Power supply system for large negative ion sources: Early operation experience on the SPIDER experiment.

• Description of the first operations of SPIDER in which the first plasma was achieved weeks before the connection of the power supplies to the Ion Source from the power supply perspective. • Description of the circuit modification with insertion of pre-load resistors which was pivotal to the ignition of the first plasma and to mitigate the occurrence of the breakdown, induced by the Radio Frequency, on the rear side of the Ion Source. • The description of the issues encountered by Tetrode-based oscillators used to drive the Ion Source. • Among the issues: the reduced power transfer to the load, well below the nominal power, the occurrence of a huge common-mode RF current due to the system layout which has a relevant impact on the source diagnostics and power supplies, the significant error on the RF power measurement to the load. In the road-map for a fully operational ITER Neutral Beam Injector (NBI), SPIDER represents a necessary step to test its negative ion source design since the required performances have never been achieved simultaneously by any previous experiment. The smaller scale of SPIDER to the full ITER NBI prototype (named MITICA, which will operate in the same test facility in Padova) allows more flexibility in the experiments and, taking advantage of its larger set of diagnostic, will provide insightful results for the entire project development. This paper is the result of the experience gained within the first two years of SPIDER operation, with the focus on the Ion Source and Extraction Power Supply (ISEPS) system. A review of the system development, of the main outcomes of the first experimental phase and the description of the still open issues, is provided. [ABSTRACT FROM AUTHOR]