A pulse rise time that is as short as possible is preferred to achieve uniform PIII [11]. The potential on control grid G1 has to be optimized. It is +200 V in our system, and the rise time of the pulse is about 1 μs. A larger G1 potential may lead to deleterious electrical oscillation. The potential on grid G2 has a slight influence on the rise time of the pulse, and no obvious difference is observed with the G2 potential changing from +1500 to +1900 V. Due to the capacitance of the plasma load and cable, a long fall time is induced after the tetrode shuts down. This will lead to the energy nonuniformity of incident ions, and surface sputtering of treated samples may happen. This fall time may last several hundreds of microseconds due to the high resistance of the plasma sheath. The hard tube or series solid-state switch (e.g., IGBTs) can be utilized to discharge the equivalent capacitor rapidly. A fall time of less than 1 μs has been reported [12]. However, this method may make the circuit of the modulator more complex and bring extra hardware cost. Therefore, a pull-down resistor is frequently utilized in practical circuits, although power consumption is needed. In our system, the fall time of the pulse is controlled by pull-down resistor R3 of 20–30 kΩ.