Superconducting magnets for Energy Buncher at FAIR

THE Energy Buncher at the low energy beamline of the superconducting fragment separator (Super-FRS) (Figure 1) at FAIR is a valuable and attractive experimental device for particle identification after secondary reactions. This device will be at the heart of the experimental facility for nuclear and reaction (NuSTAR) studies using radioactive ion beams at FAIR. The dipole, quadrupole and sextupole magnets forming the Energy Buncher have to accept fragment beams which can be transported in a large cylindrical volume. The unique feature of FAIR is the high beam intensity coupled with the ability to change the beam energy and beam species at short interval of time. The magnets, therefore, have stringent design criteria. The dipole magnets have large usable aperture of ± 380 mm horizontally and ± 100 mm vertically with maximum magnetic field intensity 1.6 T with uniformity 3 × 10. The quadrupole magnets have large usable aperture of ± 300 mm horizontally and ± 250 mm vertically with maximum magnetic field intensity 1.8 T with field quality gradient (ΔG/G) of 8 × 10. Preliminary calculations show that the required magnetic field and field gradients can be reached both by normal room temperature magnets and superconducting magnets. If we use copper conductor operating at room temperature, the size of the coil becomes large and the length of the pole stem has to be about the same as the half-aperture of the magnet. As a result, the magnet yoke will have large cross-section. So the cost of copper will be about the same as that of iron. A feasible alternative is to adopt superferric magnet technology. In a superferric magnet, the coil size is very small and the iron volume also is large. The field is iron dominated, though the coil is superconducting. Superferric magnets have the combined advantage of a very compact coil and low operating cost. The superferric magnet technology has been adopted at various laboratories like MSU, A1900 fragment separator, RIKEN, BigRIPS fragment separator, in-flight separator for RIA in USA, etc.. All of these facilities work at lower beam energies and hence the magnets are much smaller compared to those of the Super-FRS. As a constituent member of FAIR, India is seriously considering a proposal to supply hardware for building this accelerator facility. It is indeed a privilege and technological challenge for India to design superconducting magnets for use at FAIR. The experience by Indian scientific community in building the superconducting cyclotron at VECC-Kolkata and other superconducting devices will provide necessary knowhow for this project. The project itself is, however, unique in terms of the size of the magnets, and has stringent requirement in addition to building a large number of them with guaranteed quality control.