Halo Scraping, Diffusion and Repopulation MD

Beam halo measurements in the LHC were conducted through collimator scrapings in an MD carried out on the 22nd June 2012 for the first time at 4 TeV. The time evolution of losses during a collimator scan provides information on halo diffusion and population. Four scans were performed with two collimators in the vertical and horizontal plane in B1 and B2 respectively, before and after bringing the beams into collisions. During an inward step, the beam losses measured at the BLMs for the first 3 seconds are believed to be dominated by multi-turn halo removal by the collimator jaw. However, a good comparison was found between fits of the diffusion model and the subsequent loss decay. In addition, the fitted diffusion coefficients compare well to the coefficients estimated from the core emittance growth rates as a function of action.