Large horn magnets at the KEK neutrino beam line - part 2

We report on the status of the latest operation of the horn magnets. Our tw o types of large horn magnets were installed in the neutrino beam line at t he KEK-12GeV Proton Synchrotron (KEK-PS) and have been operated since March 1999. These two focusing magnets were designed to be excited at pulsed-hig h currents of up to 250 kA. One of the special characters of our horn magne t is a built-in pion-production target at the most upstream part of the inn er conductor. This configuration enable us to increase the neutrino flux as high as possible with low-energy primary protons from the KEK-PS. Our horn s have a coaxial-shape structure with a large diameter, i.e. a large volume of the magnetic field, in order to collect pions as many as possible. We e stimate that the neutrino flux is enhanced by a factor of 14 by using this horn system. Both horns have been excited over 10(6) times at pulsed curren ts 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 foun d have a fatal mechanical damage. In the spring run, the performance of our hem magnet system was almost sufficient as the pion focusing device to con firm a long-baseline neutrino oscillation experiment.