Large Aperture Hexapole Magnets for the Super-FRS LEB Spectrometer

It is proposed to use a magnetic spectrometer [1,2], located behind the Super-FRS [3] in the LEB cave, for experiments with exotic beams at low and intermediate energies (5 to 200 MeV/A) and stopped beams. For the HISPEC/DESPEC [4] experiments where exotic ions are measured in coincidence with gamma spectroscopy at the secondary target MF10, a good mass identification is necessary. This requires a spectrometer with high momentum-acceptance for particle identification via magnetic rigidity analysis and tracking. A first-order ion-optic design for the spectrometer was presented in Ref. [2]. Higher-order optic calculations have suggested the need for large-aperture hexapole magnets to correct 2-order aberrations. With no hexapole correcting magnet, the focal plane of the spectrometer is extremely inclined: at 73° from the perpendicular to the central trajectory. This is typical for large-acceptance spectrometers [5]. Such an inclined focal plane might cause difficulties for certain experiments. A hexapole magnet placed after the dipole can be used to correct the focal plane angle back to the perpendicular by cancelling the matrix element (x|ap). However, this has the effect of introducing other aberrations. In particular, the geometrical aberration (x|aa) becomes large and has to be corrected by another hexapole before the dipole where the dispersion is still zero. The layout and 2-order ion optics of the spectrometer, as presently considered, are shown in Fig. 1.