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The ELISE test facility hosts a RF negative ion source, equipped with anextraction system which should deliver half the current foreseen for the ITERNeutral Beam Injector, keeping the ratio of co-extracted electrons to ionsbelow 1. An important tool for the suppression of the co-extracted electrons isthe magnetic filter field, produced by a current flowing in the plasma grid,the first grid of the 3 stage extraction system. To boost the sourceperformances new concepts for the production of the magnetic filter field havebeen tested, combining the existing system with permanent magnets attached onthe source walls. The topologies of these new magnetic configurations influencethe beam particles trajectories in the extraction region, with consequences forthe overall beam optics. These effects will be characterized in this article bystudying the angular distribution of the beam particles, as measured by theBeam Emission Spectroscopy (BES) diagnostic. The behavior of the beam will bestudied also through the measurements of the currents flowing on the groundedgrid (the third grid) and on the grid holder box surrounding its exit. The mainfinding is that the broader component of the beam increases when the magneticfield is strengthened by permanent magnets, i.e. in the cases in which most ofthe co-extracted electrons are suppressed.

Influence of the magnetic filter field topology on the beam divergence at the ELISE test facility