Theoretical And Experimental Researches On Field-Breakdown Triggered Vacuum Switch

Triggered Vacuum Switch (TVS) is an important switch apparatus in the pulse power technology. Its mechanism of turning on and off is one of the forefront topics for study. The initial plasmas' generation and development, which have an influence on the TVS's operation characteristics (such as trigger reliability, delay characteristic, trigger accuracy, work life, etc.), becomes the key of turning on a TVS successfully. Based on Field-Breakdown Trigged Vacuum Switch (FTVS), the mechanism of initial plasmas'formation and diffusion during the turning on is proposed in this paper, and the influence of initial plasmas to the main gap switching-on is discussed, especially to the trigger reliability, delay, and trigger accuracy. Some FTVS samples are designed and fabricated. The results of this paper can support the basic research on the pulse power control switches, and the trigger system designed in this paper can be used in the fields of pulse power applications.The electrons emission is the first step of FTVS initial plasmas'generation, due to high-field emission in FTVS. Firstly, the initial plasmas'generation process in FTVS vacuum trigger gap is described from two aspects, macroscopic and microscopic view. Secondly, based on filed emission mechanism, a cathode spot heat conduction model of vacuum discharge is set up to deduce the microscopic transport characteristics of initial plasmas by mathematics model building method and thermodynamics motion equations. The microscopic transport characteristics of initial plasmas are embodied mainly in the trigger delay of FTVS. So, the trigger delay of FTVS is calculated from the heat conduction model, and the calculated results are accordant with the typical test results, which can prove the correctness of model. The research of initial plasmas'microscopic transport characteristics affords theory support to the optimum design of FTVS.Two kinds of FTVSs with different trigger structure are fabricated, i.e. needle TVS-1 and T-shaped TVS-2. Experiments are done to analyze and compare their basic work performances, such as the influence of trigger energy to the trigger system delay and its jitter time, the influence of trigger energy to the minimal break-over voltage, the influence of main gap voltage to the trigger delay, jitter time and successful trigger rate, etc. Results show that the T-shaped TVS-2 has more advantages than the needle TVS-1 from all aspects. Meanwhile, a special FTVS coated with no trigger material is fabricated to simulate the work life of FTVS coated with trigger material when the trigger material is burn out. Two kinds of trigger pulse controllers are developed respectively to build up trigger control systems for different FTVSs. Car ignition coil is used to form a negative high voltage pulse, while color TV rotary transformer is used to form a positive high voltage pulse. The optical fiber is adopted between the control circuit and trigger circuit, which can separate the high voltage source and signal system. With high peak voltage and enough continuous-flow energy, the controllers can turn on the special FTVS coated with no trigger material effectively. The trigger voltage and trigger energy during the FTVS's work process are discussed. The density of initial plasmas is due to the trigger energy, so enough trigger energy is prerequisite to turn on FTVS steadily.In order to get the electrical characteristics of the fabricated FTVS, an experiment platform based on LC oscillating circuit is set up, whose main parameters are adjustable. Experiment results show that, the fabricated FTVS has a wide work voltage range of 0.3-40kV and a work current of 60kA/10C above. FTVS has high work reliability, and its effective trigger rate can reach 100% in the normal work voltage range. Due to high voltage of trigger pulse and limited capability of pulse transformer, the trigger system delay is long, about tens of microseconds. However, the trigger delay of FTVS is very short, about hundreds of nanoseconds. It concludes two parts, wait time t1 and collapse time t2, which are agreement with the heat conduction model calculation steps. Results also show that, t1 is due to the structure of FTVS and trigger pulse; however, t2 decreases with the increase of main gap voltage. If the main gap is negative, both of t1 and t2 are about tens of microseconds. It is indicated that the trigger is much more difficult under negative voltage of main gap.The conducting process of FTVS is an arcing process in practice. The initial plasmas' generation and development images at the arc starting are recorded by a high speed camera. According to the different trigger energy, the different developments of initial plasmas are discussed during the arc starting process. Results show that both of enough high pulse and abundant trigger energy are needed to ensure the arc starting steadily. Whole arcing process is also recorded and some characteristics during the arcing are discussed. The arcing time of FTVS in a LC circuit is tested. Results show that the arcing of FTVS is steady and the arc will not extinguished until current crosses the zero. If the current ratio at the first zero-crossing of half-wave is less than 164.7A/μs, the FTVS works as a diode.