Impact of the beam pipe design on the operation parameters of the superconducting magnets for the SIS 100 synchrotron of the FAIR project

The SIS 100 accelerator of the Facility for Antiprotons and Ion Research (FAIR) at GSI Darmstadt will be the world's second fast ramped synchrotron utilising superconducting magnets in heavy ion research facilities. The request for high current Uranium beams requires vacuum of extremely high quality that can be achieved in long term operation only by cold vacuum chambers acting as a cryogenic pump. Its mechanical stable design options are strongly limited by AC loss generation and field distortion problems. Previous R&D indicated that cooling tubes, keeping the vacuum chamber below 15 K, create large additional eddy currents and thus deteriorate the field with a sextupole. This effect is most dominant at the start of the ramp. The ramp rate of the correctors is limited by the maximum available voltage and as by the heat created on the ramp up and the cooling efficiency of the Nuclotron-type cable. Thus we investigate different means to simplify the vacuum chamber design keeping its temperature below 15 K in the area where the highest suction pumping is required with alternative cooling methods as well as on the compensation margin the sextupole correctors can provide. This work was partly supported by the BMBF.