| Plasma transport in edge and scrape-off layer(SOL)areas of tokamak is a major topic currently in fusion plasma physics research.The anomalous transport,of which the transport coefficients are much higher than those of classical and neoclassical theories,leading to serious particle and heat loss,restricts the global confinement of plasma,and may bring massive heat load or even damage to the first wall and divertor of tokamak.Theory,experiment and numerical simulation researches have proved that the plasma turbulence,which originates from micro-instabilities,plays an important role in anomalous transport,and the intermittence of turbulent transport is characterized by the large-scaled coherent structures,e.g.plasma filaments/blobs,which are generated by the self-organization processes of plasma turbulence.During the high-confinement-mode(H-mode)that observed firstly in ASDEX tokamak in 1982,the turbulent transport was suppressed and the global confinement was improved significantly,and therefore H-mode has been selected as the preferred operation mode of International Thermonuclear Experimental Reactor(ITER)and even the thermonuclear fusion plants in the future.However,in the following H-mode experiments,the edge localized modes(ELMs)are commonly observed,which behave the similar large-scale and intermittence properties of filaments/blobs.Violent ELMs burst will cause serious heat erosion and damage to the first wall and divertor,while on the other side the ELMs can be restrained and modulated via optimized discharge conditions and some active control methods,and the regulated ELMs can play as favorable carriers to extract heat flow and exhaust impurity moderately from the bulk plasma.Because of the strong toroidal magnetic field,the turbulent structures mentioned above are highly elongated and vary little along the magnetic field,while their movements across the magnetic field dominate the loss of particle and heat.To investigate the spatial-temporal dynamic properties of these turbulent structures and the corresponding transport processes,and reveal the underlying physical mechanisms,2-dimensional(2-D)turbulence diagnostic with high spatial and temporal resolutions in radial-poloidal plane is necessary.In HL-2A tokamak,several diagnostics have been developed and utilized for edge and SOL plasma fluctuation measurements,e.g.Langmuir probes,Beam Emission Spectroscopy(BES),Phase Contrast Imaging(PCI),microwave reflectometry,Doppler Backward Scattering(DBS)and Electron Cyclotron Emission Imaging(ECEI),but there remain some shortcomings of these diagnostics especially for local plasma fluctuation measurements at edge and SOL.A new Gas Puff Imaging(GPI)diagnostic with high spatial and temporal resolutions has been developed in HL-2A tokamak.The optimized optics and high-performance camera ensure the spatial resolution up to 2.5mm×2.5mm in radial-poloidal plane,and the temporal resolution up to 3 ?s,which are sufficient to observe and record the structures and dynamics of plasma turbulence,filaments/blobs and ELMs in the edge and SOL areas,achieving an advanced performance in the world,and the observing area is movable(covering an area of a=25?45cm and Z=-5?15cm)to fit different magnetic configurations.In conclusion,the GPI diagnostic can be complementary to the diagnostics mentioned above to investigate edge and SOL plasma fluctuation and transport properties.Further,after the 2017-2018 experimental campaign of HL-2A tokamak,the first independent-developed Multi-Color Gas Puff Imaging(MC-GPI)diagnostic in China has been built on the basis of GPI system and tested on a linear plasma device(Peking University Test device,PPT).MC-GPI diagnostic utilizes neutral helium emission line ratio method for simultaneous electron density and temperature measurements.And the 2-D distributions of measured line ratios as well as the inferred ne and Te are qualitatively consistent with the Langmuir probe measurements,proving the new diagnostic's feasibility and basic performance.Several functional issues have been revealed during the test and will be improved for the coming 2020-2021 campaign of HL-2A tokamak.During the 2017-2018 experimental campaign of HL-2A tokamak,the GPI diagnostic has been put into initial experiments,and the measurements compared with those of Langmuir probe prove the reliability of the diagnostic.And then the spatial evolutions of plasma fluctuation and filaments were investigated with the GPI diagnostic.The analyses in frequency domain show that an m/n=2/1 tearing mode causes significant 4?5kHz plasma fluctuation and the corresponding relative fluctuating strength varies at different radial positions.As the observing point moves radially from edge area to SOL,the high frequency fluctuation decays,and the overall fluctuation gathers to low frequency band and shows non-linear dispersion in SOL.Meanwhile,the statistical investigation of filaments/blobs reveal that these large-scale coherent structures emerge around 20mm inside the last close flux surface(LCFS)and grow to be dominate in plasma fluctuation at larger minor radius,which drives the PDF of fluctuation away from gaussian distribution and leads to the significant intermittency. |
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