Magnet Design for a Very High Energy Synchrotron Employing a Separated Function Magnet Lattice

The authors conclude from an investigation that for a very high energy proton synchrotron a separated function magnet lattice is superior to the conventional combined function lattice. In addition, the direct and indirect effects of the resultant smaller radius make it an economically attractive alternative. With long straight sections employed, there is no fundamental objection to a closed magnet structure. Indeed the shielding automatically provided appears to outweigh any disadvantages. The expected almost total dependence on high intensity external beams of primary protons requires a very high degree of beam optical excellence. A lattice utilizing quadrupoles and "window frame" dipoles can be constructed with very small aberrations in the azimuthal integrals of their fields. The optical properties can be very accurately controlled for a range of dipole fields extending from lower values than practical with combined function magnets up to considerably higher values. Quadrupole aberrations allowed by symmetry are of high multipole order and can be made extremely small. Only even order dipole aberrations are allowed. However, these are very small from very low fields up to values approaching 20 kG. For higher excitations appreciable sextupole appears, followed by 10-pole. However, since widely distributed sextupole lenses are desirable for parameter control, there is no strong argument against exciting the dipoles to fields with moderate sextupole aberration. In contrast to the end effects of combined function magnets, there is no significant variation of beam resonant frequency with radius, other than the "chromatic" variation with momentum.