Effects of radial transport on divertor power and particle flux widths under different operational regimes in EAST

Abstract

Here, a study of the effects of radial transport on the outer divertor particle and power flux widths (λ<sub>js</sub> and λ<sub>q</sub>) is carried out using plasmas under different operational regimes in the Experimental Advanced Superconducting Tokamak (EAST). In the EAST experiments, the λ<sub>js</sub> values measured during the grassy intra- and inter-ELM phases are similar, and the averaged value of λ<sub>js</sub> during grassy ELMy discharges is found to be smaller than that of the intra-ELM phase of type-I ELMy discharges but larger than that of the inter-ELM phase of type-I ELMy discharges. Simulations of scans of the radial particle and heat transport coefficients, D and χ<sub>e</sub>, performed using the BOUT++ transport code show that both λq and λjs increase with D and χe, especially when D or χ<sub>e</sub> is larger than a threshold value, indicating that background turbulence starts to matter in the determination of λ<sub>q</sub> and λ<sub>js</sub> when D or χ<sub>e</sub> surpasses the threshold value. A comparison between the simulation of the D scan and the experimental results shows that the different values of λ<sub>js</sub> obtained under different plasma operational regimes are probably due to the different intensities of background turbulence, which is beyond the scope of Goldston's heuristic drift-based model as reported by Goldston et al (2012 Nucl. Fusion 52 013009), since their model was derived by assuming that turbulent transport does not exceed a certain level. The ratio of λ<sub>js</sub> to λ<sub>q</sub> is greatly affected by the radial transport; however, simulations carried out using Scrape-off Layer Plasma Simulation (SOLPS) show that divertor geometry and plasma density have big influences on λ<sub>js</sub>, and thus could also affect the ratio of λ<sub>js</sub> to λ<sub>q</sub>.