Study On The Characteristics Of High-energy Backstreaming Particles In High-current Ion Sources

High-energy backstreaming particle is an unavidable problem within the operation of high-current ion sources.Since higher power and longer pulse neutral beam injector is being developed for magnetic confinement fusion reaction,high-energy backstreaming particle is one of the research focuses of high-power,long-pulse and high-current ion source.This dissertation mainly analyzed the physical mechanism of high-energy backstreaming particles,and carried out the experimental research based on the high-current ion source experimental platform.Via the diagnostic means,this dissertation explored the thermal deposition distribution of high-energy backstreaming particle,and carried out the study of the characteristics of high-energy backstreaming particle,therefore the optimization methods of reducing their production and harm are studied,so as to meet the goal of the high-power,long-pulse,safe and stable operation of high-current ion sources.According to the collision theory,this dissertation theoretically analyzed the physical processes of high-energy backstreaming particle generated in the ion source beam extraction system and optimized the calculation model,based on the structure of the positive ion source for Neutral Beam Injector of Experimental and Advanced Superconducting Tokamak(EAST-NBI)and the negative ion source for Neutral Beam Injector of Chinese Fusion Engineering Test Reactor(CFETR).It is estimated that the power of backstreaming electron accounts for about 7.4%of the extraction positive ion beam power,and the flux of backstreaming positive ion accounts for about 2.8%of the extraction beam current.Based on the measurement of thermal and electrical parameters,the diagnostic methods of backstreaming electron are enriched and improved through the combination of theory and experiment.By using water flow calorimeter system,the thermal parameters of arc chamber and grids are obtained,thus the power magnitude of backstreaming electron and its main deposition position are qualitatively measured.By using the suppressor grid current as the electrical parameter to characterize the backstreaming electron,the lower limit of the suppressor negative high voltage setup values are calculated theoretically at different extraction voltages.The gradient grid current is also used as the electrical parameter to characterize backstreaming electron and then the magnitude of gradient grid current is calculated theoretically and compared with the experimental results.Therefore,the characterization problem and the diagnostic method of backstreaming electron have been solved.Based on the existing experimental conditions,optimization schemes were proposed,including reducing the yield of backstreaming electron production and the harm to the arc chamber.Through theoretical calculation,the relationship between the divergence angle and the perveance is derived.It is experimentally verified that the increase in the perveance increases the backstreaming electron production,while the influence of the divergence angle on the backstreaming electron production is weak.Through physical analysis and simulation,the influence of arc chamber magnetic field,especially the top center magnet,on the distribution of backstreaming electron deposition is studied.Finally,after theoretical calculation for the influence of the gas flow rate on the backstreaming electron,the relevant experiments for the influence of the gas flow rate on the backstreaming electron were designed and complished,and the positive effect of controlling the gas flow rate on reducing the backstreaming electron yield was verified.This dissertation established a space vacuum gradient calculation model for the negative ion source accelerator,and theoretically studied the influence of vacuum gradient on negative ion stripping loss.Referring to the international research method for backstreaming positive ion and combining the structure of CFETR neutral beam injector negative source prototype,diagnostic schemes for measuring the distribution of positive ion deposition power on the back plate of arc chamber by the use of thermocouple,and detecting copper pollution source and deducing the flux of backstreaming positive ion and its power by the use of sputtering probes were proposed.