• M. Meddahi, S. Bart Pedersen, A. Boccardi, A.C. Butterworth, B. Goddard, G.H. Hemelsoet, W. Höfle, D. Jacquet, M. Jaussi, V. Kain, T. Lefèvre, E.N. Shaposhnikova, J.A. Uythoven, D. Valuch
  CERN, Geneva
• A.S. Fisher
  SLAC, Menlo Park, California
• E. Gianfelice-Wendt
  Fermilab, Batavia

Unbunched beam is a potentially serious issue in the LHC as it may quench the superconducting magnets during a beam abort. Unbunched particles, either not captured by the RF system at injection or leaking out of the RF bucket, will be removed by using the existing damper kickers to excite resonantly the particles in the abort gap. Following beam simulations, a strategy for cleaning the abort gap at different energies was proposed. The plans for the commissioning of the beam abort gap cleaning are described, and the first results from the beam commissioning are presented.

• A. Jeff, S. Bart Pedersen, A. Boccardi, E. Bravin, T. Lefèvre, A. Rabiller, F. Roncarolo
  CERN, Geneva
• A.S. Fisher
  SLAC, Menlo Park, California
• C.P. Welsch
  The University of Liverpool, Liverpool

Synchrotron radiation is currently used on LHC for beam imaging and for monitoring the proton population in the 3 microsecond abort gap. In addition to these existing detectors, a study has been initiated to provide longitudinal density profiles of the LHC beams with a high dynamic range and a 50ps time resolution. This would allow for the precise measurement both of the bunch shape and the number of particles in the bunch tail or drifting into ghost bunches. A solution is proposed based on counting synchrotron light photons with two fast avalanche photo‐diodes (APD) operated in Geiger mode. One is free‐running but heavily attenuated and can be used to measure the core of the bunch. The other is much more sensitive, for the measurement of the bunch tails, but must be gated off during the passage of the core of the bunch to prevent the detector from saturating. An algorithm is then applied to combine the two measurements and correct for the detector dead time, after pulsing and pile‐up effects. Initial results from laboratory testing of this system are described in this paper.

• T. Lefèvre, S. Bart Pedersen, A. Boccardi, E. Bravin, A. Goldblatt, A. Jeff, F. Roncarolo
  CERN, Geneva
• A.S. Fisher
  SLAC, Menlo Park, California

The LHC beam dump system relies on extraction kickers that need 3 microseconds to rise up to their nominal field. As a consequence, particles crossing the kickers during this rise time will not be dumped properly. The proton population during this time should remain below quench and damage limits at all times. A specific monitor has been designed to measure the particle population in this gap. It is based on the detection of Synchrotron radiation using a gated photomultiplier. Since the quench and damage limits change with the beam energy, the acceptable population in the abort gap and the settings of the monitor must be adapted accordingly. This paper presents the design of the monitor, the calibration procedure and the detector performance with beam.