Development Of CO₂ Dispersion Interferometer On EAST And Experimental Study Of Polarimeter-interferometer On EAST

Electron density is one of the most important parameters of fusion plasma.The reaction rate of fusion is proportional to the square of plasma density.The operation of plasma at high density is the goal that every fusion device hopes to achieve.However,through a series of experiments,it is shown that the plasma has a certain upper limit of density.Breaking through this upper limit of density,the plasma will inevitably break down.This phenomenon greatly limits the progress and development of fusion engineering.On this basis,it is particularly important to study the plasma density limit and realize the operation of high density plasma.For electric density measurement of high density discharge plasma,the traditional interferometer is affected by mechanical vibration and density fringe inversion calculation errors.The measurement accuracy and accuracy are poor,and the new interferometer has been developed rapidly.Among them,carbon dioxide dispersion interferometer has become one of the hot spots in plasma electron density diagnosis recently.Based on the design of the maximum average chord limit density(1020 m-3)of EAST device,a 9.3 μm carbon dioxide dispersion interferometer system based on modulation amplitude ratio is designed and developed.Because the laser beam is coaxial,the system eliminates the effect of mechanical vibration directly in principle,and does not need the support of complex and expensive shock absorber system.At the same time,based on carbon dioxide laser interferometer,there at most one density fringe jump under the current condition of electron density discharge.In this paper,firstly,the diagnostic principle of dispersion interferometer is introduced,puts forward the conceptual design of dispersion interferometer system on EAST,and analyses the possible error sources of the system.Then,the development scheme of the system is introduced,including optical design and bench test.In the optical design part,the optical path design of the system and the parameter design of the main components of the system are elaborated.Subsequently,on the basis of the system design scheme,the desktop experiments are carried out to verify the system.The measurement of electron density in this system is based on the phase analysis between two second harmonic beams.Therefore,it is necessary to study the double-frequency crystal which generates the second harmonic.By measuring and analyzing the second harmonic,the relationship between the frequency doubled laser and the placement angle of the frequency doubled crystal under different power of the original laser is verified;the feasibility of the new system is tested by using the wedge instead of the plasma,and the feasibility of the system is verified by the short-distance feasibility test,and then the same design parameters as those of the EAST system are tested on a 1:1 platform,which verifies the feasibility of the system running on the device.The installation of system on EAST includes three aspects:mechanical support,detection and acquisition,and data processing.In data processing,on the one hand,the original signal of the storage system can be used for off-line calculation to provide high-resolution analysis of density calculation results;on the other hand,the original signal can be quickly processed by hardware with FPGA programming to realize real-time calculation of plasma electron density,meet the requirements of PCS real-time control and lay the foundation for EAST experimental analysis between two shotsIn addition,the plasma vertical displacement is measured by the polarization interferometer(POINT)system on EAST,which provides a useful non-inductive method for measuring plasma displacement for EAST and future fusion reactor.Based on the high time-resolution measurement of current distribution in POINT system,the relaxation process of plasma spontaneous current under neutral beam injection on EAST is studied.Based on the measurement results of electron density and Faraday rotation angle of polaimeter interferometer system,the plasma electron density,current density and equilibrium magnetic reconstruction(EFIT)were calculated and compared to verify the existence and change of plasma vertical position in the discharge process.Through the change of plasma electron density disturbance and current distribution in H-L mode conversion process driven by NBI,it is found that the relaxation time of current distribution is less than current diffusion time,and there exists a plasma spontaneous current relaxation process.The effect of the non-linear coupling of plasma electron density disturbance and current transport on current relaxation process is revealed.