A cylindrical plunger (probe) that moves into the accelerator-switch side cavity about 1/3 from the gun end, and thus turns the remaining 2/3 of the accelerator into a drift tube, where little acceleration occurs. Use of the energy switch allows applying a much higher power level to an effectively shorter accelerator. This shorter distance permits acceleration of much higher beam currents needed to achieve the nominal dose rate of 600 MU/min. Selection of energy type for a treatment.