PRINCIPLE OF CORRECTION OF ASYMMETRIC MAGNETIC FIELDS IN BENDING MAGNETS ∗

The generation of a high quality electron beam by a race-track microtron (RTM) requires highly precise magnetic fields in the two reversing magnets. At the RTM cascade MAMI (Mainz Microtron), a precision of 10−4 for the ver-tical field component By was achieved by symmetrical sur-face coils placed at the upper and lower pole surface in each of these magnets. For the Harmonic Double Sided Microtron [1], the fourth stage of MAMI, the correction must be extended to asymmetric field errors. The more complicated machining of the pole surfaces of its inhomo-geneous end magnets [2] leads to a higher risk for the dis-tortion of the mid plane symmetry. In addition, the correc-tion of deflection errors by external dipoles is more difficult because the path length in the dispersion region decreases with the turn number. Therefore, a numerical method has been developed to calculate the complete set of symmet-ric and antisymmetric field components from a measure-ment of the By distribution on both sides of the midplane. From this the distribution of the field components parallel to pole surfaces is extracted and compared with those of the ideal magnet field configuration. The difference determines the necessary current distribution to correct the field errors. The method has been tested successfully in 3D-simulations by means of TOSCA.