# An Electromagnetic Circuit Design to Improve a Multi-Stage Coil-Gun's Energy Conversion Efficiency
## 5 Prototype Fabrication and Measurement  
We fabricated a prototype based on the results of the FEA and design. We further measured its projectile velocity. Figure 5 shows the overall view of the nine-stage coil gun prototype used in the measurement experiment and the experimental setup. Particularly, non-metallic materials were used to prevent projectile velocity variation arising from eddy currents. Considering their durability, we constructed the launch tube using carbon pipes. \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline \multirow{2}{*}{**Stage**} & **Number of Turns in the Axial Direction, N** & **Number of Turns in the Radial Direction, M** & **Number of Turns in the Radial Direction, M** & **Initial Velocity (m/s)** & **Final Velocity (m/s)** & **Velocity Increase (m/s)** \\ \hline
1 & 26 & 9 & 3000 & 0 & 33.7 & 33.7 \\ \hline
2 & 26 & 7 & 2400 & 33.7 & 51.8 & 18.1 \\ \hline
3 & 26 & 6 & 2400 & 51.8 & 63.2 & 11.4 \\  
We configured the coil gun system's circuit such that the charging and discharging of the nine stages could be controlled. The capacitor was also charged by boosting a 12 V DC power to 400 V with a booster converter. We also developed and used a charging device to charge the nine-stage capacitors without errors. The projectile's position when the coil gun discharges acts as one of the most important design variables. In a multi-stage coil gun, a discharge system that can operate the circuit when the moving projectile has reached a certain position is needed. Here, we used an SCR thyristor for the discharge circuit, and the discharge control signals were outputted using an Arduino, a commercial development platform. However, since the signals outputted from the Arduino were insufficient to operate the SCR, a method of amplifying the output signals with a MOSFET device and an external power source was used. For the used SCR in the experiment, we configured the circuit using VS-80 RIA80, a lead-type SCR manufactured by VISHAY. At room temperature, the relevant model's operating current should be at least 6 mA; however, it should not exceed 120 mA. The operating voltage should be at least 0.25 V and should not exceed 3 V. The digital signal output of the Arduino Mega model is 5 V at 500 mA. Based on the figures, we can operate up to 4 SCRs with the Arduino's own output. However, in this study, we must operate a total of 9 SCRs. Thus, we used the above method. Figure 6 shows a schematic diagram of the discharge circuit's connection. The charging device's 12 V DC power supply was boosted to 400 V, which is the capacitor's charging capacity, using a booster converter boost circuit. Since a total of nine capacitors of the nine-stage coil gun should be charged, we developed a device that connects and disconnects the capacitors during charging using a relay to reduce the errors caused by the difference in the charging amounts of individual capacitors. We designed it  
Figure 5: The nine-stage coil gun with certain solenoid coil and capacitors highlighted. to unite the capacitors before charging and separate them after it to separately discharge them. Figure 7 shows a schematic diagram of the charge circuit. The charging-discharging device used in the experiment is shown in Figure 8.  
As for the discharge control signals, a method of adding a delay between the discharge of each coil gun circuit was used. The discharge of the stage 1 coil gun was controlled via an external switch's input. After outputting the first stage coil gun's SCR operation signal, that of the second stage circuit was outputted after a 2900 us delay. The delay value was calculated considering the projectile velocity for the optimal position from the finite element analysis and was adjusted by conducting an experiment based on it. Figure 6: A schematic diagram of the multi\(-\)stage coil gun discharging circuit. Figure 7: A schematic diagram of the multi\(-\)stage coil gun charging circuit.