| An ideal tokamak magnetic configuration is two-dimensional structure consisting toroidal and poloidal magnetic field. The deviation from the tokamak magnetic field coil shape and the mounting position can lead to radial magnetic field generating three-dimensional structures,which is called error field. The error field would hinder the rotation of tearing modes, indicating that the rotating of plasma can be more easily locked. The locking of large magnetic islands by error fields is often observed in tokamak, leading to severe confinement degradation or even to disruptions. If magnetic island is driven and rotates again during the locked mode(LM), namely unlocking in common sense, the disruption of plasma would be avoided or delayed. So it is of great significance to study the error field and the control of tearing modes in tokamak even fusion reactor in the future, for the purpose of avoiding disruption caused by mode locking.Error field and external resonant magnetic perturbation(RMP) field will generat a periodic magnetic moment on tearing magnetic island, the role of the moment always has slowed the relative motion of RMP and the island, the island tends to make the magnetic island locked in error field or RMP. Based on this, after tearing mode grow up even mode locking, a suitable rotating RMP is applied to drive the LM to rotate and unlock it, preventing or delaying tearing mode disruption. The experiment in this paper is mainly built on J-TEXT tokamak. Through the analysis and compensation of error field, experimental research about LM unlocking by rotating RMP has been carried out.Firstly, we use the scaling about LM and penetration threshold with the macroscopic plasma parameters and the interaction of error field and tearing mode to obtaine magnitude and phase of error field in J-TEXT tokamak. The amplitude and phase are 0.84Gs and 256.1°, respectively, under a typical discharge conditions (Ip is 60kA, Bt is 1.6T) in J-TEXT. We also use software such as Ansys to research coils misalignment and lead connector. It is found that toroidal field coils misalignment and lead connector is a main sources of error field in J-TEXT. According to the magnitude and phase of error field measured, we carried out experimental studies utilizing RMP coils to correct error field and obtained a good compensation effect.Secondly, a research aiming for LM unlocking by rotating RMP is carried out in J-TEXT tokamak. It is found that the unlocking of LM is sensitive to the amplitude of static RMP (or error field) and the frequency of rotating RMP. The unlocking process contains two stages, i.e. the oscillating stage and the unlocking stage. In the oscillating stage, the rotating RMP with amplitude that is not strong enough causes the LM to oscillate around its locked phase and produces magnetic fluctuation to behave as a standing wave-like structure in poloidal direction. When the amplitude of the rotating RMP is strong enough, it first causes the LM to oscillate and then transforms to mode unlocking quickly, namely the unlocking stage. Further analysis shows that the unlocking of LM is determined by the torque balance between the viscous torque and the electromagnetic torques exerted by both the static and the rotating RMP. In addition, the unlocking process is sensitive to both the amplitude and the frequency of the rotating RMP as well as the amplitude of static RMP. Nonlinear numerical modeling based on reduced MHD equations is also performed to understand the unlocking process, and numerical results qualitatively agree with the experimental ones.Furthermore, a large amount of eddy current will be induced in the conductor, resulting in a strong attenuation of the magnetic field of the RMP coils. Thus, to reduce the screening effect of the eddy current on the magnetic field, the insertion of soft magnetic material blocks with high permeability between coils and the conductor is proposed. |
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