A pair of cavities defined within a hollow elongate accelerator body include a first resonant cavity having a first resonant slot through an outer wall thereof, and a second resonant cavity having a second resonant slot through an outer wall thereof. The first resonant slot and the second resonant slot are separated by a void region that extends between the outer wall of the first cavity and the outer wall of the second cavity and is bounded in part by an inner surface of the hollow elongate member. The first and second cavities are coupled to each other through a dual slot coupling structure that includes the first resonant slot, the void region, and the second resonant slot.