Status of the Nevis Synchrocyclotron Modification

After 20 years of operation as a conventional synchrocyclotron producing 0.4 μA time average internal beam current of protons (increased to 1.6 μA in 1967) of ~ 385 MeV, we stopped operation in September 1970 for the major modification for which we have been planning since 1965. The major study program and the conversion project are supported by a $ 3.9 M grant from the NSF. The conversion retains the basic cyclotron magnet (2000 tons of Fe and ~ 300 tons of Cu coils), but adds a 10-in. thick Fe band around the outside magnet perimeter to lower the return path reluctance. A new larger vacuum chamber is used, with auxilliary excitation coils. Pole iron within 30 in. of the median plane is replaced by a new configuration which includes N=3 symmetry sector iron with a small median plane gap. = 18 kG near the center and 20 kG near 80 in. radius. Strong azimuthal magnetic field flutter begins at < 2 in. radius to give strong focusing νz, and νr∦1. We expect to obtain ~ 550 MeV proton energy, with high extraction efficiency for a 10 to 40 μA time average external beam (~ 50% duty factor), when operation starts later this year. Additional details are given in companion papers in these Proceedings. The system will still be a synchrocyclotron and will have a 300 Hz FM repetition rate, with a new RF system, etc.