Research On High Power Two-electrode Spark Gap Switch

Switch is one of the key components in the pulsed power system. The performances of the switch directly influence the output characteristics of the system. As one kind of high current closed switches, the spark gap switch with two graphite electrodes has the advantages such as large charge transfer, high current capability, long lifetime span, stability and low cost. The paper mainly studies the graphite electrode erosion, electrical contact erosion, static characteristics and triggering characteristics in the high current application of spark gap switch with two graphite electrodes.Firstly, the erosion mechanism of graphite electrode under the arc was analyzed. The erosion mechanism of the graphite electrode is investigated based on the theory of thermodynamics and validated by experimental result. An erosion mechanism was developed to explain the erosion of graphite electrode under pulsed discharge condition. Based on the analysis of the erosion mechanism and thermodynamics theory, the erosion of the graphite electrode has the characteristics as follows:(1) for the graphite electrode, the duration of heating process is less than a nanosecond. Thus the duration of heating process can be ignored for a microsecond or longer discharge process. (2) the relationship between the graphite electrode erosion and charge transfer can be considered linear. This relationship can be used to predict the life of graphite electrode.The electrical contact between the graphite electrodes and electrode erosion was studied in this paper. Computing model of contact temperature rise is proposed based on contact surface model and heat-conduction equation. The model can theoretically illustrate the relation between the electrical contact surface temperature rise and material physical properties as well as switch mechanical parameters. The model analyzing conclusion indicated that the static contact temperature rise was influenced by material physical properties and engineering manufacturing accuracy. Appropriate contact area, contact pressure and low surface roughness contributed to reducing temperature rise and preventing static welding. The structure and static operating characteristics of the spark gap switch were then studied in the paper. A structure with combined conical and cylindrical shape is proposed in order to reduce the electrode installation stress. Crimp style installation and insulation structure at the bolt area can avoid the erosion between the metal sleeve and the metal electrode holder. Based on above construction, switch operating voltage is in the range of the 40% to 60% of the self-breakdown voltage. Initial value of working air pressure of the gap is 130kPa in the spark gap, which is an optimal value to reduce the self-fire probability and refuse act rate.A spark gap switch has been developed. The experimental results show that the switch can bear the impulse of the large current with the peak of 570kA. In the lifetime test with the peak current of 316kA of and charge transfer of 103.4C each shot, the switch remained well after 1500 shots.At last, the trigger characteristics of spark gap switch was studied. The switch on process of the spark gap was simulated. The simulation results showed that a 50 ohm resistor should be installed at the output of the Marx generator to facilitate the breakdown of a spark gap. Based on above result, operation characteristics of trigger device were analyzed. The results indicated that the operating point of the Marx generator should be close to its breakdown curve. For the sphere gap without trigger signal, a grounding resistance connecting to the copper coil can improve the breakdown probability, and broaden the working pressure range of the Marx generator.