Initial results from the SPIDER beamlet current diagnostic

SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) is the full-scale prototype for the ITER Heating Neutral Beam (HNB) source. Driven by four driver coil pairs powered by oscillators the SPIDER source plasma exhibits fluctuations in a range of frequencies that have been observed by Langmuir probes and Source Emission Spectroscopy. These fluctuations are believed to influence the extracted beam current, meniscus shape and beam features. Beam operation in SPIDER, with the eventual aim of achieving 100 keV, 46 A hydrogen negative ion beam for one hour, has mainly been performed in volume operation. Recently, with the introduction of caesium, SPIDER has moved to the surface operation, with a noted increase in beam current. Electrical and calorimetric measurements of the beam current have shown discrepancies between the two, possibly due to background gas interactions. Taking advantage of the reduced number of beamlets due to the Plasma Grid mask a series of Hall effect current transducers and AC current transformers have been installed in SPIDER. These Beamlet Current Monitors (BCMs), positioned downstream of the Grounded Grid, provide a direct measurement of the current of up to five individual beamlets, from DC up to 5MHz frequencies. This contribution describes the initial results from the BCM with and without caesium, focussing on the characterisation of the beamlet direct current component.