Influence of plasma grid-masking on the results of early SPIDER operation

SPIDER is the prototype negative ion source of the heating neutral beam injectors for ITER. As required by the ITER injector design, the SPIDER beam source is entirely contained inside the vacuum vessel, so that voltage holding depends on the background gas pressure. During the recent operation, the number of extraction apertures was limited in order to routinely operate the ion source with its nominal filling pressure while minimizing the vessel pressure. To close the apertures, a machined molybdenum sheet was installed downstream the plasma-facing electrode of the accelerator. In this contribution, we discuss the implications of this configuration, highlighting possible influences on the experimental results obtained so far, and the differences that can be expected in the future high-current operation (with all 1280 apertures). Among other topics, the possible effects on the use of caesium, the negative-ion density at the extraction region, the perveance of the extractor and the profile of neutrals along the accelerator will be discussed.