Design and Development of the T2K Pion Production Target

The T2K experiment will utilise the highest pulsed power proton beam ever built to generate an intense beam of neutrinos. This uses the conventional technique of colliding the 0.75 MW 30 GeV proton beam with a graphite target and using a magnetic horn system to collect pions of one charge and focus them into a decay volume where the neutrino beam is produced. The target is a two interaction length (900 mm long) graphite target supported directly within the bore of the first magnetic horn which generates the required field with a pulsed current of 320 kA. This paper describes the design and development of the target system required to meet the demanding requirements of the T2K facility. Challenges include radiation damage, shock waves resulting from a 100 K temperature rise in the graphite material during each beam spill, design and optimisation of the helium coolant flow, and integration with the pulsed magnetic horn. Conceptual and detailed engineering studies were required to develop a target system that could satisfy these requirements and could also be replaced remotely in the event of a target failure. T2K ν FACILITY AT J-PARC The Tokai-to-Kamioka (T2K) [1] experiment currently being commissioned in Japan will direct the world’s most intense conventional neutrino beam from Tokai to the Super-Kamiokande far detector [2] some 295 km distant, in the study of νμ → νe oscillations. The neutrino beam is directed at a small off-axis angle to the detector in order to generate a quasi-monochromatic νμ beam tuned to the oscillation maximum at 0.5 – 1 GeV. The νμ beam is a tertiary beam produced from the decay of pions, which are themselves generated by the interaction of a 30 GeV proton beam with a two interaction length graphite target. A system of three magnetic horns focuses the required sign of pions in a forward direction. Consequently the νμ beam generated by the decay of pions as they traverse a decay volume is also well focussed. In order to maximise the capture of forward momentum pions, the target is supported directly inside the bore of the first magnetic horn. The proton beam enters the target station through a proton beam window which separates the beamline vacuum from the target station and decay volume filled with helium at atmospheric pressure. Between the window and the first horn assembly containing the target is a graphite baffle/collimator to protect the target and horn in the event of a miss-steered beam. The remnant proton beam is deposited in a hadron absorber or beam dump situated 100m downstream of the target at the far end of the decay volume. Approximately one third of the beam power is transmitted into the kinetic energy of secondary particles including pions, another third is deposited in the beam dump and the remainder into the decay volume walls and target station shielding. Less than 5% of the proton beam power is deposited in the target itself as heat. The beam window, baffle, target and magnetic horns are supported beneath shielding modules to permit replacement and to accommodate a potential change in off-axis angle for the facility if required. The components installed for Phase I have been designed for operation at an average beam power of 750 kW. The T2K roadmap foresees an upgrade to 1.66 MW by 2014 and there is an ambition to achieve 3-4 MW within the lifetime of the facility. Consequently there is a need to begin the design of replacements for these components for higher power operation. Since the target station, decay volume and hadron absorber are fixed installations and cannot be maintained or replaced after activation, they were all designed for operation at the highest envisaged beam power of 4 MW. BEAM EFFECTS ON GRAPHITE TARGET Table 1: Beam and Target Parameters Proton beam kinetic energy 30 GeV Average beam power 750 kW Protons per pulse 3.3 × 10