Experimental investigation of impurity transport in the core and edge of the wide pedestal QH-mode regime on DIII-D

Wide Pedestal Quiescent H-mode (WPQH) is a promising scenario for future fusion reactors, offering intrinsically ELM–free operation and high energy confinement [1]. We have performed a detailed investigation of impurity transport in the WPQH pedestal. The particle confinement time τ_imp of carbon injected by impurity powder dropper and nitrogen from gas puff is 0.09 and 0.25s, respectively. In contrast, τ_imp of fluorine and calcium injected by laser blow-off system is 1.0 and 3.2s, close to the prediction from a neoclassical model. The primary cause of high τ_imp is a low diffusion and high inward convection at the wide edge transport barrier, comparable to inter-elm transport coefficients observed in ELMy discharges, however, without the particle sink from ELMs. The n = 2 resonant magnetic perturbation (RMP) reduced calcium τ_imp by 40%, while H₉₈ and β_N degraded only by 5%. Nevertheless, the carbon density remained unchanged. Low carbon τ_imp and insensitivity of carbon content to the RMP indicate that the high Z_eff is likely a consequence of a significant carbon source rather than unfavorable pedestal transport.

[1] K.H. Burrell et al., Nucl. Fusion 60 086005 (2020).