Longitudinal Density Monitor

The Longitudinal Density Monitor (LDM) is the principal instrument for the measurement of satellite and ghost bunches in the LHC. The LDM uses single photon counting of synchrotron light. The very high energies reached in the LHC, combined with a dedicated undulator for diagnostics, allow synchrotron light measurements to be made with both protons and heavy ions. The arrival times of photons are collected over a few million turns, with the resulting histogram corrected for the effects of the detector’s deadtime and afterpulsing in order to reconstruct the longitudinal profile of the entire LHC ring.

The LDM has achieved a dynamic range in excess of 1e5 and a time resolution of 90 ps. The results obtained form a crucial part of the LHC’s luminosity calibration procedures.

The Longitudinal Density Monitor was designed and commissioned by Adam Jeff as part of the DITANET Marie Curie action.