SPACE CHARGE AND WORKING POINT STUDIES IN THE CERN LOW ENERGY ION RING

Abstract

The Low Energy Ion Ring (LEIR) is at the heart of CERN’s heavy ion physics programme and was designed to provide the high phase space densities required by the experiments at the Large Hadron Collider (LHC). LEIR is the first synchrotron of the LHC ion injector chain and it receives a quasi-continuous pulse of lead ions (Pb54+) from Linac3, exploiting a sophisticated multi-turn injection scheme in both transverse and longitudinal planes. Seven of these pulses are injected and accumulated, which requires continuous electron cooling (EC) at low energy to decrease the phase space volume of the circulating beam in between two injections. Subsequently, the coasting beam is adiabatically captured in two bunches, which are then accelerated and extracted towards the Proton Synchrotron (PS). To achieve the ion intensity requirements of the High-Luminosity LHC (HL-LHC), i.e., 8.1× 108 ions/bunch at LEIR extraction, the major LEIR intensity limitation had to be overcome. Past studies showed that up to 50% of the beam was lost after radio-frequency (RF) capture
and during the first part of acceleration and, furthermore, the total extracted intensity was reported to be limited to a maximum of 6× 108 ions/bunch. Various possible sources limiting the LEIR intensity reach were identified, such as hardware related problems, effects related to direct space charge or collective instabilities of the ion beams. In the framework of the LHC Injectors Upgrade (LIU) project, dedicated studies were started at the end of 2015 to understand the underlying loss mechanism and to find possible mitigation measures. These studies revealed the important interplay of betatron resonances and direct space charge effects during the bunching process. In this paper, the results of more recent experimental and
simulation studies, which further emphasize the importance of space charge effects, are summarized and the impact of these studies on the LEIR performance increase is discussed.