MINIMIZING DISPERSION IN FLEXIBLE-MOMENTUM-COMPACTION LATTICES

Medium-energy accelerators are often confronted with problems during t ransition energy crossing, such as longitudinal microwave instability and nonlinear synchrotron motion. These problems can be avoided by an accelerator having a negative momentum-compaction factor. A modular me thod for designing a lattice with adjustable momentum-compaction facto r is presented. The dispersion excursion of the basic flexible-momentu m-compaction module can be minimized to less than the maximum dispersi on of the focusing-defocusing (FODO) lattice containing the same FODO cells. The phase advance of the module can be adjusted to be an odd mu ltiple of quarter betatron waves. We found that a lattice composed of such modules possesses excellent chromatic properties with excellent t unability, smaller systematic stop-band widths, and smaller sextupole distortion functions.