Measurement And Investigation Of The Plasma Density Fluctuation Based On The J-TEXT Far-infrared Polarimeter-Interferometer

To achieve a sustained and economical nuclear fusion reaction,plasma confinement performance must be improved,but the anomalous transport can seriously degrade the confinement performance,so the research on plasma transport is essential for magnetic confinement fusion research.Experiments show that the anomalous transport is closely related to the microscopic turbulence in the plasma,and the plasma density fluctuation is an important characterization of microscopic turbulence.Therefore,the measurement and investigation of the plasma density fluctuations is very significant.The far-forward collective scattering（FFCS）measurement method has been developed on the 17-channel far-infrared polarimeter-interferometer system（POLARIS）on J-TEXT takamak to measure the plasma density fluctuation.The improved data processing method can recover the low-frequency density fluctuation which is interfered by the filter in the mixer and the IF bandwidth previously.The density fluctuations related to the tearing mode instability are observed in experiments.The system can provide a measurement of density fluctuations with maximum measurable wave number:k⊥<1.45cm^-1 and time response up to 400 kHz.Further,a method of identifying the propagation direction of the plasma density fluctuation is developed.The optical trace of software shows that when the laser refraction effect is large enough,it’s possible to use the heterodyne coherent scattering measurement of the POLARIS to identify the propagation direction of the density fluctuation.The result of a density ramp-up experiment show that when the density and refraction effect increases,the heterodyne scattering spectrum changes from symmetrical distribution to asymmetrical,verifying the above simulation results.The bias voltage modulation experiment further validates the reliability and accuracy of this technique.Using the FFCS system described above,we observed a quasi-coherent density fluctuation which propagated in ion diamagnetic direction.Finally,the optimized design scheme and software modeling results of the focusing elements are given.The use of a rotating quadric lens eliminates the problem of spherical aberration,and the design of a rotating parabolic reflector allows the system to receive scattered light with larger angles.