1. LHC Studies Working Group – 03 July 2012 Beam Scraping and Diffusion + Asynchronous Dump MD G. Valentino, R. W. Assmann, F. Burkart, L. Lari, S. Redaelli, B. Salvachua + colleagues from Fermilab: V. Previtali, G. Stancari, A. Valishev

2. Beam Halo Scraping MD Study Motivation: Time evolution of beam losses in collimator scan gives information on: –Halo diffusion, halo population, emittance growth, lifetime, … Measurements will profit from the new 12.5 Hz BLM data for greater resolution. Diffusion model (developed by G. Stancari for Tevatron data) could predict loss spikes before, during and after collimator steps, both inward and outward important for collimator alignment automation. MD Programme (22/06/2012, 0400 – 0800) Started off at 4 TeV with squeezed, separated beams (1 nominal bunch per beam). Retracted IR7 TCPs and TCSGs to 7 σ. Moved in vertical IR7 B1 TCP and horizontal IR7 B2 TCP in steps of 5 µm to 20 µm every 10-40 s. Half gap: 7 σ 3 σ (nominal beam size). Moved out TCPs in steps of 20 µm to 100 µm when the losses reach steady-state (halo repopulation). Brought beams into collisions, repeated inward and outward steps. 2 hours of data-taking in requested beam conditions. Gianluca Valentino2

3. Preliminary Results (1) Gianluca Valentino3 Inward Step Outward Step 12.5 Hz BLM [Gy/s] Time [s] Signal: D = 8.62E-11 µm 2 /s D = 1E-9 µm 2 /s Losses decay faster than expected! Non-diffusion beam physics? Other fit functions to be explored… where Loss Rate: Distribution function gradient at collimator: Lengthy repopulation

4. Preliminary Results (2) Diffusion model prediction: particles lost linearly with 1/√t Diffusion coefficient can be estimated from linear fit: Gianluca Valentino4 Inward Step Outward Step Distribution Fit: D = 1.0E-9 µm 2 /s Linear Fit: D = 1.15E-9 µm 2 /s Non-linear behaviour

5. De-bunched Beam Dump Gianluca Valentino5 Asynchronous dumps provide loss maps which aid in identifying possible critical points in the machine for realistic error scenarios (see preliminary simulations by L. Lari presented in IPAC’12). Test scenario at 4 TeV (assuming maximum error superimposition): -Tight collimator settings -1.5 mm orbit bump away from IR6 TCDQ and TCSG -TCSG.B5L7.B1, TCSG.A5L7.B1, TCSG.B4L7.B1, TCLA.A7R7.B1 retracted by 1mm. -TCTH.4L1.B1 moved to 8 sigmas from 9 sigmas (~1.1 mm). After scraping MD, dumped B2 and de-bunched B1. Abort gap intensity at time of asynchronous dump = 7.8E9 protons Preliminary Results: -High losses in TCTH.4L1.B1 as predicted -Losses in IP6 due to nature of orbit bump -Full analysis to be presented by L. Lari in collimation meeting