Large horn magnets at the KEK neutrino beam line. II

For pt.I see Yamanoi et al., Proc. on Magnet Technology, p.711 (1997) We report on the status of the latest operation of the horn magnets. Our two types of large horn magnets were installed in the neutrino beam line at the KEK-12 GeV Proton Synchrotron (KEK-PS) and have been operated since March 1999. These two focusing magnets were designed to be excited at pulsed-high currents of up to 250 kA. One of the special characters of our horn magnet is a built-in pion-production target at the most upstream part of the inner conductor. This configuration enable us to increase the neutrino flux as high as possible with low-energy primary protons from the KEK-PS. Our horns have a coaxial-shape structure with a large diameter, i.e. a large volume of the magnetic field, in order to collect as many pions as possible. We estimate that the neutrino flux is enhanced by a factor of 14 by using this horn system. Both horns have been excited over 10/sup 6/ times at pulsed currents of 175 kA-250 kA. Some beginning problems were found in the peripheral apparatus of the horn magnets, and the built-in production target was found have a fatal mechanical damage. In the spring run, the performance of our horn magnet system was almost sufficient as a pion focusing device for a long-baseline neutrino oscillation experiment.