the Structure of the Nucleon

In February 2007, the fourth stage of the Mainz Microtron, MAMI-C, started operations with a first experiment. The new Harmonic Double-Sided Microtron delivers an electron beam with energies up to 1.5GeV while preserving the excellent beam quality of the previous stages. The experimental program at MAMI is focused on studies of the hadron structure in the domain of non-perturbative QCD. In this paper, a few prominent selections of the extensive physics program at MAMI-C will be presented. 1 The new 1.5GeV Harmonic Double-Sided Microtron as the Fourth Stage of MAMI The Mainz Microtron MAMI is a unique facility in Europe to study the hadron structure with the electromagnetic probe at small momentum transfers [1]. MAMI comprises a cascade of three race-track microtrons (RTM), delivering since 1991 a high-quality 855MeV, 100μA cw-electron beam [2], which was energy upgraded by a fourth stage, MAMI-C, in recent years [3]. Realizing the fourth stage as another RTM was evidently impossible: the two 180-bending magnets would have had a weight of approx. 2× 2000 tons for an end-point energy of 1.5GeV. However, for the next higher polytron configuration, the Double-Sided Microtron (DSM), this weight is reduced by a factor of four. By operating at 4.90GHz, the first harmonic of the fundamental 2.45GHz radio-frequency (rf), the necessary coherent energy gain per turn for the two normal conducting linacs is reduced to get a moderate power consumption. For achieving a simple transverse optics the strong vertical fringe field defocusing of the four 90-bending magnets is compensated