The Kinetic Simulation Study Of The Scrape-off Layer Plasma With Electrode Biasing

Around 1982,F.Wagner and other researchers firstly discovered the High-confinement mode(H-mode)on the German ASDEX tokamak(Tokamak).Studies showed that when the neutral beam heating power exceeds a certain threshold,Tokamak plasma will spontaneously form a boundary pedestal,the boundary turbulent transport is suppressed,and the energy confinement time increases exponentially,which greatly improves the restraint performance of the device.However,the improvement of the H-mode constraint performance is accompanied with a brand-new instability,namely Edge-Localized Modes(ELMs).This is an instability of quasi-periodic eruption at the boundary,which will lead to periodic collapse of the boundary pedestal.A large num-ber of particles and heat generated by the collapse will be sprayed onto the plasma wall material in a very short time scale,which seriously shorten the material Lifes-pan and threaten the safety of the device.How to reduce and control the amplitude of ELMs bursts while maintaining the H-mode has been one of the most important topics in fusion research during the past three decades.At present,the experimental methods for controlling ELMs mainly include Resonant Magnetic Perturbations(RMPs),pel-let injection,plasma shaping,and plasma rotation control.Among them,RMP,as a mainstream control method,relies on the three-dimensional perturbation magnetic field generated by the external coil.In the future fusion reactor,due to the presence of neu-tron irradiation,the RMP coil in the vacuum chamber may not be suitable for ELMs control.Therefore,it is of great significance to explore RMP generation methods that do not rely on external coils.Around 2012,the researchers from ASIPP found that the Lower Hybrid Wave(LHW)has mitigation effect on ELMs through the LHW modulation experiment on the EAST(Experimental Advanced Superconducting Tokamak)tokamak,and proposed a possible physical mechanism,that is,LHW drives the fast electrons in the Scrape-off Layer(SOL)and the three-dimensional perturbation magnetic field generated by the fast electron current is similar to the perturbation field generated by the RMP coil and has the potential to affect the behavior of ELMs.Additionally,some theoretical studies have also shown that it is possible to drive a sufficiently strong current within the SOL to control ELMs through target bias and other methods.However,the relevant experi-ments and theoretical analysis of active current drive in SOL are still in the initial stage.This paper,for the first time,studies the characteristics of the SOL currents generated by divertor biasing using kinetic simulation methods,which is capable of obtaining a self-consistent potential and current distribution in SOL area.This article can be roughly divided into three parts:The first part mainly in-cludes the construction of the kinetic simulation model.For example,how to build a Vlasov-Darwin system and couple it to a particle simulation(Particle-in-cell,PIC)program,how to solve the Darwin electromagnetic field equations,and construct a two-dimensional real space three-dimensional velocity space(2D-3V)PIC simulation program,how to analyze the numerical stability of the program and realize related nu-merical problems such as parallelization.The second part mainly includes verifying the correctness of the PIC simulation program.By simulating several classic kinetic phenomena and instabilities,and com-paring the results with the linear kinetic theory,the reliability of the simulation program is analyzed.The kinetic simulations listed in this article include,two stream instability,Landau damping,the electron Bernstein waves and the Bohm sheath.The third part is the main content of this thesis,which includes the one-dimensional theoretical analysis of the biasing driven SOL current and the simulation results from the two-dimensional PIC simulation program.Theoretical analysis and simulation results show that the current driven by the bias voltage disperses in the SOL and is not a fixed current parallel to the magnetic field lines.Due to the presence of the plasma sheath,most of the biasing voltage will be shielded by the electron sheath near the electrode and can not enter the plasma.Apart from that,the current system must reach a state of global ambipolar transport before it can reach equilibrium,which will cause the current to decay in the direction of the parallel magnetic field lines.In a two-dimensional sim-ulation within a idealized geometry,this attenuation is linear,which gradually decays from the electron saturation level to the ion saturation level.Both PIC simulation and the experimental results from the electrode biasing experiments on the J-TEXT Toka-mak indicate that the biasing driven SOL current decays in the direction parallel to the magnetic field.Finally,this thesis will analyze and discuss the feasibility of generating SOL currents in the realistic tokamak configuration,and make reasonable predictions and suggestions for the SOL biasing experiments based on the simulation results.