Overview of Cs evaporation control and monitoring during partial beam operation of the ITER HNB ion source prototype SPIDER

SPIDER is the full-scale prototype of the ITER heating neutral beam ion source. In order to reach the current density requirements, the production of H−/D− ions is greatly enhanced by covering the source's surfaces with caesium, which lowers their work function, thus transforming them into effective conversion surfaces for atoms and positive ions into negative ions. In particular, a sufficient and uniform Cs coating is required at the plasma grid, the first electrode of the multi-grid accelerator facing the plasma. In the case of SPIDER, Cs evaporation is controlled by three Cs ovens located at the rear part of the source. This contribution presents the control and monitoring of Cs evaporation in SPIDER during the SO-2 experimental campaign (made up of the S24, S25, and S26 sub-campaigns), during which only 1/4th of the ion source was operated, resulting in unusual caesiation conditions. The use of Cs enabled the extracted negative-ion current density to reach values up to 210 A/m2 with extracted electron-to-ion ratios of the order of 1. The estimations of Cs consumption from simulations made with the AVOCADO code are compared with the cumulated consumption measured after the campaigns, the data obtained from laser absorption spectroscopy (LAS) and post-campaign inspections of the source are discussed to obtain information on the uniformity of Cs coverage and Cs transport.