Neutron emission from beam-target reactions studied at the ELISE neutral beam test facility

Neutron measurements are proposed for the SPIDER and MITICA Neutral Beam Injection (NBI) prototypes in Padua as a means to diagnose the footprint of the deuterium beam when it hits the beam calorimeter. Neutron emission is here due to reactions between the beam and the adsorbed deuterons in the target and thus depends on the deuteron absorption level in the beam calorimeter. We have investigated such process at the ELISE facility of the Max Planck Institut für Plasmaphysik in Garching. ELISE is a "half size" ITER NBI test facility capable to accelerate negative hydrogen and deuterium ions up to 60 kV and to perform plasma operations of the radio frequency ion source for up to 1 hour. A first measurement campaign was carried out in 2014 during the initial deuterium operations of ELISE with a liquid scintillator detector installed in vicinity of the dump and that was used as global neutron monitor. The detector had standard neutron/gammaray discrimination capabilities and known response function obtained from a dedicated calibration at a neutron generator in Peking University. The collected data showed an increasing neutron rate as a function of time, until a saturation level is reached, and were generally in agreement with calculations based on local mixing model of deuterium deposition in the target up to a concentration of 20% of the copper atoms. Deviations up to 40% from the predicted neutron yield were however observed at the highest beam currents (10 A). These could be either due to neglected spatial profile variations of the beam power deposited on the target or indicate physics mechanism beyond the local mixing model, such as diffusion of deuterium at saturation due to temperature effects. A new, dedicated experiment is presently ongoing at ELISE to understand neutron emission at the highest beam currents, which are of special relevance to validate calculations for SPIDER/MITICA and, more generally, to predict neutron emission from beam target reactions, such as in compact neutron sources. The experiment makes use of detailed beam diagnostic data from calorimetry and infra-red systems, which can be combined to track changes in the beam power deposition profile on the dump. Preliminary results from the new experiments will be presented and compared with data at low beam currents.