Achromatic focusing of a 3.5 MeV proton beam with magnetic dipoles, quadrupoles and sextupoles

In magnetic lens systems that produce a proton microbeam, chromatic aberrations limit the spot size. A lens system is studied containing horizontal and vertical dipoles, and quadrupoles, to perform stigmatic focusing. In the dispersive foci of the dipoles, sextupoles are situated to correct the chromatic aberrations. It is our goal that the lens system can produce a spot with a diameter of 25 μm. A beam with an energy spread of 0.3% will be focused by this system to a spot of ca. 40 x 30 μm2. The remaining spot enlargement is due to geometric aberrations of the lens system introduced by the sextupoles. (orig.)