**2. Design and Fabrication of a Three-Electrode Planar Spark Gap High-Voltage Switch Based on Copper Foil**

#### *2.1. The Design of Switches*

The three-electrode planar spark gap high voltage switch was composed of two main electrodes and trigger. The main electrode includes the cathode and anode, as shown in Figure 1. The main electrode is semicircular in shape and placed in the opposite position. The trigger electrode is located in the middle of the two main electrodes. Its principle is to load high DC voltage between the cathode and anode of the switch. When a specific pulse

voltage signal is applied to the trigger electrode, a high voltage gap is formed between the cathode and the trigger electrode. A certain number of ions or electrons are produced by the breakdown field strength, and the particles and gas undergo the collision multiplication process, resulting in the instantaneous conduction of the anode and cathode of the switch. In Figure 3, *a* represents the width of the trigger electrode, *b* represents the gap width of the main electrode, and *R* represents the radius of the main electrode.

**Figure 3.** Integrated exploding foil initiator based on three-electrode planar spark gap high-voltage switch. (**a**) Copper foil three-electrode planar spark-gap high-voltage switch integrated with EFI. (**b**) Three-electrode planar spark-gap high-voltage switch based on copper foil.

As can be seen from Figure 3, the key structures of the planar three-electrode switch include the main electrode radius (*R*), the main gap (*b*), the trigger gap (*(b* − *a)/2*) and the trigger electrode width (*a*). The diameter R of the two main electrodes is a semi-circular structure with a diameter of 4.0 mm ± 0.5 mm. The purpose of this design is to ensure that a uniform electric field exists between the electrode gaps as much as possible, so as to improve the service life of the switch. With the input of the trigger signal, the switch conduction first occurs in the trigger gap. Therefore, reducing the trigger gap is beneficial to improve the working reliability of the switch, but the gap is too small, which affects the working voltage range and safety of the switch. Combined with the machining accuracy, the structural design parameters for the plane three-electrode spark gap high-voltage switch are shown in Table 1.

**Table 1.** Switch Structure Parameters.


#### *2.2. Switch Fabrication and Characterization*

In the three-electrode planar spark gap high voltage switch, a magnetron sputtering coater was used to sputter copper film on the surface of the substrate. The coating photoresist is spined on the surface of the copper film. Then the photoresist surface is covered with a photoresist mask, which is exposed under a strong light to develop the substrate. Finally, the final switch is formed after being etched with FeCl3 etching solution. The switch is shown in Figure 4.

**Figure 4.** Physical diagram of three-electrode plane spark gap high-voltage switch. (**a**) Planar switching with different trigger electrodes. (**b**) Planar switching with different main electrode gaps.

The microscope stage micrometer is used to test the switch structure size according to the parameter requirements in Table 1. The test results are all within the design range requirements.
