Standing wave particle beam accelerator with switchable beam energy

In an accelerator for accelerating a particle beam, a chain of resonant electromagnetic cavities coupled in series along an axis and resonant at approximately the same frequency,

• a cylindrical coupling cavity coupled to each of at least two intermediate adjacent cavities through irises,
• conductors extending parallel to the axis into said coupling cavity with their ends spaced from one another,
• at least first and second probes mounted for independent radial insertion into said coupling cavity at a radial angle with respect to one another, and with their ends adjacent and selectively coupled to one or the other of said conductors to change the distribution of electromagnetic fields in the cavity whereby the electromagnetic field coupling between said two adjacent cavities is changed with the selective insertion of said probes to thereby change the energy of the particle beam from a first value with both probes retracted and uncoupled to a second value with only one probe inserted and coupled to one conductor and a third value with only the other probe inserted and coupled to one the or the other conductor.