Abstract: Variable energy selection is accomplished in a side cavity coupled standing wave linear accelerator by shifting the phase of the field in a selected side coupling cavity by .pi. radians where such side coupling cavity is disposed intermediate groups of accelerating cavities. For an average acceleration energy of E.sub.1 (MeV) per interaction cavity, and a total number of N interaction cavities, the total energy gain is E.sub.1 (N-2N.sub.1) where N.sub.1 is the number of interaction cavities traversed beyond the incidence of the phase shift. The phase shift is most simply accomplished by changing the selected side cavity configuration mechanically in repeatable manner so that its resonant excitation is switched from TM.sub.010 mode to either TM.sub.011 or TEM modes. Thus, the total energy gain can be varied without changing the RF input power. In addition, the beam energy spread is unaffected.