Design concept of the magnetic horn system for the T2K neutrino beam

The design concept of the magnetic horn system for the T2K neutrino oscillation experiment is described. We proposed a three-horn system with optics similar to a conventional two-horn system, aiming to focus relatively low-momentum pions under a high beam intensity environment. Good focusing efficiency was obtained with compact horns and a rather large inner conductor radius for the first horn. The geometry of the decay volume was also optimized. The intensity and quality of the obtained neutrino flux is satisfactory to achieve the T2K physics goals.