## 2 Facility Description
### Piston and Projectile  
The schematic drawing of the piston is shown in Figure 2. The front and rear part are made of polytetrafluoroethene (PTFE), which can prevent abrasion of the pump tube and minimize friction losses. The diameter of the piston at positions a, b, and c in Figure 2 is 108 + (0.1-0.2) mm at room temperature before the experiments. The diameter at position d is manufactured to be 108 + (0.5-0.9) mm, which can prevent gas leakage from the pump tube to the detonation tube and make full use of the compressed light gas. Although the piston will freeze before the experiments, the diameter at position  
Figure 1: Schematic diagram of the two-stage light gas gun (not to scale, units in mm).  
d can still satisfy a close fit with the pump tube. To further minimize friction losses, the diameter of the piston is locally reduced as shown in Figure 2. The deformable front and rear parts of the piston have a weight of 4.114 kg and 0.774 kg, respectively, and they are replaced after each test. The PTFE surrounds a steel metallic weight, with a diameter of 105 mm. The length of the steel weight is selected in order to change the weight of the piston and influence the compression process. In the present study, a steel weight of 5.552 kg is selected. Thus, the total weight of the piston is 10.44 kg, with a total length of 680 mm.  
Due to the geometric variation in the longitudinal direction as the piston moves past the piezoelectric transducers, the pressure around the sensors and the voltage of the piezoelectric transducers change. Typical pressure periods as the piston move past the sensors P3 and P8 are shown in Figure 3. Points d, e, and f in Figure 3 represent time when the corresponding point on the piston passes the sensor. In Figure 3a, the rising of the pressure at point d indicates the passing of the piston, as well as the arrival of the high pressure detonation gas. For the pressure sensors at the right section of the pump tube, they are already exposed to the high pressure light gas at the arrival of the piston. There are significant pressure changes due to the geometric variation in the longitudinal direction of the piston, as shown in Figure 3b. Thus, the pressure histories can be used to calculate the piston velocities at different positions.  
Different shapes and weights of projectile can be used in the gas gun. However, only the muzzle velocity value and the measuring method are described here. A laser diagnostic system and high-speed photography were used to measure the projectile velocity. The laser diagnostic system can measure the projectile muzzle velocity directly, while the high-speed photography is an auxiliary method to verify  
Figure 3: Pressure periods in the pump tube during the piston movement passing through the sensor.  
(a) P3 sensor; (b) P8 sensor.  
Figure 2: Schematic diagram of the piston (not to scale, units in mm).  
the accuracy of the results of the laser diagnostic system results. The photography optical window is 3320 mm away from the launch tube exit. The snapshots in Figure 4 demonstrate the high-speed photography results processed by a script written in Python that incorporates edge detection and shape fitting methods. The velocity of the projectile is calculated using Equation (1):  
\[v_{pro}\ =\ \frac{\Delta d}{\Delta t}\frac{R_{pro}}{R_{red}} \tag{1}\]  
where \(\Delta d\) is the distance between the center of the red circle in two images, \(\Delta t\) is the time interval between two sequential frames, i.e., 13.3 us in the present case; \(R_{pro}\) and \(R_{red}\) are the radius of the actual projectile and the radius of the red circle in the image, respectively. \(R_{red}\) and \(\Delta d\) have the unit of pixel and their values are related to the resolution of the high-speed camera. However, their values are detected from the images and have no influence to the actual projectile velocity calculation. The projectile in the present is a cylinder with a spherical head, as shown in Figure 4, and the diameter of the sphere is 30 mm.