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

Abstract

A study of the effects of radial transport on the outer divertor particle and power flux widths (λ js and λ q ) is carried out using plasmas under different operational regimes in the Experimental Advanced Superconducting Tokamak (EAST). In the EAST experiments, the λ js values measured during the grassy intra- and inter-ELM phases are similar, and the averaged value of λ js 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 e, performed using the BOUT++ transport code show that both λ q and λ js increase with D and χ e, especially when D or χ e is larger than a threshold value, indicating that background turbulence starts to matter in the determination of λ q and λ js when D or χ e surpasses the threshold value. A comparison between the simulation of the D scan and the experimental results shows that the different values of λ js 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 js to λ q 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 js , and thus could also affect the ratio of λ js to λ q .

Characteristics of grassy ELMs and their impact on the divertor heat flux width

“…Heat fluxes of the grassy ELMs are found to be only 1/20th-1/10th of those with large type-I ELMs and they are comparable with inter-ELM levels [4]. BOUT++ simulations for DIII-D, EAST, ITER and CFETR show consistent trends and demonstrate consistent divertor heat flux width broadening and peak amplitude reduction in the grassy ELM regimes [12][13][14][15][16]. A 60 s steady-state long pulse high β p EAST grassy ELM discharge has been achieved with high performance in 2019 [17].…”

“…One possible reason for the difference is caused by RF heating, which can change the magnetic topology. Magnetic topology changes induced by lower hybrid waves (LHW) can lead to splitting of the strike point and result in a dramatic broadening of the heat flux width [14,31,32]. The LHW effect is not considered in our model.…”

Modeling study of divertor particle and heat flux asymmetries for EAST H-mode discharges

“…The E × B and diamagnetic drift terms in the model could be switched on/off by the corresponding switches. The BTC has been applied to the edge plasma simulations of discharges from tokamaks like DIII-D 34 , C-mod 36 and EAST 34 , 35 and so on. The good performance of the code has proved it to be an effective tool for tokamak edge plasma simulation.…”

“…The good performance of the code has proved it to be an effective tool for tokamak edge plasma simulation. Detailed information about the BTC can be found in 35 . Note that the grid is simplified in the divertor region, which may not be a big issue since benchmarks among BTC, SOLPS and the experiment have already been done in previous work 34 , 35 .…”

How fluctuation intensity flux drives SOL expansion

“…Recently, both experiments and simulations show that in small/grassy edge localized modes (ELMs) regimes, the ELM size remains small but the SOL width λ q is broadened, while high plasma confinement persists, in contrast to the case of type-I ELMs [17][18][19][20][21][22][23]. Here ELMs stand for the edge-localized modes, being characterized by quasi-periodic relaxation events occurring at the edge pedestal H-mode plasmas.…”