*2.3. Measurement of Variable Polarity Plasma Arc Pressure*

To understand the plasma arc pressure, and thus to analyze its influence on the digging process, the measurement platform is established as shown in Figure 5. Two water tanks are used for cooling

plasma torch and the copper, respectively. Arc pressure is measured by the diffusible silicon pressure transducer (Beijing HuaKong xingye technology development Co., Ltd., Beijing, China). The range is from 0 to 5 kPa. A hall sensor is used to get the current synchronously. The measuring frequency is 10,000 Hz. The plane diagram of pressure transducer device is shown in Figure 6. The copper with 6 routes of cooling water is outside. A square tungsten plate is in the center. A small hole with 1 mm diameter is in the center of tungsten plate. Thermally conductive silicone is smeared between the pressure transducer and the cooling copper, which is used to quickly transfer the heat of pressure sensor to the cool copper. The base metal moves with the speed of 1 mm/s for measuring the radial distribution of pressure. The parameters are same with that in Section 2.1. The EP current changes from 150 A to 200 A.

**Figure 4.** Diagram of the observation results of the small hole boundary.

**Figure 5.** Experimental system of measuring variable polarity plasma arc pressure.

**Figure 6.** Diagram of measurement structure cross section.
