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This study investigates the dynamic evolution of edge-localized modes (ELMs) in the China Fusion Engineering Test Reactor (CFETR) under the influence of a biased divertor target plate system using integrated numerical simulations. By combining magnetic field line tracing with the three-dimensional equilibrium code HINT and the nonlinear MHD instability code MIPS, the feasibility of employing a biasing system as an ELM control technique for CFETR is systematically evaluated. The results demonstrate that, for an optimal bias configuration, a bias-driven scrape-off layer (SOL) current of 1000 A can significantly alter the pedestal pressure distribution and reduce the saturated kinetic energy of ELM-related instabilities by approximately 70%.
ELM control in H-mode operation is essential for future tokamak reactors such as CFETR, as uncontrolled Type-I ELMs can impose intolerable transient heat loads on plasma-facing components. Although resonant magnetic perturbations (RMPs) are among the most effective ELM control techniques, their implementation in reactor environments is challenged by limited installation space and severe neutron irradiation. In parallel, the biased divertor approach provides a more reactor-compatible alternative by generating helical currents in the SOL without the need for in-vessel coils. In this work, a coupled HINT – MIPS modeling framework is employed to assess the impact of bias-driven SOL currents on three-dimensional MHD equilibrium and edge instabilities in CFETR.
The simulations are based on a 13 MA hybrid H-mode equilibrium. A filament current model combined with magnetic field line tracing is used to calculate the spatial distribution of bias-driven SOL currents along magnetic field lines, as illustrated in Fig. 1(a). The resulting three-dimensional magnetic perturbations are then obtained using the Biot – Savart law. Several representative bias configurations are examined, including“+ + + + + + ++”, “++ ++ ++ --”,“++ -- ++ --”,“+- +- +- +-”, and“-+ -+ -+ -+”. Analysis of the resonant magnetic spectra and magnetic topology reveals that the configuration with all electrodes biased positively exhibits the strongest resonant component at toroidal mode number n=4 maximizing the edge Chirikov parameter (Fig. 1(b)). This configuration is therefore identified as optimal for further investigation.
Using the HINT code, three-dimensional nonlinear resistive equilibria are calculated for different SOL current amplitudes. The bias-driven magnetic perturbations lead to the formation of magnetic islands at rational surfaces and stochastic magnetic fields near the plasma edge, resulting in significant modifications of the pressure profile. The magnitude of pressure redistribution increases with SOL current amplitude as shown in Fig. 2. These equilibrium changes directly affect the pedestal pressure gradient and thus the stability of edge MHD modes.
After establishing the initial 3D equilibrium, the MIPS code is used to simulate MHD instabilities. This code solves the full set of MHD equations in cylindrical coordinates. Fig.3(a) shows the time evolution of MHD instability kinetic energy, comparing cases with and without the n=4 SOL helical current.
Subsequently, the MIPS code is applied to simulate the evolution of edge instabilities based on the reconstructed three-dimensional equilibria. As the SOL current increases from 0 to 1000 A, the linear growth rate and saturated kinetic energy of ELM-related instabilities decrease markedly, with the most pronounced stabilization occurring between 0 and 600 A (Fig. 3(a)). Further increases in SOL current yield diminishing returns, suggesting a combined effect of nonlinear pedestal modification and the intrinsic nonlinear dependence of ballooning-type instabilities on pedestal structure. Pressure perturbation analyses (Fig. 3(b,c)) confirm that the dominant modes are ballooning-like and that their amplitudes are strongly suppressed at higher SOL current levels.
These results clearly demonstrate the potential of biased divertor systems for effective ELM control in CFETR. The generation of SOL helical currents provides a promising and reactorrelevant pathway for mitigating edge instabilities and reducing transient heat loads in H-mode operation. Future work will extend this study using the MARS-F code to incorporate detailed resistive plasma response effects.-
Keywords:
- CFETR /
- Divertor /
- Biased target plate /
- Nonlinear simulation /
- ELM
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