Measurement And Investigation Of The Sawtooth Perturbation Behaviors On J-TEXT Tokamak

Sawtooth is a macro-instability of plasma occurring in the core area, related with the magnetic perturbation (current disturbance) within surface of q=1. It will lower plasma core parameters, degrade plasma confinement properties, but can also rule out impurities from the core. To successfully achieve precise control of the sawtooth, improving the core plasma confinement, and ruling out a particle produced from fusion reactions, it is necessary to conduct further exploration and research on particle transport and magnetic perturbation in sawtooth behaviors. Three-wave polarimeter-interferometer can measure electron density and core magnetic field simultaneously with a high temporal resolution. If sawthooth can be observed, undoubtedly it is the best method to carry out these studies. I have participated in all the construction work of J-TEXT polarimeter-interferometer (POLARIS), and implement the measurement of sawtooth behaviors successfully by optimizing its accuracy. The main contents of this paper are as follows:Eliminating vibration is the key to achieving high-precision measurements of the polarimeter-interferometer. According to experiences obtained from vibration eliminating pre-test on HCN interferometer, we achieved effective vibration damping during design and construction of POLARIS based on inertial vibration resistance, medium vibration reduction principle, rationally allocate large mass base and rubber vibration isolator. Later, on account of the diagnostic principle, we arrange the layout of reference chord the same as probe chords, so that the impact of the residual vibration can offset in phase comparison angle measurement. Eventually, the influence of vibration is reduced to noise level, with the density of～1°(1.4×1016m-2), and the Faraday rotation angle of～0.1°.Collinear alinement is a basic requirement to obtain accurate Faraday rotation angle and is one of the largest difficulties for achieving three-wave polarimeter-interferometer. Because the additional phase of plasma density is larger about2orders of magnitude than that of the Faraday rotation angle, if the two probe beams are not collinear, measurement error will be introduced into the Faraday rotation angle from density gradient. Collinear adjustment of POLARIS is completed by using a laser beam analyzer. Then, a polarizer is added in the light path creatively to modulate the two beams’ polarization state from circularly polarized to the same linear polarized before they pass through the plasma, and for the first time achieved the direct measurement of collinear error.Through the above optimization, POLARIS successfully measure accurate sawtooth perturbations on both the electron density and Faraday rotation angle signals.Because polarimeter-interferometer only measures chord integral amount, to get the distribution of electron density, current density and safety factors, we also need to perform invertion of the measured data. in POLARIS, an inversion program based on Equilibrium reconstruction is also developed.On this inversion program, a modification on the diagnosis refraction effect of inversion procedure is made to realize the measurement error of the current density by J-TEXT polarization interferometer to be less than10%in the-0.6≤ρ <0.6area. This result is consistent with the accuracy requirements for current density measurement of the polarimeter-interferometer which is planned be to developped on the International Thermonuclear Experimental Reactor (ITER).By using the density sawtooth signal measured by POLARIS and the density distribution obtained from the inversion, research on the particle transport during the sawtooth has been done. Results show that when the sawtooth collapses the plasma core particle transport coefficient will explosively increase and the particle transport coefficient near the outside will correspondingly increase, which leads to the fact that the core particle density distribution becomes flat and the change of density near the edge is not obvious; moreover, when the sawtooth collapses the outward transport of the core particles can only reach the surface of q=2, which will become a challenge for ruling out core a particle through sawtooth.Finally, we conduct a preliminary analysis on the Faraday rotation angle. The study on the reverse of adjacent Faraday rotation angle signals at the low field side indicates that ～5kA local current perturbation exists between the two chords, and the current perturbation has the same direction with the plasma current, featured with poloidal mode as n=0. The sawtooth diversity presented on the Faraday rotation angle signal from POLARIS also shows that magnetic perturbation during the sawtooth are more complex than the existing theory and imagination, and we need to conduct deeper research on measurement techniques and theoretical models.