Simulation Study On Ion Source Discharge Characteristics And Extraction System Of RF Ion Source Neutron Tube

Neutron tube is a kind of neutron source equipment with the advantages of low cost,light weight,simple operation and maintenance,easy protection and suitable for the field.It is mainly composed of four parts:ion source,acceleration system,target and air pressure regulation system.Its working principle is to release neutrons through the nuclear reaction of deuterium and tritium,forming instantaneous neutron flow pulse.Neutron tube is widely used in the fields of basic scientific research,military engineering,industry and agriculture because of its strong neutron penetration.At present,the main ion source used in domestic neutron tube is panning ion source.Panning source neutron tube has been recognized at home and abroad for its stable ion beam density,but its neutron yield is difficult to exceed 10~9n / s.With the development of nuclear technology,the neutron tube driven by RF source abroad has been favored by more and more researchers because of its higher neutron yield,stable beam flow,long service life and other advantages.The ion source is an important part of the neutron tube,and the advantages and disadvantages of the ion source will greatly affect the performance of the neutron tube.Therefore,it is necessary to carry out the research work of the neutron tube RF ion source.The process of plasma formation by neutral gas ionization is affected by many factors.In order to deeply understand the discharge characteristics and the law of beam extraction of RF ion source used in neutron tube,starting with the discharge principle of RF inductively coupled plasma,the theoretical model of hydrogen discharge simulation is established.The important factors that affect the particle density in the process of plasma discharge are analyzed and deduced by using Maxwell equations and other formulas.Combined with the two-dimensional axisymmetric model and three-dimensional model in COMSOL software,the corresponding geometric structure is designed respectively,and the relevant parameters affecting the structure composition are found.Using these parameters,the single variable method is adopted,and through a large number of simulation experiments,the influence rules of the structure parameters changes on the discharge characteristics and beam extraction are summarized,and some valuable conclusions are drawn.The specific research work is as follows:(1)In the aspect of theory,based on the discharge principle of RF ion source,the database of collision reaction cross-section in hydrogen discharge is established by basic collision reaction,and the important parameters in the simulation process are calculated by combining the database and Boltzmann equation two approximate modules,which provides theoretical support for the simulation of neutron tube RF ion source.(2)In the aspect of simulation,using the low temperature plasma module of the Comsol multi-physical field simulation software,the discharge characteristics of the neutron tube RF ion source are studied,and the hydrogen ion density distribution of the RF inductively coupled plasma is simulated from a macroscopic point of view.By changing the coil power,discharge air pressure,the coil type and the coil size,the influence of the structure parameters of the ion source on the distribution of hydrogen ion density is obtained.(3)The beam extraction structure of neutron tube RF ion source is designed,and the hydrogen ion extraction structure is studied by using the charged particle tracking module of Comsol software.The simulation experiment is carried out by changing the diameter of outlet hole,outlet voltage and acceleration gap.According to the simulation results,the influence of these parameters on the electric field and the ion track distribution is summarized.Through the analysis of a large number of simulation data,the optimal parameters of spiral antenna are turn spacing of 5mm,turn number of 4,coil diameter of 3.5cm.The best parameters of the coil antenna are 6 mm turn spacing,3 turns number.And the maximum value of hydrogen ion number density can be 5.0410~9/m~3 and 4.9610~9/m~3 under the conditions of 1W and 1Pa on-line power and discharge air pressure,respectively.When the diameter of the outlet hole of the plasma electrode is 6mm,the diameter of the outlet hole at the upper end of the Faraday cylinder is 8mm,the acceleration gap is 14 mm,and the extraction voltage is-120 kv,the focus surface can be closer to the plasma electrode,and the distribution area of the outgoing beam at the bottom of the Faraday cylinder is larger.In this paper,the discharge characteristics and beam extraction characteristics of the neutron tube RF ion source are summarized by using the method of simulation experiment,and the general rules of the RF ion source are mastered,which is of great significance to the optimization of the corresponding parameters and the design of the specific structure of the neutron tube RF ion source in the future.