Experimental Study Of Edge And Pedestal Structure On EAST Tokamak

In order to obtain high fusion power,future fusion devices(such as ITER and CFETR)will mainly operate in high confinement mode(H-mode),which is often ac-companied with edge localized mode(ELM).The characteristic of the H-mode is the edge transport barrier,which is also called as pedestal.This structure of pedestal is particularly important for fusion plasma.Firstly,due to the stiffness of the tempera-ture profile,the pedstal temperature determines the core temperature,and then strongly affects the overall energy confinement and fusion power.Secondly,it is found that the ELM size is closely related to the experimental parameters.Therefore,the accu-rate measurement and experimental study of the edge and pedestal structure will help to understand the physical mechanism of the pedestal formation,and provide the opti-mization scheme for the steady-state operation of the future fusion reactor.This paper is mainly about the accurate measurement of edge and pedesal structure,including:(1)The preliminary work of this paper is the construction and upgrading of 30-110 GHz Q,V,W-band X-mode fast-sweeping microwave reflectometry and 20-40 GHz O-mode fluctuation reflectometry on EAST in 2018.For the fast-sweeping microwave reflectometry system,the incident microwaves of three subsystems are conupled to the same transmission line through the quasi-optical(QO)combiner system,which is based on the frequency selective surface(FSS).And the received microwaves are decoupled by using another QO system.This work is the pre-research work for the advanced mi-crowave diagnosis of the future fusion deviecs.The results show that the fast-sweeping microwave reflectometry based on the optical combiner system can greatly reduce the area of the diagnostic window and obtain a relatively high time-resolved electron density profile.Besides,we developed a new multichannel density fluctuation reflectometry with O-mode polarization in 2018,which can simultaneously measure electron density fluctuations at 4(radial)×2(poloidal)positions.The measuring positions of the O-mode density fluctuatio nreflectometry are all located in the plasma edge(or pedestal)region,which helps to provide much more effective information for the physical re-search of edge turbulence,L-H transition,I-phase and so on.(2)The algorithm work of this paper includes three parts:firstly,by analyzing the beat signal(or time of flight)measured by the fast-swepping reflectometry,a method has been applied to observe the spatial and temporal evolution of density perturbations from the edge to the plasma center,named as TOF method.The density perturbation study from the TOF method appears to be more straightforward than looking at the induced perturbation into the reconstructed density profile.Secondly,Microwave re-flectometry diagnostics have been widely used to measure density profiles in fusion plasma.However,the high sensitivity of the diagnostics to plasma turbulence often results in large radial deviations in the edge density profile and causes difficulty in pro-file evaluation.To improve the performance of profile evaluation,a modified RANdom SAmple Consensus(RANSAC)method has been applied to fit the density profiles mea-sured by reflectometry on the experimental advanced superconducting tokamak.Com-pared with the traditional least-squares method,the modified RANSAC method is much more efficient and robust in fitting the experimental profiles.Thirdly,a combination of RANSAC and a genetic algorithm(GA-RANSAC)is used to further optimize the profile evaluation procedure.The results show that this GA-RANSAC method yields better performance and stabler convergence than the modified RANSAC alone.(3)Experimently,an axisymmetric magnetic oscillation(AMO)at frequencies of 5-10 kHz has been observed in L-mode edge or before L-H transitions on EAST.The conditions of AMO’s appearance is the heating power is marginal to the power thresh-old.The AMO can be measured in many diagnostics,such as reflectometry,Mirnov probes,divertor Langmuir probes,fast bolometer,soft x-ray arrays and D-alpha filter-scopes.The AMO’s magnetic fluctuation is toroidally axisymmetric(n=0)and likely an m=2 standing wave in poloidal direction.The density fluctuation of AMO is nearly in phase,suggesting not a radial traveling wave.Further analyses show that the AMO have an obvious modulation effect on the magnetic/density fluctuations,and then,edge density/density gradient and particle flux onto the divertor are also modulated by the AMO.Besides,the bicoherence analyses show a strong nonlinear interaction between the AMO and the background turbulence.And another nonlinear interactions between the AMO and three other coherent fluctuation with frequencies(～4,～11,～15)kHz are also found.The mechanism of 4 kHz generation and energy transfer is still not-known and need to be studied in further.The frequency scaling of the AMO is fm ∝(▽ne/Ip2)-1,suggesting the AMO frequency might be associated with the perpendicular flow veloc-ity.However,lacking of Er,the phase relationship between the turbulence and E × B flow need to be investigated in the further experiments.