# Design and Experiment of Discharge Control Methods for Three-Stage Coil Gun Experiments
## 1 Introduction  
An electromagnetic launcher (EML) accelerates a projectile using electrical energy. EMLs are broadly classified as rail guns and coil guns according to the method used to convert electromagnetic force into kinetic energy [1]. In rail guns, Lorentz force is applied to a projectile located between two rails through which current flows, and coil guns accelerate projectiles using the electromagnetic force generated by a current applied to a cylindrical solenoid coil [2]. EMLs do not use chemical energy such as that from gunpowder, so they have no restrictions on firing conditions as long as there is a charging environment with a power supply. For this reason, EML systems have received extensive attention in research for defense and space-launch vehicles [3; 4]. In particular, recent research has focused on applying an EML to an aircraft catapult as an accelerator that allows fighter jets to take off from aircraft carriers, as well as the actual deployment of an EML on an aircraft carrier. An electromagnetic injection device has also been developed to replace compressed vapor injection devices, which are currently used on U.S. aircraft carriers [5; 6]. A coil gun primarily comprises a solenoid coil to generate electromagnetic forces and a direct current (DC) power supply to apply power to the solenoid coil. Coil guns typically include a capacitor as a power supply to instantaneously apply high voltage and high current. Devices such as SCRs, IGBTs, and FETs are used to maintain the stability of the high voltage and high current flowing in the charging circuit for the capacitor and in the coil gun circuit [7]. Single-stage coil guns have limited capability for acceleratinga projectile due to the magnetic saturation point of the solenoid coil. Addressing this limitation in acceleration performance requires a multi-stage coil gun design. A coil gun circuit with suitable configuration for acceleration according to the launcher velocity can greatly accelerate a projectile in theory. To realize an efficient acceleration in the coil gun, the components of the coil gun must be analyzed in terms of their relationships to the launcher velocity. Recent efforts have been made to optimize the coil gun components, which include the solenoid coil, the shape of the projectile, and power supply such as a capacitor, which act together to determine the launch velocity [8,9]. One of the design elements of the coil gun system is the distance between the projectile and the solenoid coil during discharge. In the coil gun device utilizing capacitors, the current applied to the solenoid coil is not constant, and the magnitude of the applied electromagnetic force constantly changes according to the position of the projectile [10]. The current that moves through the electric subsystem is underdamped due to the capacitance, inductance, and self-resistance of coil winding, so discharge must be controlled according to the position of the moving projectile. For a multi-stage coil gun, the configuration of the discharge control system to discharge the coil gun is problematic when the projectile reaches an intended position because discharge must be controlled according to the position of the moving projectile. [PERSON] used coil guns with two different types of control circuits: one based on physical contact between a projectile and a discharge circuit, and another with a photodetector [11]. The latter is based on the detection of infrared rays reflected by the projectile, but it recorded large errors during secondary acceleration. The physical contact method also has disadvantages: the position of the projectile needs to be readjusted every time the switch is turned on, and an impact from turning the switch may cause damage. These problems were addressed by configuring a coil gun system with a discharge control circuit based on a delay [12].