model that treats cold (Franck-Condon) neutrals and charge-exchange neutrals self-consistently. We test this predictive model on a dataset of DIII-D H-modes spanning a range of 3-10 x10¹⁹ m^-3 in pedestal density, and find good agreement with experimental n_e,ped measurements from the Thomson Scattering diagnostic (RMSE ~25%). Further, we compare neutral density profile predictions from the model to simulations from the KN1D code [3] and experimental measurements from the LLAMA (Lyman-Alpha Measurement Apparatus) diagnostic, in order to robustly validate the model’s two-fluid assumption for neutrals in the edge region.

[1] P.B. Snyder et. al., Nuclear Fusion 51(10), 2011

[2] S. Saarelma et. al., Nuclear Fusion 63(5), 2023

[3] B. LaBombard, KN1D: A 1-D space, 2-D velocity, kinetic transport algorithm for atomic and molecular hydrogen in an ionizing plasma, 2001.