IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 2, FEBRUARY 2011 737 Analysis on Useful Lifetime of High-Power Closing Switch With Graphite Electrodes Lee Li, Fuchang Lin, Cai Li, Hu Guan, Liu Ning, and Zhou Zhengyang Abstract—High-power closing switches with graphite electrodes have been used in power-supply system for a large laser pump. The useful lifetime of this kind of switch is an interesting and important issue. Previous researchers have observed that the graphite erosion loss is proportional to the amount of transfer charge in a high-power pulsed discharge. Therefore, the erosion rate of graphite may be used for predicting and estimating the electrodes’ useful lifetime. According to the mathematical model proposed in this paper, the lifetime of a closing switch with graphite electrodes is dependent upon several key parameters, including the dimensions of electrode, density, the erosion rate of graphite electrodes, undervoltage ratio, working temperature, and the amount of energy transfer. A mathematical formula describing the relationship between lifetime and those parameters has been deduced. Based on a newly developed spark-gap switch which can support over 100-C single transfer charge, this proposed lifetime model has been verified. This paper may quantitatively perform the function of guidance in engineering applications of graphite electrode switches in large pulsed-power facilities. Index Terms—Charge transfer, erosion rate, graphite electrode, power conditioning. I. I NTRODUCTION H IGH-POWER power-supply systems are necessary for driving large laser pumps. The power-supply module is one of the most important units in a laser pump. With the parallel operation of multiple units, it can generate the required pulse for the xenon flashlamps within a laser amplifier, which lead to the high energy gain of laser beams from the amplifier. High voltage, high current, and high-coulomb transfer clos- ing switches play a special role in a power-supply module. The most practical closing switches to date are spark gaps due to their relatively simple structure, robustness, easy field maintenance, and low cost. A typical high-current spark gap is the ST-300A used in the National Ignition Facility. This switch is of a two-electrode design using high-density graphite electrodes, supports over 300-kA peak current capability with 150-C charge transfer per discharge shot, and is relatively inexpensive and reliable [1], [2]. Its cotwin, the ST-4198 switch, which was developed for electromagnetic launch, can operate at a maximum peak current of 850 kA or a maximum single charge transfer of 350 C [3]. However, the main drawback of Manuscript received May 29, 2010; revised October 6, 2010 and November 23, 2010; accepted November 29, 2010. Date of publication January 17, 2011; date of current version February 9, 2011. The authors are with the College of Electrical and Electronic Engineer- ing, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail: leeli@mail.hust.edu.cn; fclin@mail.hust.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2010.2097280 Fig. 1. Structure of the two-electrode spark-gap switch. spark-gap switches is limited useful lifetime. The lifetime of the ST-300A switch is about 1500 shots (i.e., the total charge transfer is about 150 kC), and the ST-4198 only supports about 100 shots. It is well known that the lifetime of a spark-gap switch is related directly or indirectly to the erosion of the electrodes. How would one design and manufacture a spark- gap switch with a long lifetime? In other words, what is the mathematical principle of graphite electrode lifetime in high- current discharge? In this paper, the erosion features of graphite electrode are analyzed first. Then, it is proposed to use the erosion rate to estimate the lifetime of graphite electrodes. This paper presents a practical method of measuring and calculating the erosion rate. Next, the mathematical model of electrode lifetime is deduced. This model represents a series of key parameters related to useful lifetime. Reasonable parameter configuration will be helpful for long-lifetime design. Lifetime testing of a spark gap with graphite electrodes verifies the correctness of the proposed lifetime model. II. OVERVIEW OF SPARK-GAP SWITCHES In order to make the issue more clearly understood, the structure of the spark-gap switch is shown in Fig. 1. The spark-gap switch generally adopts a coaxial structure and high- density graphite electrodes. The coaxial structure can balance the strong magnetic force, reduce shake, and restrain the arc channel on the center of the switch. As presented in the ST-300A [2], the main components include two main elec- trodes, the up and down metal electrode holders, an insulator envelope, insulator rods, etc. There are two airports in the down holder, which can be used for air charge and insulation recovery of repetitive discharging. Materials eroded from the electrodes exit the switch as a gas via air charge too. Due to the lack of a separate trigger electrode, this switch must 0093-3813/$26.00 © 2011 IEEE