A Simplified Magnetic Field Tapering and Target Optimisation for the Neutrino Factory Capture System

In a Neutrino Factory, a 4 MW proton beam with a kinetic energy between 5 and 15 GeV interacts with a free floating liquid mercury jet target in order to produce pions which after capturing are let to decay forming a muon beam, input to the front-end accelerator system of the facility. The baseline capturing layout consists of a series of normal and superconducting solenoids producing a tapered magnetic field from 20 T, near the target, down to 1.5 T at the entrance of the drift pion decay section. An alternative layout is studied, where the magnetic field is rapidly squeezed from 20 T to 1.5 T using only three solenoids. This new layout showed to produce similar, and even slightly better performance than the baseline, having the additional advantage of being simpler and could potentially be made more robust to radiation. In this paper we report on further optimisation studies taking into account the beam interaction path length in the mercury jet and shape fluctuations of the jet.