Experimental Study Of Plasma Current Density Profile Evolution Of Lower Hybrid Current Drive On EAST

In tokamak plasma,a weak or reversed magnetic shear configuration is an important candidate for the future fusion reactor,it is helpful to build a hollow current profile,establish an internal transport barrier(ITB),improve the plasma confinement,and realize the long-pulse discharge.In experiments,some supporting heating systems,including NBI(Neutral Beam Injection),ECRH(Electron cyclotron resonance heating),ICRH(Icon cyclotron resonance heating),and LHCD(Lower hybrid current drive,are used to control the plasma current due to the Ohmic current driving efficiency is limited,consequently,achieving the weak or reversed magnetic shear configuration.Among the heating systems,LHCD is proved to be the most efficient way to drive the plasma current,so it is necessary to research the plasma current driven by LHCD,which is the key to realize high performance plasma.On EAST(Experimental Advanced Superconducting Tokamak),both L and H mode plasma have been achieved through LHCD,however,some related questions including where is the current deposition by LHCD or how it changes along with the discharge are still not clear due to the absence of effective measurement diagnostic systems.With 11 measurement chords,POINT(Polarimeter-INTerferometer)system was built in 2014 and can measure the plasma electron density and Faraday rotation angle simultaneously,it was updated to 11 measurement chords from formal 5chords in 2015 experiment campaign.The plasma current can be obtained by Faraday rotation as it is related to the poloidal magnetic field arising from plasma current.Besides,POINT system has a high time resolution and are capable to run both in long-pulse operation and multiple heating systems configuration,all these advantages make POINT system an effective way to measure the plasma current.The topic of this thesis mainly on plasma current measurement by POINT.Firstly,the basic measurement theory of POINT system and some POINT measured data are demonstrated.Secondly,stray light error on Faraday effect measurement is introduced in detail,a theory model is built to describe the error,and based on which we update a light isolator and invent an algorithm to decrease the measurement error arising from stray light.Thirdly,plasms current profile is obtained with the combination of POINT measurement and EFIT reconstruction code,which provides the fundamental of plasma current control.In addition,a quick method with POINT measurement to speculate the current deposition by LHCD is introduced.Fourthly,some LHCD plasma current profile results with POINT measurement are shown,in which the discharges have different parameters,which provides a reference for the future plasma experiments.Based on the reliable current profile and the LHCD current evolution study,the current profile improvement has been done in EAST long-pulse operation,the current profiles and transportation characters related to ITB discharge and good performance plasma are also obtained.In long-pulse discharges with improved current profile,with a flat center q profile and q0 is slightly less than 1,1/1 tearing mode and internal kink mode are observed,the non-linear interaction between these magnetic perturbation and current transportation is possibly one reason for the flat center q profile.Moreover,the characters of GAM density fluctuations in H mode are also observed with POINT system,the poloidal number is verified to be 1 with POINT density measurement.POINT system provides the basic measurement of plasma current and makes it reachable to control plasma current in the future.