Investigation Of Edge Coherent Modes And No-ELMs Regimes In H-mode Of Tokamak

The first High-confinement(H)mode was obtained on the German ASDEX installation in 1982,and this finding became an epoch-making breakthrough in the entire Tokamak research.The H-mode confinement was selected as the standard reference operating scenario for ITER,since its relatively high energy confinement and particle level,as well as good stability repeatability.One of the most important features in the H-mode operation is the edge localized mode(ELM)eruption.Because of the transient giant heat flux problem caused by ELMs has not yet been well solved,the control of ELMs is still a hot issue in the international tokamak research.This paper research relies on the EAST superconducting tokamak device,and utilizes the high spatial-temporal resolution methods for the edge profiles and fluctuations measurements.It mainly revolves around the characteristics of high-performance no-ELM operation scenarios,edge coherent modes,and parameter space of them.There are two main parts included:Diagnostic setup and physical research.(1)A radial edge Helium Beam Emission Spectroscopy(He-BES)diagnosis is successfully established on EAST.Diagnostic gas injection control and signal acquisition procedures are written in the LABVIEW language,which make it possible to obtaine the high spatial-temporal resolution measurements of important physical quantities such as boundary density,temperature,and electrostatic fluctuations.(2)A systematic study of the spontaneously generated no-ELM operating regimes on EAST is performed,as well as the respective parameter space.The spontaneously no-ELM operation produced on the EAST can be simply divided into three types:① Low heating power,high collisionality,and high recycling no-ELM operation.Significant feature is that the edge Da signal continues to rise after entering the H-mode.The parameter space of the scenario is:4.6GHz low hybrid current drive(LHCD)heating or synergistic heating of the counter neutral beam injection(Ctr-NBI)and electron cyclotron resonance heating(ECRH),when additional heating power form 2.45GHz LHCD,ion cyclotron resonant frequency(ICRF)or Co-NBI will break this no-ELM operation.For the plasma configuration:outer gap = 3.5-5.5 cm and stretch ratio κ= 1.55-1.63 and so on;② Low heating power,high collisionalitv.low recycling no-ELM operation:One of the outstanding features of’ this no-ELM is the continuous decreased edge Da signal after the L-H transition until the ELM completely disappears.The experimental conditions needs in this scenario is extensive lithium coating or real-time lithium powder injection conditions,in addition,this operation also shows a clear density threshold,and it is easier to be obtained at low density;③ High heating power,low collisionality no-ELM operation:This scenario is the highest heating level attainable on the EAST nowadays,but unfortunately it is just transient since of the lack of transport channels,so it cannot be maintained for steady-state.(3)There are quite a lot of fluctuations in the plasma edge.This paper systematically summarizes the multiple edge modes on EAST.It mainly includes four common modes:① Edge Coherent Mode(ECM)with electrostatic properties,which is the dominant mode in low collisionality,high recycling no-ELM scenario.When the edge radial electric field(Er)increases,ECM will radially shift and spectrum space broaden,then it become the dominant mode for type II ELMs and low recycling no-ELM H-modes.The magnetic signal is very weak,and the propagation is in the electron diamagnetic drift direction.It is a balloon mode feature with a high toroidal mode number(n～17-19).Studies have shown that it may share the nature of dissipative trapped electrons mode,strongly depending on the collisionality and pressure gradient.ECM is measured to bring strong particles and heat flux,provide a key transport path at the edge,so as to suppress the ELMs;②Magnetic Coherence Mode(MCM):The MCM existing parameter range spans almost the entire EAST H-mode discharge,and is currently only lost during the high-frequency grassy-like ELMy H mode.MCM is the dominant mode under high heating power,low collisionality no-ELM operating conditions.Its electrostatic signal is very weak,and its propagation in the electron diamagnetic drift direction.In contrast to ECM,MCM are thought to contribute little to the edge transport,while it is found that it can significantly change the poloidal distribution of heat flux;③ High-Frequency electromagnetic coherence Mode(HFM),which propagates in the electron diamagnetic drift direction.It is found that this mode has no obvious particle transport near the mid-plane of the low field side,while there is obvious particle transport at the high field side,and it is guessed that this mode may be generated in the high field side area;④ Electromagnetic precursors and post oscillations.An electrostatic precursor fluctuation can be observed during the type III ELMs interval.The precursor frequency range is about 100-200 kHz.It exhibits moderate toroidal modes nature(n～5-10),and the poloidal wavelength is larger than 10 cm.The radial wavelength is about 1cm.In addition,in the critical ELM gap(between type Ⅰ and type Ⅲ),an electromagnetic post mode with a frequency of 50 kHz can be observed,which propagates in the direction of the electron diamagnetic drift and observes that the post mode can be significantly elongated the ELM fall time.(4)The high-frequency and small-amplitude Type Ⅱ ELMs appearing on the EAST is studied.It is found that the appearance of such small ELMs is mainly related to the enhancement of pedestal Er.The enhancement of Er can suppress the instability of the ECM in the pedestal region,and shift the ECM peak position outwards.At the same time,the ECM frequency rises and the spectrum broadens into a wide spectrum fluctuation feature(called BB).It is found that BB exhibits burst characteristics,which the burst frequency is coincided with the ELM frequency,furthermore the burst frequency increases with Er enhancement.Finally,it is observed that this small ELM operation mode is compatible with Internal Transport Barriers(ITB)and low-torque operation.It is a possible alternative operating scenario for the next-step fusion experiments.The above research are not only lays a good foundation for small or no ELMs operations research in EAST,but also provides important reference for the exploration of ELMs control methods under the ITER conditions.