Experimental investigation of beam-target neutron emission at the ELISE neutral beam test facility

Neutron measurements are proposed for the SPIDER/MITICA Neutral Beam Injection (NBI) prototypes in Padua. 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 "half size" ITER NBI ELISE facility of the Max-Planck Institut. A first measurement campaign was carried out in 2014 during the initial deuterium operations of ELISE with a liquid scintillator detector used as global neutron monitor. The collected data were generally in agreement with calculations based on the local mixing model of deuterium deposition in the copper target. However, deviations approaching 40% from the predicted neutron yield were also observed at the highest beam currents (10 A) and could be due to neglected effects such as spatial profile variations or physics mechanism beyond the local mixing model In order to clarify the apparent discrepancy between experiment and predictions, a second high current (> 10 A) measurement campaign was held during deuterium operations in 2015 and took advantage of a significantly improved determination of the beam current and profile thanks to recent developments in the calorimetry and infra-red diagnostic systems. Our experimental results show that neutron emission is in general very sensitive to the Cesium conditioning phase of the radio-frequency source, which is an essential ingredient to obtain high currents with a negative ion source. In this contribution,we present a detailed analysis of the data focusing in particular on the relation between neutron emission and the beam current. The implications of our results for understanding neutron emission from beam-target reactions are finally addressed and used for an updated evaluation of the emission expected at SPIDER/MITICA. This work was set up in collaboration and financial support of F4E.