Measurement And Analysis Of The Magnetic Perturbations At Plasma Edge On J-TEXT Tokamak

In tokamak plasmas,magnetic perturbations can have a large effect on the macroscopic behavior of plasmas.Two effects are of particular importance.First,magnetic perturbations can generate a large-scale magnetic field through current driven by a dynamo effect,and then destroyes the plasma confinement,sometimes may cause the disruptions in tokamaks.Second,magnetic fluctuations can cause magnetic field lines to wander stochastically.Energy and particle transport can then arise from particles streaming along the stochastical magnetic field lines.Such as in tokamaks,we found the transport flux observed in the experiments is much larger than the value predicted by the(new)classical theory.It indicates the existence of anomalous transport.The experimental results show that the electrostatic fluctuations and the magnetic fluctuations may cause the anomalous transport.Anomalous transport is one of the most important problems to be solved in magnetic confinement fusion.In this paper,the magnetic perturbations at plasma edge(r/a>0.9)werestudied on the J-TEXT tokamak.Before the physical experiment,we have established three sets of experimental systems:The high frequency magnetic probes calibration system,the rectangular array of magnetic probes(RAMP)and the fast reciprocating magnetic probes(FRMP)system.This high frequency magnetic probes calibration system is consisting of a high frequency power supply,Helmholtz coils,amagnetic probe and the oscilloscope.the magnetic probe was inserted in the high frequency magnetic field generated by the Helmholtz coils.The voltages of the Helmholtz coils and the magnetic probe were recorded simultaneously.By implementing the Fourier transform,two analytical lumped-circuit models,with respect to these separated circuits,are constructed to obtain the transfer function between each other.Herein,the precise lumped-circuit parameters(including the resistance,inductance and capacitance)of the magnetic probe can be determined by fitting the experimental data to the transfer function.The rectangular array of magnetic probesis composed of 26 2D magnetic probes which are arranged at the different poloidal position in the vacuum vessel.Each probe can measure the local magnetic field in R and Z direction,simultaneously.This experimental results can be used as the input of EFIT code for the equilibrium reconstruction of the plasma.And this system also can be used for calculating the magnitude and displacement of the plasma current.The fast reciprocating magnetic probes system includes three part:The magnetic probes array,the mechanical pneumatic device and the data acquisition.The magnetic probe array is composed of four 2D magnetic probes atdifferentradialpositions.Each probe can measure the local poloidal and radial magnetic field,simultaneously.the probesarray is mounted at the top of the pneumatic device whichcan move along the radial direction.The pneumatic device can scan the radial range of 5cm with the speed of 1m/s.In the experiment,the FRMP system was successfully used to measure the magnetic perturbations at plasma edge on J-TEXT tokamak.The physical experiment consists of the following parts:1.Measurement of plasma boundary magnetic field topology.When the 3/1 magnetic island exists at plasma edge,the magnetic field topology in the plasma was measured by FRMP.The measurement results were compared with the inversion results of thepolarimeter-interferometer.The results show that the FRMP can accurately measure the magnetic topology at the plasma edge,And the results are compared with the simulation results.2.Experimental study on the particles transport at the plasma edge induced by the magnetic fluctuations.By use of the FRMP system and Langmuir probes,the radial profile of the electron density fluctuations,the poloidal electric field fluctuations,the radial magnetic fluctuations and the toroidal current fluctuations were measured successfully.The particle flux particle flux caused by the electrostatic fluctuations and the particle flux caused by magnetic fluctuations are calculated based on the measurement results,respectively.The results show that the electrostatic fluctuations are dominant in the transport induced by the fluctuations at plasma edge on J-TEXT tokamak.3.Experimental study on the interaction between the external local magnetic perturbations in the reciprocating probe and the plasma.As an exploratory experiment,the preliminary experimental results are obtained.In the experiment,we observed that the local magnetic perturbations can affect the frequency of m/n=2/l tearing mode.When there is no tearing mode,the local magnetic perturbations can excite the m/n=3/1 tearing mode.