Numerical Investigations On High Power Radio-Frequency Inductively Coupled Negative Hydrogen Ion Sources Based On A Three-Dimensional Fluid Model

A neutral beam injection(NBI)prototype has been put forward to China Fusion Engineering Test Reactor(CFETR)in the Southwestern Institute of Physics(SWIP).The negative hydrogen ion beam extracted from the negative hydrogen ion source(NHIS)for CFETR NBI prototype is expected to have current of 20 A and energy of 200-500 keV for 3600 s.In order to optimize the design of NHIS,a three-dimensional(3D)fluid model,which includes the electromagnetic field module,magnetostatics module and plasma module,is developed within the finite element analysis software COMSOL to simulate the NHIS with a single driver,double drivers and four drivers.In order to improve the computational efficiency,the drift diffusion approximation is adopted for the conservation equations of electron momentum and ion momentum in the 3D model.Besides,the effective collision frequency is used in the electron transport coefficient,and the high electric field ion mobility is used in the ion transport coefficient,to make sure the validity of this model at low pressures.First,the simulation and experimental research progress of low pressure inductively coupled plasmas and high power NHIS have been reviewed in the introduction.Then,the effects of magnetic filter field,magnetic shield and discharge parameters on the plasma spatial distribution are analyzed by the 3D fluid model for magnetized plasmas.The main conclusions are as follows.(1)The influences of the magnetic filter field,pressure and power on the plasma characteristics are demonstrated in chapter 3.The magnetic filter field is generated by the magnetic filter(MF)with permanent magnets whose properties are determined by remanence.It is found that the electron density and electron temperature in the xz-plane become much more asymmetric when the magnetic field is enhanced.However,the plasma parameters in the yzplane are symmetric no matter the magnetic field is applied or not.Besides,the electron density throughout the whole chamber first increases and then decreases with magnetic field,while the electron temperature at the bottom of the expansion region first decreases and then almost keeps constant.For different remanence of magnets,the electron density in the driver region is the highest at |Bre|=6 kGs,and the electron density at the bottom of the expansion region reaches the maximum value at |Bre|=2 kGs.As the pressure increases,the symmetry of the electron temperature in the xz-plane becomes much better.(2)As mentioned above,the magnetic filter field reduces the electron temperature and electron density in the expansion region.This is because the magnetic field in the driver region,which is stronger than 10 Gs,affects the generation and transport of electrons.Therefore,the"magnetic shield" is proposed in chapter 4.In this strategy,the magnetic shield material(high relative permeability)is applied to the wall of the chamber to reduce the penetration of the magnetic field into the driver rgeion.The results show that the application of magnetic shield material to the interface between driver and expansion regions can effectively increase the electron density,i.e.,the maximum value at the bottom of expansion region increases by about 30%.Meanwhile,the electron temperature in the expansion region remains below 2 eV.Besides,the simulation results agree well with experimental measurements of SWIP.(3)The effects of magnetic filter field on the plasma distribution in multi-drivers NHIS are presented in chapter 5.First,the influence of magnetic filter(MF)position on the spatial distribution of plasma parameters in double drivers NHIS is indicated.Due to the E × B-drift,the maximum of the electron density shifts towards the left of the drivers.Besides,the electron density of the driver Ⅱ is higher than that of the driver Ⅰ.This is because electrons are more constrained by the strong magnetic field in the driver Ⅱ,so that it becomes more difficult for them to transport to the expansion region.In addition,the electron density in drivers increases first and then decreases while MF drifts to the driver region,whereas the value at the bottom of expansion region decreases gradually.In order to optimize the spatial distribution of the electorn density,the power parameters could be modulated.Meanwhile,a plate is applied to the expansion region to modify the electron temperature distribution there.The results indicate that the distributions of the electron temperature and electron density in the expansion region are more homogeneous when the power of the two drivers are 40 kW and 42 kW,and the height of the applied plate is from 10 cm to 15 cm.For four drivers NHIS,when the magnetic field is generated by the input current on plasma grids(PG),the electron temperature is homogeneous in most regions except for the left side of the bottom expansion region.Besides,the electron density exhibits higher values on both sides of the bottom expansion region.When the PG current increases from 1000 A to 5000 A,the electron temperature at the bottom of the expansion region decreases first and then increases slightly;the homogeneity of the electron temperature in the expansion region is improved;the electron density in the expansion region decreases by about 50%,because the electron transport between the driver and the expansion regions is constrainted.Finally,a brief summary of this dissertation and the prospects of further work are given in chapter 6.