Initial Results From the SPIDER Beamlet Current Diagnostic

Source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER) is the full-scale prototype for the ITER heating neutral beam (HNB) source. Beam operation in SPIDER, with the eventual aim of achieving 100-keV, 46-A hydrogen negative ion beam for 1 h, has mainly been performed in volume operation without cesium. Recently, with the introduction of cesium, SPIDER has moved to the surface operational regime, with a noted increase in beam current. Electrical and calorimetric measurements of the beam current have previously shown discrepancies between the two, possibly due to background gas interactions in the accelerator and upstream of the STRIKE calorimeter. Taking advantage of the reduced number of open apertures due to the plasma grid (PG) mask, a series of fluxgate current transducers have been installed in SPIDER to measure the current of individual beamlets. These beamlet current monitors (BCMs), positioned downstream of the grounded grid (GG) around individual grid apertures, have provided direct measurement of the current of up to five individual beamlets during the recent SPIDER campaigns with and without cesium. This contribution describes the installation of the BCM dc sensors, analysis of the dc signal, and initial characterization of the beamlet direct current component in volume and surface operational regimes.