HOLLOW BUNCHES PRODUCTION

Abstract

Hollow bunches address the issue of high-brightness beams suffering from transverse emittance growth in a strong space charge regime. During the Proton Synchrotron (PS) injection plateau, the negative space charge tune shift can push the beam onto the Qy = 6 integer resonance. Modifying the longitudinal bunch profile in order to reduce the peak line charge density alleviates the detrimental impact of space charge. To this end we first produce longitudinally hollow phase space distributions in the PS Booster by exciting a parametric resonance with the phase loop feedback system. These inherently flat bunches are then transferred to the PS, where the beam becomes less prone to the emittance growth caused by the integer resonance. During the late 2016 machine development sessions in the PS Booster we profited from solved issues from 2015 and managed to reliably extract hollow bunches of 1.3 eV s matched longitudinal area. Furthermore, first results to create hollow bunches with larger longitudinal emittances towards the LHC Injector Upgrade project goals proved successful. Here, we present the ingredients to the established hollow bunch procedure, showing the few involved changes to the PS Booster cycle for nominal LHC beam production. We also discuss challenges related to the competing phase loop and radial loop feedback systems: their mutual responses can effectively inhibit the excitation of the parametric resonance which should deplete the core of the initially parabolic bunches.