Study on the properties of deuterium ions in a composite cathode vacuum arc discharge

Abstract

Vacuum arc discharge with metal-deuteride cathode can generate supersonic deuterium ion jet, which finds important applications in vacuum arc ions sources. In this study, a 1D3V spherical PIC-DSMC cathode spot model with titanium-deuteride cathode is developed to investigate the ionization and acceleration of deuterium ions from vacuum breakdown to steady-arc stage. The effects of cathode deuteration concentration on the deuterium ion fraction and kinetic energy are also analyzed. The results show that the released deuterium atoms start to be ionized at about tens of nanometers far away from cathode and then be fully-ionized at about 2 μm as cathode potential drop gradually building up. The proportion of deuterium ions in plasma is approximately same as the proportion of deuterium atoms in cathode material. Velocity separation of deuterium and titanium ions occurs due to the acceleration of electric field during the vacuum breakdown stage, however, the predominant ion-ion coulomb collisions wipe out this separation at the steady-arc stage. Shifting the deuterium atoms concentration in cathode under the constant arc burning voltage produces an approximately equal current density, and increases the velocities of all ion species. The higher ion kinetic energy is gained by reducing the ohmic heating dissipation, which is facilitated by the lower plasma resistivity under the increased deuterium ion density.