| In tokamaks,H-mode is usually accompanied with a periodic burst of boundary plasma,which is called the edge localized mode(ELM).The bursts of ELMs generate a large amount of heat and particles pulsing out of the last closed flux surface(LCFS)and into the scrape-off layer(SOL).The impulsive heat flux can degrade the plasma confinement and erosion the divertor target plates.Now,how to effectively control ELMs is important for the future fusion reactors.Recently,the ELMs are completely suppressed by the ion cyclotron resonant heating(ICRH)during H-mode,which is first observed in EAST.This amazing finding suggests a new approach for ELM control.Therefore,it’s very crucial to explore the physical mechanisms and key factors of the ELM suppression by ICRH,which is of great significance for the future fusion.In order to study the mechanisms of ELM suppression by ICRH,both numerical simulation and experimental analysis have been applied.First,the experimental phenomenon of ELM fully suppressed by ICRH is presented,and three main factors that may affect the ELM control are proposed;Then,the effects of these factors on the ELM are analyzed by using BOUT++.The results reveal that the RF sheath plays a key role in the ELM suppression.The specific contents of this paper are as follows:(1)The phenomenon of ELM suppression by ICRH is introduced in detail,and the factors of ELM control are put forward.In this experiment,the following changes occurred:the ELM is changed from Type-I ELM to no/small ELM;as the stored energy increases,the pedestal pressure becomes higher;the shear flow is enhanced by RF sheath;the intensity of coherent mode(CM)is enhanced and the boundary transport is improved;and etc.Based on these changes,the possible reasons for ELM suppression are:the difference of pedestal instability,the RF sheath,and the enhanced CM.(2)Based on the two-fluid model in BOUT++,the impact of pedestal instabilities on ELM is discussed.Here,the pedestal instability and ELM energy loss(ELM size)before and during ICRH are analyzed.The results show that:(a)After ICRH,the linear growth rate becomes smaller,but the pedestal still remains unstable,the dominant mode is the peeling-ballooning mode(P-B mode);(b)After ICRH,the ELM size is reduced from 3.4%to 2.1%,which still belongs to relative large ELMs,and is inconsistent with the experiment Therefore,the difference of pedestal instability is not the key factor for ELM suppression by ICRH.(3)By combining the RF sheath model and two-fluid model in BOUT++,the impact of RF sheath on ELM is studied.First of all,the radial electric field Er is selfconsistently calculated by the RF sheath model;Then the characteristics of ELM are explored based on the three-field reduced model with the RF sheath Er.The results demonstrate that:(a)after considering the RF sheath,ELM size is reduced from 2.1%to 0.36%,much closer to the no/small ELMs,which indicates that RF sheath plays a key role in the ELM control;(b)The larger E×B shear rate in pedestal and SOL induced by RF sheath reduces the linear growth rates,enhances the nonlinear mode coupling,and breaks up the original large-scale filaments into small-scale turbulence,which can fundamentally suppress the ELM;(c)Furthermore,the parameter scanning indicates that there is an effective range of the sheath potential,which can reasonably explain why it is difficult to achieve the ELM suppression using ICRH in previous experiments;(d)Finally,the physical mechanism of RF sheath controlling ELM and the existence of effective window of sheath potential are confirmed by the experiments.(4)By extending the two-fluid model in BOUT++the impact of enhanced CM on ELM is analyzed.Modifying the three-field reduced module,an extend perturbation is added as the CM.The simulation results indicate that:(a)with the CM,the ELM is effectively mitigated and the energy loss is reduced by about 45%,which shows that the enhanced CM makes a contribution to the ELM control;(b)After considering CM,the linear single-n dominant mode shifts to multiple-n modes,and the nonlinear mode coupling becomes stronger.Stronger mode coupling leads to the lack of dominant filamentary structures and the reduced ELM energy loss.(c)In addition,the sensitivity studies of the ELM size to different CM parameters are conducted,which reveals that the CM amplitude A,phase θ,and wave number kr are the key factors setting the ELM energy loss,while the normalized toroidal spectrum Pkz(n)has little effect.In conclusion:for the experiment of ELM suppression by ICRH in EAST,RF sheath is the crucial factor of the ELM control,CM also has a contribution,while the difference of pedestal instability has little impact.This work can provide some hints for the development of ELM control by ICRH.ICRH is expected to become a new ELM control technology. |
