A dual-plunger energy switch assembly for standing wave linear particle beam accelerators capable of operating in a higher energy mode and a lower energy mode employs two mechanical plungers that can be extended different distances inside two side cavities of the linear accelerator. When the linear accelerator is operated in the higher energy mode, both plungers are retracted out of the side cavities. To achieve high output while the linear accelerator is operated in the lower energy mode, the two plungers are radially inserted into the two side cavities to adjust the electromagnetic accelerating field along the length of the accelerator, e.g., one plunger is inserted into a side cavity so that the plunger touches the smile surface of the side cavity, while the second plunger is inserted into a second side cavity so that the plunger is adjacent to, but not touching, the smile surface of the side cavity.