A microtron is a cyclotron in which the kinetic energy of electrons is increased by a constant amount per field change (one half or a whole revolution). They are designed to operate at constant field frequency and magnetic field strength despite notable relativistic effects at higher energies. Thus, power consumption can be much reduced.
In a microtron due to their different relativistic mass the electrons in different passes of the acceleration fly on different paths through the bending magnet dees. The time needed for that is proportional to the pass number. The slow electrons need one electric field oscillation, the faster electrons an integer multiple of this oscillation.
In the racetrack-microtron by tailoring the field in the dee all electron-packets can be made to enter and exit at the same position so that they can be fed conveniently through a compact linac. A racetrack microtron does not have the electric field of the classic cyclotron between its dees. Instead, there is a linear accelerator near the edge of the gap between the dees. The remainder of the gap is used for focussing devices. The electron is readmitted to the linear accelerator after each revolution. This procedure can be repeated until the increasing radius of the particle's path makes further acceleration impossible. The particle beam is then deflected into an experiment area or a further accelerator stage. The world's largest racetrack-microtron is the Mainz Microtron[1].
Quasi-cw high energy electron beam.