Reduction of Effective Emittance by Stacking Beamlets in Phase Space

Multiple aperture extraction systems are commonly used to produce high current ion beams. One disadvantage of such systems is the dilution of the beam brightness by the grids. Typically, for the seven beamlets from a hexagonal array with a central aperture, the beamlets occupy only one-quarter of the area of the ellipse enclosing them. Beamlet steering by aperture displacement is commonly used in neutral beam injectors to reduce the size of the beam. For realistic beams, typical of high current-density extraction systems, beamlets can be stacked in phase space to reduce the effective emittance of the beam. This paper presents the concept of the technique and the experimental verification. In initial tests, the emi ttance of a 150 mA hydrogen beam from three in-line apertures was halved using the technique. The reduction was not from reduction in the source size as this decreased less than eight percent.