Measurement of stripping losses in the negative ion source SPIDER

The negative ion source SPIDER at Consorzio RFX (Italy) is a crucial experimental step towards the development of the Heating Neutral Beam (HNB) injectors for ITER. The goal of SPIDER is to operate and optimize a full-scale negative ion source in hydrogen and deuterium, with particles accelerated up to 100 keV. One of the processes having a considerable effect on the global efficiency of HNB systems is the partial neutralization of negative ions by gas collisions during their acceleration. This process is commonly referred to stripping losses and it can account up to 29% of the extracted current in a full-scale accelerator. The electrons generated by the neutralization of partially-accelerated negative ions can also contribute significantly to the total amount of heat deposited onto the acceleration grids. The investigation of stripping losses in SPIDER can provide useful methods to extrapolate and estimate also for the full scale HNB injector, called MITICA, currently under manufacturing. The purpose of this work is to develop a tool to estimate stripping losses in the accelerator of full scale HNB injectors based on beam emission spectra. The Beam Emission Spectroscopy (BES) technique has been employed to investigate stripping losses during the first SPIDER experimental campaign with caesium evaporation, characterized by larger extracted current densities compared to pre-caesium operations. Experimental data interpretation required synergy with beam models, necessary to account for the beam-gas interaction processes in the accelerator. In this work we describe the method of analysis and provide first estimates of stripping losses in SPIDER from data obtained during the first caesium operation campaign.