Experimental Study Of Particle Transport On EAST

The study of plasma particle transport and confinement has always been one of the most important and fundamental research topics in magnetic confinement fusion physics.The characteristics of confinement and transport not only directly affect the plasma density,but also have a large correlation with plasma energy storage and total energy confinement time.For future nuclear fusion devices,maximizing fusion output requires high-density and high-energy-confinement plasmas.The main purpose of studying plasma confinement and transport related physics is to obtain a high-confinement mode and steady-state plasma in future fusion reactors and to try to control it.Density in tokamak has always been the most basic physical quantity.In order to observe the density variation in the EAST device,a three-chord HCN interferometer and an eleven-chord POlarization-INTerferometer(POINT)have been developed to measure the line-integrated density.The POINT system has high time resolution and can not only be used for density feedback,but also be very important for studying the relevant density fluctuations.In addition,microwave reflectometers and Thomson scattering system are capable of providing local measurements of density.The development of these related diagnostics provides reliable data supporting for the study of particle transport.The study of plasma particle transport has always been a difficult point.On the one hand,it is not easy to experimentally determine the particle source of particle transport relative to energy transport.On the other hand,there is a convection term in the particle balance equation.It is almost impossible to separate convection and diffusion term from the equilibrium state of the plasma.A method for measuring the particle transport coefficient using Supersonic Molecular Beam Injection(SMBI)has been developed on EAST.Compared with gas puffing,SMBI has the advantages of high fueling efficiency and short response time.In addition,the pellet injection can increase the density sharply,so the particle transport coefficient can be obtained by the relationship between the particle flux and the density gradient.But this method requires specific experimental conditions.Using the density modulation method,the particle transport coefficients under different confinement modes are studied.In the ohmic discharge,the boundary plasma transport coefficient is significantly larger than that of the core,but the difference is more pronounced in the H mode and the L mode than the ohmic discharge.Here,the core diffusion coefficient Dc= 0.2 m~2/s,and the boundary is De= 0.8 m~2/s.In the H mode,the core diffusion coefficient is relatively small.For convection velocity,the difference in the three cases is more pronounced relative to the diffusion coefficient.First,the boundary convection velocity is many times larger than the core.The H mode has the strongest inward convection,and the convection velocity of the ohmic discharge is the smallest.This is closely related to the formation of the Edge Transport Barrier.Density pump-out caused by resonant magnetic perturbation(RMP)is an important particle transport phenomenon.During this period,particle confinement are degraded and even H-L transitions are triggered.The particle transport coefficients before and after the density pump-out under H mode were obtained by density modulation.In the plasma core where the particle source can be ignored,the change in particle transport coefficients should be consistent with the change in the inverse density scale length.In pedestal region,RMP results in an increase in diffusion coefficient,but inward convection decreases,indicating that more particles are lost through the last closed flux surface.By comparing the neo-classical transport coefficients,it is found that the transport coefficient obtained by modulation is one or two orders of magnitude larger,indicating that the anomalous transport plays a major role in the density discharge caused by RMP.After the RMP,the potential well of the boundary radial electric field decreases,and the corresponding E×B shear rate decreases at the position of ρ=0.97.By analyzing the boundary density fluctuations,it was found that the plasma density fluctuation increased significantly during the RMP application,which is consistent with the decrease of the shear rate,indicating that the density pump-out phenomenon is associated with an increase in turbulent transport.By analyzing the operational region of EAST in recent years,discharges exceeding the Greenwald density limit were observed.Most of these discharges are accompanied by the MARFE phenomenon.Stronger magnetic perturbations usually occur in discharges close to the density limit under ohmic conditions,and the density fluctuations measured by the POINT also increase.These phenomena also occur in the discharge exceeding density limit under the H mode,accompanied by the occurrence of the divertor detachment.Under the MARFE condition with strong radiation,due to the increase of the density gradient,the electromagnetic wave will have a large refraction angle therein,which may result in the loss of the interferometer signal.