Influence of lower hybrid wave injection on peeling-ballooning modes

Abstract

The high-confinement mode (H-mode) significantly enhances the energy and particle confinement in fusion plasmas compared to the low-confinement mode (L-mode), and it is the basic operation scenario for ITER and CFETR. Edge localized modes (ELMs) often appear in H-mode, helping to expel impurities to maintain a longer stable state. However, the particle and energy bursts from ELM eruptions can severely damage the first wall of fusion devices, necessitating control of ELMs. Experiments in EAST and HL-2A tokamaks have been conducted with ELM mitigation by lower hybrid wave (LHW), confirming the effect of LHW on ELMs, but the physical mechanism of ELM mitigation by LHW is still not fully understood. This paper investigates the influence of LHW injection on the linear and nonlinear characteristics of peeling-ballooning mode (P-B mode) in the edge pedestal region of H-mode plasmas in tokamaks, based on the BOUT++ code. The simulations consider both the conventional main plasma current driven by LHW and the three-dimensional perturbed magnetic field generated by the scrape-off layer helical current filament (HCF) on the P-B modes. The linear results show that the core plasma current driven by LHW moves the linear toroidal mode spectrum towards higher mode numbers and lower growth rates by reducing the normalized pressure gradient and magnetic shear of the equilibrium. Nonlinear simulations indicate that due to the broadening of the linear mode spectrum, the core current driven by LHW can reduce the pedestal energy loss caused by ELMs by globally suppressing different toroidal modes of the P-B mode, the three-dimensional perturbed magnetic field generated by LHW-driven HCF can reduce the energy loss caused by ELMs by promoting the growth of modes other than the main mode and enhancing the coupling between different modes. The study finds that the P-B mode promoted by the three-dimensional perturbed magnetic field generated by HCF have a mode number threshold, and when the dominant mode of the P-B mode is far from the mode number threshold driven by the three-dimensional perturbed magnetic field, the energy loss due to ELMs is more significantly reduced. These results contribute to a deeper understanding of the physical mechanisms in ELM control experiments by LHW.