Research Of The Formation Of The Magnetic Island And Its Application For The Divertor Configuration In The Magnetic Confinement Plasma

The instability of the tearing mode is a key issue in magnetic confinement fusion plasma,the resulting magnetic islands can significantly degrade the confinement performance of the plasma and even cause large disruption.The threshold of the magnetic island excitation must be considered in the design and operation of large devices such as ITER and DEMO.It is expected that the parametric dependence of the magnetic island excitation threshold from present devices can be applied in ITER.This requires plenty of experiments to demonstrate its reliability as well as a better understanding of the process of excitation.On the other hand,the magnetic island divertor configuration can be formed by cutting the edge magnetic island via target plates.The power decay length of the scrape-off layer（SOL）in the island divertor configuration is large.This advantage makes it one of the methods to alleviate the problem of the peak thermal load of the divertor exceeding the material limit.The study of the transport in the boundary magnetic island region is the key to better understanding and application of the magnetic island divertor configuration.To clarify above problems,this thesis carried out the researches on the magnetic island excitation threshold and the edge transport characteristics of the magnetic island divertor on the J-TEXT tokamak and the Wendelstein 7-X stellarator device,respectively.On J-TEXT,a new resonant magnetic perturbation（RMP）coil system is established to meet the requirements for magnetic island excitation in a wide range of plasma parameters.The electromagnetic and thermal characteristics of the RMP coils are figured out by finite element simulation.According to the simulation results,the cross-sectional structure and size of the conductor,insulating layer,and protective layer of the coil are designed.The graphite armor structure and the coil outlet adapter to resist plasma strikes have also been improved.The new RMP coil is compact,the current limit is 5 k A,can generate much stronger magnetic field,and can be applied in the vacuum chamber.The effective RMP component generated by the new system is 2.33 times that of the previous one,and the flexibility has also been greatly enhanced.This set of coils has been widely applied in J-TEXT experiments since 2017.Using the new RMP system,the magnetic island excitation experiments were carried out in the full operating parameter range of J-TEXT.The dependence of the magnetic island excitation threshold on plasma density,toroidal field,edge safety factor,heating power,and other parameters is obtained.It is found that the magnetic island excitation threshold changes non-monotonously with the increase of density and safety factor,which is contrary to the empirical scaling n_e¹.Disentanglement of density and rotation dependence of the excitation threshold was performed for the first time.The result indicates that the excitation threshold depends linearly on the rotation but only weakly on the density as n_e^-0.18.It may explain the different density scaling on different devices.Further experiments were carried out to excite the magnetic island with the high-frequency rotating perturbation field.It also proves the linear dependence of the excitation threshold of the magnetic island on the rotation,and the plasma shielding response and the excitation process were dynamically observed.To identify the pattern evolution during the excitation of the magnetic island,a method of identifying the weak pattern evolution based on the shortest path algorithm was developed.Even the signal-noise ratio is 1,the certain degree is 80%.This method is not only suitable for the cases in this thesis,but also generally suitable for pattern recognition in the time series of stationary and non-stationary processes.On W7-X,the turbulence and transport characteristics of the SOL in the island divertor configuration were studied using the combined probe.In the process of studying edge magnetic fluctuations,this thesis has developed a method based on circuit models to achieve low-frequency magnetic probe signal correction.A local broadband spectrum electromagnetic turbulence of frequency 80～200 k Hz was found in the far SOL with high density gradient and flat temperature profile.The particle flux and heat flux caused by electrostatic fluctuations also peak in the electromagnetic turbulence region.It is found that as the plasma current increases from-1 k A to 2.6 k A,the position of the electromagnetic turbulence will move obviously and the resulting particles and heat flux will increase.Tracing the magnetic field lines found that the electromagnetic turbulence position is located in the long magnetic field line connecting length area outside the edge magnetic island.The position change caused by the change of the magnetic field structure in different plasma currents corresponds to the electromagnetic turbulence position in the experiment.The above results prove that the magnetic field structure can affect the abnormal transport level and play an important role in edge transports and profiles.The work of this thesis focuses on the magnetic island.On J-TEXT tokamak,it gives an explanation of the divergence on density scaling of the field penetration threshold.On W7-X stellarator,it demonstrates that the electro-magnetic turbulence is crucial for the edge transport.In addition,the work is related to two types of magnetic confinement devices across tokamak and stellarator,which makes it important for the research of advanced magnetic field configuration of magnetic confinement fusion in the future.