Suppression of space-charge effects in transport through an RFQ using an annular beam

High intensity proton linacs (HIPLs) have severe space-charge issues that can lead to emittance blow-up and the production of beam halos, both of which lead to limitations in the operable beam current. Hollow or annular beams are known to have a small spacecharge force (for a given current). Here we present preliminary studies on the production of such annular beams in the Low Energy Beam Transport (LEBT) line, and the subsequent transport of such a beam through a Radio- Frequency Quadrupole (RFQ). We show, using three-dimensional particle-in-cell simulations, that such an annular beam experiences a smaller emittance blow-up as well as reduced beam halo. Starting with an nns normalized emittance of 0.2 πmn-mrad, after transport through the RFQ the emittance blows up to 0.39 πmm-mrad for a Gaussian beam, but only to 0.26 πmm-mrad for an annular beam. Similarly, the halo parameter for the annular beam is only 0.4 as compared to 1.4 for the Gaussian beam. Thus, annular beams suffer lesser deterioration due to space-charge forces in transport through the RFQ, and may therefore be a better choice for HIPLs. (author)