Mainz Microtron
The Mainz Microtron (German name: Mainzer Mikrotron), abbreviated MAMI, is a microtron (particle accelerator) which provides a continuous wave, high intensity, polarized electron beam with an energy up to 1.6 GeV. MAMI is the core of an experimental facility for particle, nuclear and X-ray radiation physics at the Johannes Gutenberg University in Mainz (Germany). It is one of the largest campus-based accelerator facilities for basic research in Europe. The experiments at MAMI are performed by about 200 physicists of many countries organized in international collaborations.
The scientific research at MAMI focusses on the investigation of the structure and dynamics of hadrons, particles consisting of quarks and gluons bound by the strong force. The most important hadrons are protons and neutrons, the basic constituents of atomic nuclei and, therefore, the building blocks of ordinary matter. Electrons and photons interact with the electric charges and the magnetization of quarks inside a hadron in a relatively weak and well understood way providing undistorted information about basic hadronic properties like (transverse) size, magnetic moments, distribution of charge and magnetism, flavor structure, polarizabilities and excitation spectrum. At MAMI the full potential of electroweak probes is explored in an energy region characteristic for the first hadronic excitations and with a spatial resolution in the order of the typical hadron size of about 1 fm.
The MAMI accelerator consists of four cascaded microtrons, an injector linac, a thermal source for unpolarized electrons and a laser-driven source for electrons with 80% spin polarization. The operation principle is based on the continuous wave (cw) microtron technique. There the beam is recirculated many times through a normal-conduction linear accelerating structure with a moderate energy gain per turn. Due to constant, homogenous magnetic bending fields the length of the beam path is increasing with energy after each turn. The magnetic fields, the radio-frequency (rf) used to accelerate the electrons and the energy gain per turn have to be adjusted to meet the microtron coherence condition, i.e. the condition that the length of each path is an integer factor of the rf wavelength. This microtron scheme makes efficient use of the rf power and the inherent strong longitudinal phase focussing guarantees excellent beam quality and stability.
In each of the first 3 stages the recirculation is archived by two homogeneous 180° bending magnets. The electron tracks are reminiscent of the race track of an antique arena which is the origin for the name "race-track-microtron (RTM)". The third stage, MAMI-B, started operation in 1990 and delivered a beam for experiments with energies up to 882 MeV and 100 A cw for more than 97800 h until the end of 2007. The quality of the beam is very high: an energy spread of 30 keV and an emittance of 25 nm*rad is achieved routinely. The bending magnets of MAMI-B are approximately 5 m wide and weigh 450 t. At this point the mechanical limit of the RTM concept has been reached, leaving MAMI-B to be the biggest microtron in the world.
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