A standing-wave linear accelerator structure has an electron gun; a first cavity axially adjacent to the electron gun, into which electrons are injected directly from the electrode gun; a pancake cavity disposed adjacent to the electron gun on a side of the first cavity opposite the electron gun; and a plurality of accelerating cavities including both on-axis cavities and side-coupled cavities, disposed serially after the at least one pancake cavity, to accelerate electrons injected from the electron gun through a central aperture formed in each of the on-axis cavities. The first cavity and the pancake cavity together form a buncher cavity. The accelerator structure omits the prebuncher and buncher cavities while retaining their functions.