Optimal Design Of The Magnetic Circuit And Extraction System Of The Microwave Ion Source Used In The Neutron Tube

Neutron tube is a kind of accelerator type neutron source,which is composed of an ion source,an acceleration system,a target,an air pressure adjustment system.It has broad application prospects in the fields of petroleum logging,explosive and drug detection,radiological medicine,and reactor physics.As one of the important components of the neutron tube,the ion source ionizes the deuterium and tritium gas to generate plasma,and the plasma is extracted to form the ion beam.The quality of the ion beam will affect the yield,life and stability of the neutron tube.The working principles of different ion sources are different,and the percentage of each ion in the generated plasma is also different.The main principle of the neutron tube is that the internal monoatomic deuterium and tritium ions react to generate neutrons,and the monoatomic ion ratio produced by the microwave ion source is more than 80%.In this paper,the microwave ion source is used as the injection source of neutron tube.The performance of the microwave ion source is affected by many parameters,especially the magnetic field and extraction system.For 2.45 GHz microwave ion source,the microwave absorption efficiency can be improved by providing appropriate magnetic field intensity at the microwave window.High density plasma can be produced by increasing the number of ECR regions on the premise of uniform magnetic field distribution.In this paper,CST Studio Suite is used to simulate the internal magnetic field of ion source with different discharge chamber materials.Through experiments and simulation software,a variety of magnetic circuit systems including single magnetic ring,double magnetic rings and multi magnetic rings are designed to optimize the axial magnetic field of discharge chamber.The magnetic field distribution of axial magnetized and radial magnetized double magnetic rings in the optimal mode is compared.Neutron tubes of different applications have different special requirements for ion beams.The extraction system is designed for high-yield neutron tube and tagged neutron tube,respectively.The high-yield neutron tube has a high beam intensity,which is easy to cause target damage and affect the life of the neutron tube.A double slits two-electrode extraction system was designed for high-yield neutron tube.By adjusting the position and size of the slits,the distribution of the beam on the target is uniform and the bombardment area of the ion beam is enlarged.The tagged neutron tube with ? detector is often used to detect the position of explosives,and the extraction beam needs to be focused to form a small beam spot.A three-electrode extraction system was designed for tagged neutron tube.The position and thickness of the extraction electrode and the voltage of each electrode are changed to meet the requirement of small beam spot.One of the most important goals of the ion source is to produce ion beam with high current density.In order to increase the intensity of the beam,an extraction system was designed in which a positive bias electrode was placed behind the plasma electrode.High density plasma is formed near the extraction hole,which can effectively improve the neutron yield.