Study On Discharge Characteristics Of Triggered Vacuum Arc Ion Source

The triggered vacuum arc ion sources (TVAIS) been intensively used to provide high current ion beams in film deposition and material surface modification, particle acceleration injection, plasma thruster and fundamental nuclear physics studies, primarily due to the low breakdown voltage, high arc current and high repeatability. On the basis of the review on the current developments of discharge theories, triggering mechanisms and triggered discharge modes for the TVAIS, the properties of the TVAIS based on the high-voltage surface flashover discharge mode are studied by using both experimental and simulative approaches.In an attempt to further understand the mechanism of the high-voltage surface flashover discharge, the electric field distribution and the electron and ion temporal distributions in the discharge process are simulated by using particle-in-cell (PIC) method on the assumption that initial electron emission based on the Fowler-Nordheim formula, Secondary Electron Emission Avalanche (SEEA) theory and the impact ionization model for a desorbed gas layer on a ceramic insulator surface. The images of secondary electron multiplication and occurring of breakdown in the surface flashover process are acquired.The characteristics of the TVAIS such as the trigger voltage, arc burning voltage and delay time are obtained for different materials and gap distances. Results show that the triggered time delay of the ion source is mainly affected by the triggering structure, triggering current density and triggering voltage. There exist little correlations with the main gap distance and the main gap voltage. The trigger voltage is in the range of7-11kV. The burning voltage of main arc is about30V. The action delay time is about20ns from the beginning of the trigger pulse to the time that the main gap voltage begins to decline, while the conduction delay time is130-210ns from the starting point of the main gap voltage declining to the time that the main gap voltage stabilizes, and increases with the distance of the main gap. The discharge mechanism is studied by combining a high speed framing camera with high temporal-spatial resolutions and discharge parameter measurements; the influence of cathode spots formation and plasma diffusion on triggered vacuum arc discharge are also discussed. The photographs of discharge process and discharge gap are performed. The factors influencing discharge are also analyzed.The total ion current of triggered ion source is measured using flat probe diagnostic method. The results demonstrate the structure of the ion source exerts a significant influence on the ion current magnitude. Ion current has linear relationship with arc current. The standard deviation of ion current falls in the range of7%-17%, on which arc current has little influence.The triggered discharge plasma is diagnosed using emission spectroscopy, from which the electron excitation temperature of the discharge plasma from titanium cathode is obtained. The influence of arc current on the emission spectrum and plasma electron temperature is also discussed. Research shows that ionization rate is very high and light intensity of atom emission spectrum is weak, and the electron temperature is about2eV.