Study of emittance measurements in a high-current multibeamlet beam

SPIDER [1] is a prototype high-current, multibeamlet, beam source of the ITER Heating Neutral Beam [2] injector. The nominal beam parameters are 1280 beam- lets, for a total of 70A H- (56A D-), accelerated at an energy up to 100keV. A heat ux up to 5 MW/m2 is expected at the calorimeter: a movable diagnos- tic calorimeter [3] will be used in short-pulses, as a direct mean to obtain the beam footprint, while a fixed beam-dump[4] is installed for steady-state opera- tion. During SPIDER operations, numerical simulations of beam extraction and accelerator will be carried out to support the experimental campaign: for the comparison between simulations and experiment, measuring the beam emittance is extremely useful, because it is the most complete characterization of a particle beam. We discuss in this paper two proposals for the additional beam-emittance measurements in SPIDER: a fixed electric-sweep scanner (ESS) is proposed for the integration with the beam-dump; a movable emittance scanner (Allison Scanner [5]) is proposed for the installation on the movable diagnostic calorimeter. The peculiarities of a multibeamlet setup are discussed and included in the calcula- tion of a synthetic signal of the emittance scanners. The constraints given by the integration in a high heat load component and the thermal design are discussed. The proposed fixed ESS can be used to reconstruct the beam divergence, even if the fixed scanner can detect only a limited section of the beamlet emittances, assuming identical single-beamlet optics. The movable emittance scanner is eas- ier to integrate in the present design, and allows a full characterization of single beamlet optics. This work was set up in collaboration and partial financial sup- port of F4E, and partially under the Fusion Researcher Fellowship granted by EUROfusion.