Experimental Study Of The Related Mechanisms In Internal Transport Barrier Plasma On EAST

Magnetic confinement nuclear fusion devices generally operate in high confinement mode(H mode),which form an edge transport barrier(ETB)at the plasma boundary.At present,H mode is the basic operating mode of the International Thermonuclear Experimental Reactor(ITER).The advanced operating mode can form an internal transport barrier(ITB)in the plasma core to obtain higher confinement and fusion gain.Although there have been a lot of experimental results and theoretical research on ITB,there are still many problems to be solved.There are a large number of discharge experiments with ITB on the "Experimental Advanced Superconducting Tokamak"(EAST),which is suitable for carrying out ITB-related research.The formation of ITB is associated with the suppression of anomalous transport in the plasma core,and the anomalous transport is usually considered to be caused by turbulence.For the local density turbulence in the plasma core,we developed an X-mode polarized W-band multi-channel(four fixed frequency,i.e.79.2 GHz,85.2 GHz,91.8 GHz,96 GHz)poloidal correlation reflectometry on EAST to provide diagnostic support for ITB-related research.The whole development process of this system includes:the selection of detection frequency,the design of microwave circuit,the test of each microwave device,the arrangement of antenna array,the acquisition and control of data,and the test and optimization of the whole system.The system has completed testing of its diagnostic capabilities during the EAST 2018 campaign and measured turbulence in both low confinement mode(L mode)and high confinement mode(H mode)plasma core on EAST.The formation of ITB is related to core transport,while extensive experimental observations from EAST and other devices suggest that ITB is also related to the safety factor(q)profile.Therefore,the core plasma is investigated for the monotonic q profile configuration with the centre safety factor(q0)<1 and the reversed magnetic shear configuration.For the monotonic q profile configuration with q0<1,it is observed by the W-band poloidal correlation reflectometry that the m/n=1/1(m and n are poloidal and toroidal mode numbers)internal kink mode before a sawtooth crash can trigger localized highfrequency density fluctuations(100-400 kHz).The poloidal wave number of these fluctuations is 3-5 cm-1,corresponding to m=36-60.The physical mechanism behind this phenomenon is that the 1/1 internal kink mode can induce a strong electron pressure gradient at a spatially localized position,and this density fluctuation will be triggered when the localized electron pressure gradient exceeds a certain threshold.This discovery will be beneficial for further understanding of a sawtooth crash,and also the physics in 3D equilibrium.In the scenario,it is generally impossible to form an ITB.For the reversed magnetic shear configuration,the study is divided into two parts:(1)In an ITB discharges with the reversed magnetic shear configuration on EAST,the core instabilities that occur during ITB formation are systematically investigated.Through the W-band poloidal correlation reflectometry system,it is found that during the formation of ITB,the density fluctuation level within ITB decreased in the early phase of ITB formation and significantly increased later.This density fluctuation level increased with the density gradient after the appearance of reversed shear Alfvén eigenmodes(RSAEs).By locating the position of RSAEs,it is found that the minimum safety factor(qmin)may play a role in the formation of ITB.These results will be beneficial to understanding the formation of ITB.(2)Double tearing mode(DTM)is a specialized magnetohydrodynamics instability in discharges with a reversed magnetic shear configuration.This configuration is usually used to obtain ITB.However,when DTM is strong enough,it will cause ITB to collapse.In addition,there are also simulations showing that the shear flow induced by DTM can trigger the formation of ITB.Given that the impact of DTM on ITB is not fully understood,we analyze the characteristics of DTM on EAST and its impact on normalized β(βN)from experiments.In a high βN(βN>1.5)discharge with ITB,DTM is formed during the βN collapse,which is associated with the appearance of two single tearing modes.The temperature perturbation caused by it is strongest between the two magnetic islands.After statistical analysis,it is found that βN decrease by 10%-30%within 0.1 s during the occurrence of DTM.Then there are two different changes in βN:recovery and non-recovery.After comparison,it is found that this may be related to the amplitude of DTM and may be related to ohmic heating.