Characterisation and Mitigation of Beam Dynamics Limitations in the CERN Low Energy Ion Ring (LEIR)

Abstract

The Low Energy Ion Ring (LEIR) is the first synchrotron of the CERN heavy-ion injector chain. At its low injection energy, the space charge tune spread of the beam overlaps several resonance lines around the working point, so the dominant resonances have to be compensated directly. This thesis studies the linear and non-linear errors of LEIR, in order to improve its performance. The measurements were performed on a dedicated low intensity cycle with a coasting beam, which gives a small tune spread, and were complemented by tracking simulations with Xsuite. First, the set tune was compared with the measured tune over a grid of working points. A constant offset was found in each plane and corrected, with a remaining error of the order of 10−3. An additional tune shift was observed when the skew sextupoles are powered, for which feed-down from a vertical offset is a possible explanation. The resonances that dominate the losses were then identified with loss maps: the third order skew resonance 3Qy = 8 and the third order normal resonance Qx + 2Qy = 7. A skew sextupole scan on 3Qy = 8 revealed two low-loss regions. Both settings reduce the losses on 3Qy = 8, but excite Qx +2Qy = 7. Normal sextupole scans showed that one skew sextupole adds an unwanted normal component. Combining the skew and normal sextupole settings strongly reduces the losses over the whole region, and gives the best compensating setting found in this work. Finally, tracking simulations give a first qualitative picture of this region. They show resonances that may be excited by space charge alone, and reproduce the mechanism of the compensation of 3Qy = 8.