Beam Characterization of Linac 4's ISO3

After the second LHC long shutdown, Linac4 will become the new injector for proton beams to CERN's chain of accelerators. It has been commissioned at a maximum beam current of 25 mA, below the initial specification of 40 mA. At present, the beam current delivered by the linac is limited by the pre-injector. Previous investigations have found that this is due to the emittance of the H− beam extracted from the ion source exceeding the acceptance of the RFQ. Detailed measurements of the beam parameters provided by the ion source at a dedicated test stand confirmed these earlier findings. By extending and adjusting the simulation model implemented using IBsimu, an agreement to the measured emittance values was obtained. An analysis of the simulation results shows that the larger-than-expected emittance is a result of aberration at the extraction electrodes. These are due to a plasma meniscus significantly more convex than predicted by the earlier simulation. Three modifications to the extraction gap were proposed to achieve a shift of the plasma meniscus back towards a more concave position. Two of the changes - a shorter extraction gap and a larger bore diameter - result in significant decreases of the emittance as confirmed by experiments.