Tail Repopulation Measurements in the PSB

The PS Booster (PSB) is the first circular accelerator in the LHC injector chain providing protons for the full CERN complex. Each of its four rings provides beams in a range of intensities varying from 40 e11 p/cycle to 0.8 e13 p/cycle. Low intensity beams are produced by transverse shaving, that is by scraping the tails, in order to tailor the intensity and transverse emittances. Eventually, tails repopulate and the beam profile reshapes, under the effect of space charge, which is dominant at low energy in the PS Booster. This paper describes the results of the measurements after the shaving process, where the tails are scraped but finally re-appear in the transverse profile, and it provides a first benchmark with space-charge simulations. It highlights the challenges encountered and the lessons learned, to guide the future experiments. The final outcome of these studies is the characterisation of the halo creation mechanism and the determination of the diffusion speed, important for the design of the future PS Booster scraping system.