Design of a racetrack microtron to operate outside the phase stability region

The beam optics design of a small 5-turn cw racetrack microtron is described. This microtron is the first stage of a two-stage microtron of 31 MeV maximum energy, and 50 μA maximum current. The microtron booster is fed by a 1.8 MeV injector linac. This configuration was chosen in order to maximize the final energy of the system with the available RF power, and, as a result, it was necessary to operate the first stage outside the ideal orbital stability region. The separation between successive return orbits is not constant, and the system presents large phase oscillations from one orbit to another. Nevertheless it was possible to find geometrical and magnetic field strength configurations that allow the extraction of a 4.9 MeV beam, within the design goals for the main acceleration step.