Neutronics Analyses for Shield Upgrading of the Compact Neutral Particle Analyzer for LHD Deuterium Plasma Experiments

The Compact Neutral Particle Analyzer (CNPA) is an E//B type neutral particle energy analyzer using permanent magnets. CNPA is installed on the diagnostic platform and connected to the outside horizontal port. Based on the preliminary shielding design, 15 cm thick borated polyethylene shields were installed before the fast deuterium plasma campaign. However, neutron induced noises could not be ignored in plasmas with the neutron emission rate higher than ∼1 × 1014 n/s. We carried out the detailed neutronics analyses for the original neutron shield by using the MCNP6 code, which takes into account the detailed structure. The neutron streaming from the beam inlet and the penetration of the bulk shield are approximately 60% and 40% contribution to the neutron flux at the detector array, respectively. Consequently, 5 cm borated polyethylene plates are added to the bulk shield and an extension shield of borated polyethylene cylinder shield with 20 cm in diameter and 35 cm in length is added around the beam inlet, where the neutron flux is expected to be 1.9 × 106 n/cm2⋅s at the detector array for the plasma with the neutron emission rate of 1.9 × 1016 n/s