Eddy current effect of magnets for J-PARC 3-GeV synchrotron

The 3-GeV synchrotron proposed in the JAERI/KEK Joint Project (J-PARC) is a rapid-cycling synchrotron (RCS), which accelerates a high-intensity proton beam from 400-MeV to 3-GeV at a repetition rate of 25-Hz. The 3-GeV synchrotron is used to produce pulsed spallation neutrons and muons. It also works as an injector for a 50-GeV synchrotron. Since the magnets for the 3-GeV synchrotron are required to have a large aperture in order to realize the large beam power of 1 MW, there is a larger fringing field at a pole end than for a usual synchrotron magnet. In addition, the 25 Hz ac field induces eddy currents in magnet components: e.g., in the coil, magnet end plates, etc. The eddy current induced in the end plates is expected to be large. Therefore, it is important to investigate the effect of large leakage field and eddy current on the beam motion around the magnet end part. We have measured the eddy loss and the eddy current field at the edges of the dipole and quadrupole magnets. In this paper, we report the comparison between the results of the measurements and the two-dimensional eddy current model developed for this study.