IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 4; August 2013 1069 1070-9878/13/$25.00 © 2013 IEEE Analysis of Breakdown Mechanism in Trigatron Switches Li Cai, Lee Li, Yunlong Liu, Bin Yu, Chaobin Bao and Fuchang Lin State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wu Han, Hubei, P. R. China 430074 ABSTRACT Analysis of breakdown mechanism in gas-insulated trigatron switches is important for the engineering design and function prediction. Based on the gas discharge theory, a physical model was proposed to estimate the breakdown time for gas-insulated trigatron switches. And the model can explain the breakdown in the trigatron switches with two modes of the breakdown mechanism, i.e., fast breakdown mode (FBM) and slow-breakdown mode (SBM). Based on Raether breakdown criterion, this paper discussed the factors affecting on the E-field enhancement. The E-field is composed of the static E-field and the dynamic E-field, which leads to transformation of breakdown mechanism. Some coefficients could be determined by experiment and simulation, which are helpful to get the quantitative solution of breakdown time. Index Terms - Trigatron switches, breakdown mechanism, E-field enhancement, undervoltage ratio. 1 INTRODUCTION BECAUSE of relatively simple design, robustness, the controllable triggering feature and low trigger voltage, the three-electrode trigatron switch could be used in application of repetitive frequency or single shot with high power [1-4]. In the past years, a great number of studies have reported the influence factors on characteristics of the trigation gap, such as effect of air pressure on range of working voltage [5-6], effect of polarity on the switching performance [7], the influence of the gas insulation parameters on the triggering characteristics [8-9]. Among these characteristics, breakdown time is strongly related to working parameters. Adjacent electrode Load Trigger pin Opposite electrode Trigger gap Main gap Figure 1. Schematic diagram of the trigatron structure. Simple structure of three-electrode trigatron switch is shown in Figure 1, it was composed of main gap (i.e. opposite electrode and adjacent electrode) and the trigger gap (i.e. trigger pin and adjacent electrode). Previous study has shown that three-electrode trigatron switch has two breakdown mechanisms [10-11]. In fast breakdown mode (FBM), the gap between the trigger pin and opposite electrode breaks down firstly, and the main gap may break down under a high degree of over-voltage. In slow-breakdown mode (SBM), the trigger gap will break down first, which initiates an electron avalanche leading to breakdown of the main gap. SBM would have a long lifetime with low erosion. It has a relatively long breakdown time with microsecond-level [12], while FBM has short breakdown time and low jitter. t d t r Trigger voltage Main gap voltage Figure 2. Formation of time delay. For the convenience of measuring and observing, the time delay can be divided into two parts: the rise time of trigger voltage r t and breakdown time of main gap t , which can be written as r d T T T   in Figure 2. As for the fast breakdown mode, it can be given that d r T T  . As for the slow-breakdown mode, it can be given that d r T T  . When d r T T  , d T T  .The breakdown time in this paper means the breakdown time of main gap t . And it can be measured by monitoring the voltage shape on the trigger pin and the HV electrode. This paper presents a physical model to Manuscript received on 17 September 2012, in final form 4 February 2013.