Inference of edge carbon transport coefficients from multi-ion measurements on DIII-D tokamak

The high intrinsic carbon concentration observed in wide pedestal QH-mode [1] poses a major challenge for this otherwise promising ELM-free scenario. Understanding the edge impurity transport is essential to identify the origin of high impurity density and for prediction for future reactors.

We present an investigation of edge and pedestal carbon transport. The transport coefficients inference is based on non-parametric Gaussian process regression combined with an analytic steady state solver of the impurity transport equation [2]. The inference is constrained by line emission profiles of all carbon ion charge states C1+ to C6+ and O5+, O8+ measured simultaneously with Dα emission on the outer midplane by the DIII-D CER system. The inferred carbon flux is an order of magnitude higher than neoclassical prediction, indicating an additional transport mechanism. Furthermore, the measured carbon ion profiles enabled us to determine the electron ionization source profile in the pedestal and compare it with the fueling profile from deuterium.

[1] Burrell K.H. et al 2016 Phys. Plasmas 23 056103

[2] Nishizawa T. et al 2022 Nucl. Fusion 62 076021