The DESIR High-Resolution Separator
The High Resolution Separator (HRS) consists of two magnetic dipoles (DI), six quadrupoles (EQ), two sextupoles (EH) and 1 multipole (E5) with the configuration QQHQDMDQHQQ as shown in Figure 1. This symmetric design helps to minimise aberrations. Large mass dispersion and small aberration coefficients are obtained by increasing the incident and exit angles at the boundary of the magnets. Two 90o bending dipoles with 36o both incident and exit edge angle are presently considered. This design is inspired by the Argonne CARIBU separator.
Figure 1: Layout of the DESIR HRS consisting of 3 quadrupoles and one sextupole at the entrance, a large dipole, and a multipole which is followed by a symmetric exit configuration.
Quadrupoles are used as the focusing mode in the y-direction and defocusing in x-direction. This offers two advantages simultaneously: high transmission as well as small image magnification to attain high resolution. The quadrupole doublet produces a ribbon-shaped beam, so y angles are small minimising y angle aberrations. The quadrupole placed before the dipole diverges in x direction and converges in y direction. The small y size minimise y aberrations and the large x area in the dipoles gives mass dispersion. The reverse matching section transforms the ribbon-shaped beam back to a circular cross section at the focal plane. The two sextupoles and the central multipole (sextupole, octupole, decapole and duodecapole) allow correcting aberrations to 5th order.
Figure 2 shows a beam envelop calculation for the x direction.
The RFQ cooler SHIRaC and the HRS will be installed in the production building of SPIRAL2. Both equipments can be bypassed, if no high-resolution mode is needed for experiments either in DESIR or for acceleration in CIME. In this case, the beam is sent directly to these installations without going through the loop containing the RFQ and the HRS (see figure 1).