On Beam Blowup in a Racetrack Microtron

This paper reports on a preliminary theoretical study of the interaction of electromagnetic fields in the accelerating structure of a racetrack microtron with the transverse motion of electrons circulating through the structure. Basic equations of the transverse beam dynamics and of buildup of the deflecting modes are derived with the help of idealized models of a microtron as well as of the beam-structure interaction process. The dependence of the starting current for beam blowup -(BBU) on the parameters of the accelerator has been studied and some features of the Microtron Using a Superconducting Electron Linac (MUSL-2) of the University of Illinois were taken into account to make numerical calculations. It was shown that the starting current is extremely sensitive to the phase correlation between the returning beams and under some conditions can be much lower than that for a linac. The use of the thin magnetic lenses on each orbit helps to achieve higher values of the starting current.