Numerical Simulation Study Of Factors Affecting The Quality Of Ion Beam In Neutron Tube

Compact neutron tube is a kind of small-sized and accelerator-type neutron generator.Because of it has many advantages such as high yield,long life and portable,compact neutron tube has a very wide application prospects and becomes research hotspots at home and abroad.The major components of compact neutron tube are metal vapor vacuum arc ion source,plasma diffusion region,extraction and acceleration system and target system.Deuterons are generated from ion source by vacuum arc discharging,after diffusing uniformly,deuterons are extracted to beams and accelerated by electric field which is built by extraction and acceleration system.Finally,deuterons bombard target and have nuclear reaction with tritons in metal target to generate neutrons.The neutron yield is not only related with energy of deuteron,which is effected by D-T reaction cross section,but also related with ion beam's quality,beam's intensity and ion beam's condition when they bombard target.With an order of magnitude of hundred keV energy,it is unavoidable that deuteron beams bombarding metal target will generate secondary electrons.Because of high charge-to-mass ratio,secondary electron current will increase accelerating power load considerably and ruin extraction grid mesh.People always use shield tube or shield net with weak backward electric field to suppress secondary electrons' emission.The emission and suppression of secondary electrons is a dynamic process.Tritium,in target,has natural radioactivity,will decay to helium spontaneously.Apart from few helium atoms will escape from target to build background gas by early period of helium releasing,most of helium atoms free in target and gather in helium bubbles in holes.When deuterons bombard target with 100keV energy,secondary electrons will emit from target,at the same time,helium atoms release from target fast for that deutrons break helium bubbles up.These helium atoms that release from target will have elastic collision,charge exchange and ionization with deuterons in acceleration region.This process will make deuterons drift off the course and can not reach target normally,or make deuterons become neutral particle,or perhaps deuterons can not gain enough energy in limit acceleration region,which will deteriorate ion beams' quality and decrease neutron yield.With the purpose on studying these two process,we use numerical simulation method to study and analyze.We build a simplified ion emission and acceleration model of compact neutron tube,simulate a dynamic process that deuterons bombard target and generate secondary with an external circuit model connecting acceleration region model,to study the effect of secondary electrons on deuteron beam's quality.The results show that when there is 55.44mA secondary electron current built,the acceleration voltage will decrease 45.135%and ion beam's quality deteriorate,the energy deuterons bombarding target decrease accordingly.Whereafter,we add a linear resistance between target and accelerating electrode to build a weak backward electric field to suppress secondary electrons.The results show that this method can suppress secondary electrons effectively and the suppression voltage will change according to the statement of secondary electrons and deuterons.Then,we build collision model of helium atoms and deuterons to simulate the process that helium atoms release from target in both static process and dynamic process,the elastic collision,charge exchange and ionization between helium atoms and deuterons,to analyze the effect of these reactions on deuteron beam's quality.The results show that after collision with helium atoms,the number of deuterons reaching target decreases 0.26%,the quality of deuteron beam deteriorates,the number of deuterons reaching electrode increases on the contrary.The effects of secondary electrons and helium releasing from target is considerable.The study in this paper help suppression secondary electrons and increase target more effective.