Particle accelerators use electric and magnetic fields to accelerate charged particles. The simplest principle is to take a high direct voltage and to accelerate the particles in its electric field. Above some 100kV the probability of electric arc accumulation increases significantly, thus limiting the attainable (accessible) energy. Therefore, the principle of linear accelerators is used for higher energies where electrons are accelerated via microwave radiation. With this principle energies can be reached where normally an electron would have to pass through several MV of voltage.
This kind of accelerator will typically only allow a few MeV of energy gain per meter length. To avoid a kilometers-long linear path, the electrons at MAMI pass through the same linear accelerator structure many times by way of a repeated deflection in magnets. These tracks bear a resemblance to the race track of an antique arena for which reason this model is called race-track-microtron, i.e. RTM. These deflecting magnets must be large enough so that even electrons of the highest energy will still be deflected completely within the magnets. The magnets used for the accelerator stage MAMI B are approximately 5 m wide and weigh 450t. At this point the mechanical limit of the RTM concept has been reached [1], leaving MAMI to be the biggest microtron in the world.
The latest accelerator stage therefore no longer uses two 180°-bending magnets and one linear accelerator, but rather four 90°-bending magnets and two linear accelerators. For this new concept of the harmonic double sided microtron, high-frequency linear accelerators for 4.90 GHz were developed and put into operation for the first time worldwide [2].
[1] H. Herminghaus et al.: The Design of a Cascaded 800 MeV Normal Conducting C. W. Race Track Microton; Nucl. Instr. & Meth. 138 (1976) 1.
[2] K. H. Kaiser et al.: The 1.5 GeV Harmonic Double-Sided Microtron at Mainz University; Nucl. Instr. & Meth. A 593 (2008) 159-170.