Study On Motional Stark Effect Diagnosis And Reconstruction Of Safety Factors In EAST

Current density distribution and the safety factor is one of the important parameters of magnetic confinement fusion plasma.The measurement of current density distribution is of great significance to the study of plasma equilibrium,confinement and magnetic instability.Motional Stark Effect(MSE)diagnosis is one of the important methods to measure the current density distribution.The current density distribution is obtained by measuring the polarization direction change of Stark spectrum combined with Tokamak plasma equilibrium inversion.This paper introduces the MSE diagnostic system based on modulation method,the spatial calibration of diagnosis and the calibration of polarization transmission characteristics in EAST device.The preliminary result of a qprofile by equilibrium constraints using the measured pitch angle is obtained.A MSE diagnostic system based on dual photoelastic modulation was developed to measure the core safety factor profile of EAST,The system includes 10 channels,covering the measurement range along the major radius from R=1.81m to R=2.27m.The spatial resolution is 0.9～2.3 cm,and the time resolution is 50ms.The MSE periscope observes the red-shifted Da light emitted by the NBI-2L.a square telecentric imaging lens and an inclined fiber array are incorporated as they can ensure a uniform illumination pattern on the fiber plane from the core to the boundary.The filter angle tuning method is used to accommodate the variation in the σ component’s center wavelength,which is caused by the beam voltage change.MSE diagnosis is the direct measurement of plasma magnetic field direction based on tokamak internal real space geometry.The measurement accuracy of MSE depends on the calibration accuracy of spatial geometry configuration.High precision calibration of spatial geometry configuration is the basis of MSE diagnosis combined with magnetic balance reconstruction.A laser tracker and a joint coordinate measuring instrument were used to establish a precision measuring network of the EAST device on the EAST,and then the laser tracker or joint coordinate measuring instrument was used to accurately calibrate the line of sight direction of MSE diagnosis,so as to obtain the accurate position of the line of sight direction and determine the projection coefficient of the measured polarization angle in the EAST tokamak.In addition,since MSE diagnosis is the measurement of polarization,the complex optical system will modify the polarization state of σ componentr leading to erroneous results,so it is necessary to evaluate the polarization-preserving ability of the optical system and calibrate the polarization transmission characteristics of polarized light in the whole optical system.Based on the Stokes vector of polarized light and Muller matrix of optical element signal analysis,a theoretical model was established.Combining with the calibration results,a polarization transmission model suitable for this system was established.The error analysis of the calibration results was completed,and the polarization Angle measurement model was determined,and the polarization Angle measurement error was less than 0.5°.For the EAST device,the equilibrium reconstruction code developed for plasma magnetic field reconstruction,safety factor and plasma current density calculation.The safety factor profile in the core is obtained by fitting and inversion of the polarization angle measured by MSE and the magnetic measurement diagnostic data.In the discharge process of 101148,at 7.9s,ECE observed the sawtooth oscillation inversion surface,and the position of q=1 surface was about the major radius R=2.01m,and the position of q=1 surface measured by MSE was about the major radius R=2.03m.It was consistent with ECE measurements.