Study On Neoclassical Tearing Modes Suppressed By Unjustified Electron Cyclotron Current Drive

Neoclassical tearing modes(NTMs)instability is an unavoidable MHD instability in all major tokamak experiments.NTMs will seriously limit the ? threshold of the device well below,and will change the magnetic field near the rational surface of tokamak,destroy the local plasma balance,enhance the radial transport of the plasma,and so on.In serious cases,they may break or transport of the plasma discharge and damage the plasma facing components.The NTMs have been studied in the past decades,a variety of methods to avoid and control NTMs have been explored.Theoretically,it is a feasible method that producing a codirectional driving current in the magnetic island to suppressing NTMs,such as electron cyclotron current drive(ECCD).The method was verified by suppression NTM by ECCD in experiment.ECCD is also planned for ITER to control NTMs.It is very important to develop the theory of ECCD suppression NTMs for the fusion plasma devices.According to the current research results,the required power of electron cyclotron wave(ECW)more than 70MW for ITER to completely suppress NTMs instability.So,it will be greatly helpful for the future tokamak reactors to improve the efficiency of ECCD to suppress NTMs instability.In this paper,the classical tear modes(TMs)and NTMs are introduced by introducing the MHD instability,and it is well known that the TMs can be controlled by suitable plasma current.However,it seen impossible to control NTMs by suitable plasma current from the theoretical study of controlling the NTMs.It is shown that the excitation of NTMs in major tokamak requires seed magnetic island and high plasma pressure to exceed the ? threshold.It is likely that the ? of the system exceeds the threshold and may stimulating the NTMs when the powerful and high heating efficiency neutral beam is added.What's more the emergence of NTMs will greatly influence the constraint of tokamak,increasing the loss of high-energy particle and the heating efficiency of the neutral beam will decrease obviously.NTMs was observed in the EAST when the neutral beam injection.At present,ECCD is one of the key methods to suppress NTMs,and the heating and current driving theory of ECW are introduced in this paper.TORAY-GA and TM1 are well known numerical simulation program to study and analyze of ECW heating and current driving and the evolution of NTMs in tokamak experiments,respectively.The system of ECW is optimized for the aiming of ECCD suppress NTMs.The launch angle of ECW is scanned for a launch position and the scanning step is 0.5°,the radial position of the rational surface is the first selection condition,and the parameters of ECCD are the second selection condition for the suitable launch angle.Finally,the optimal launch angle is determined by the test of the TM1 program with the parameters of ECCD.In the same way,the relationship between the minimum power of ECW to completely suppress the NTMs with different launch positions.The simulated results may show that the optimal launch position of ECW is not uniform for the NTM with m/n=3/2 and m/n=2/1.The simulated results also show that the radial deposition center of ECCD at the rational surface even the scanning step is 0.5°,the radial position of the rational surface changed with the addition of ECCD,and the radial misalignment between the radial deposition center of ECCD and the rational surface is the objective fact that ECCD suppresses the NTM in the experiment.In this paper,the influence of the small radial misalignment on the suppression of NTM is studied.The simulated results seen show that the suppression of NTM by ECCD does not required the radial deposition center of ECCD at the rational surface,and there is an alignment region between them.The radial deposition center of ECCD can completely suppress the NTM within 0.01a outside of the rational surface.At the same time,the simulated results also show that NTM can be easier completely suppressed by the ECCD if the radial deposition center of ECCD at the outside of the rational surface by increasing the power of ECW.There is an obvious difference between the ECCD center inner and outer sides of the rational surface by paying attention to the plasma current density distribution near the rational surface,which also leads to the different suppression efficiency of the NTM by ECCD with the ECCD center on the inside and outside of the rational surface.The radial misalignment will obviously influence the suppression efficiency of the NTM by ECCD.If it is too larger to completely suppress NTM by ECCD,and which need to be reduced.The simulated results seen shows that the radial misalignment will be reduced and completely suppress NTM by ECCD if the radial deposition center of ECCD is adjusted by the feedback of magnetic island width with enough long deposition time.In order to greatly reduce the power of ECW.In this paper,suppressing NTM at the initial stage by ECCD is studied.The simulated results show that adding ECCD at the initial stage of NTM can greatly reduce the required power of ECW.At the same time,it is also show that the timing is related to the width of magnetic island.The simulated results show that ECCD must be added before the width of magnetic island reaches 0.05a.Modulated ECCD is more efficient for NTM at the initial stage.The growth rate of the magnetic island will slow down by adding a small ratio of continuous ECCD in the early stage of the NTM.Improved the suppression efficiency of NTM by continuous ECCD and reduced the effect of radial deviation on the suppress of NTM by ECCD.