Median Plane Effects in the Eindhoven AVF Cyclotron

The median plane of the main magnetic field of the Eindhoven AVF Cyclotron does not coincide with the midplane of the cyclotron magnet at small radii. This tends to decrease the axial acceptance of the cyclotron for the injected beam. The effect of an asymmetric excitation of inner circular correction coils is described. With this it is possible to alter the position of the magnetic median plane and thereby change the area of the axial acceptance of the cyclotron. A deflection voltage on axial deflection plates in the centre of the cyclotron can shift the axial cyclotron acceptance, so as to properly match the ion source emittance. In combination with the asymmetric excitation of inner circular correction coils this parameter provides a good means to optimize the beam current in the centre of the cyclotron. Finally, numerical calculations show that there is only a small effect on the ion beam of an accelerating gap which has an inclined dee with respect to the dummy dee.