The treatment of metal and polymer materials by plasma ion implantation (PII) requires high-voltage, high average-power modulators to process relatively large-size parts in a reasonable amount of time. CROSSATRON-switch-based modulators provide all of the features desired for large-scale PII. The fast opening and closing capability of the switch eliminates the need for pulse-forming networks and high-voltage thyratrons, and allows arbitrary pulse width adjustment over the range of 2 to 100 mus of interest for PII. The CROSSATRON switch is capable of modulating high peak currents, which permits rapid charging of the relatively high capacitance of the PII load to provide fast switching times. The CROSSATRON switch requires only a low-voltage (less-than-or-equal-to 1 kV) pulsed biasing of the control grid to close and open, and has switching times of 1 mus or less. CROSSATRON switches are cold-cathode, plasma discharge devices that also eliminate the need for large filament-heater power supplies and higher grid-drive power required for hard-vacuum-tube switches. A 100-kV modulator, built at Hughes Research Laboratories for the PII program, has demonstrated reliable operation at up to 100 kW of average power, limited only by the existing power supply. The modulator is based on the 120-kV, 8455H CROSSATRON switch, which provides hard-tube-like modulation at peak currents of up to 1000 A and pulse repetition frequencies (PRFs) of over 1 kHz.