Numerical Simulations Of Plasma Breakdown Process On Tokamaks And Reversed Field Pinches

Magnetic confinement fusion is a technique that uses a strong magnetic field to confine hot plasmas to achieve nuclear fusion and release thermonuclear fusion power.At present,different configurations have been built in the world to study fusion plasma,including magnetic mirrors,stelarators,reversed field pinches,tokamaks and so on.The start-up process of these configurations starts with a cold neutral gas.By changing the current of poloidal coils,a loop electric field is induced,which will heat free electrons in the vacuum chamber to collide with neutrals,causing electron avalanche,generating plasmas.By controlling the poloidal coils,a better magnetic field profile is obtained,which will confine the plasma,achieving fully ionization of the neutrals and ramping up of the plasma current.Even if the fusion experiments have been studied for decades,there is no complete theory to explain plasma behaviors during this process,especially the breakdown phase.The Townsend discharge cannot fully explain the breakdown under strong magnetic field conditions.Also due to the limitation of diagnostic technology,there are no diagnostic tools designed for plasma breakdown,the experiment data about breakdown is limited.In response to these problems,plasma breakdown process is studied on the tokamak and reversed field pinch configurations,aiming to provide experimental references and theoretical foundation for experiment researchers.Based on the conditions of plasma breakdown on the magnetic confinement fusion devices,a numerical simulation code has been developed to simulate the plasma breakdown process using particle-in-cell coupled Monte Carlo Collision method(PIC/MCC).This method will be described in detail in chapter 2,as well as the physical assumption adopted in the thesis.The PIC algorithm includes charge accumulation,particle push,solving field equation and processing boundary.The MCC method includes particle sampling and post-processing,collisions between electrons and molecules,and collisions between ions and molecules.The tokamak breakdown driven by pure Ohmic heating was studied under the ideal conditions.The plasma density,D? emission,electron and ion energy loss,averaged electron and ion energy,the evolution of plasma dynamical behaivors were given by the simulation for the first time.It was discovered that the operation mechanisms were different under low pressure and medium pressure operations.Both mechanisms have suitable device conditions and parameter spaces.In addition,by carefully analyzing the simulation results,three stages can be distinguished:fast breakdown stage,transition stage and slow breakdown stage.Plasmas behave differently in those three stages and the main heating mechanisms of electrons and ions are also different.Based on the tokamak breakdown driven by pure Ohmic heating,the effects of pre-filling pressure and external induced field on plasma breakdown were systematically studied.Three discharge modes were found:no-breakdown discharge,successful breakdown and runaway discharge.The plasma behaviors in different discharge modes and the corresponding parameter spaces were specifically analyzed.In addition,the discharge parameters of a tokamak breakdown were compared with the traditional Townsend discharge and it was found that the Townsend discharge couldn't fully explain the breakdown process on the tokamak.Also the International Thermonuclear Experimental Reactor(ITER)breakdown was simulated.When the electric field is 0.3V/m,the range of pre-filling pressure for successful breakdown became narrow,which is consistent with Lloyd's theoretical prediction.At the same time,by analyzing the volt-second consumptionof breakdown,it was found that the optimal pre-filling pressure was about 1.33×10-3Pa,which is in good agreement with ITER's design.The plasma breakdown on the reversed field pinch was investigated.Based on the parameters on the KTX device,the effects of error magnetic field,toroidal magnetic field and loop voltage on the breakdown process were studied.It was found that the influence of the error field was not negligible.In a proper range,the loop voltage could be as large as possible,since it is helpful to achieve a large current and benefits for plasma confinement without runaway generation.Besides,the behaviors of plasma under different parameters were analyzed.Compared with the experiment data on KTX,the simulation results were roughly consistent.Also the breakdown on the tokamak was compared with that on the reversed field pinch,it was found the plasma breakdown on the reversed field pinch was much easier and the ranges of possible breakdown parameters were much wider.