A Longitudinal Density Monitor for the LHC

At the Large Hadron Collider (LHC), the world’s largest and highest energy particle accelerator, ion bunches circulate in two counter-rotating beams and are brought into collision. Each bunch is confined within a bucket by the longitudinal focusing effect of the radio frequency (RF) cavities. The RF period is 2.5 ns, while the minimum bunch spacing is 25 ns. Thus, 9 out of every 10 buckets should be empty, as well as additional gaps to allow for the rise-time of injection and dump kickers. In practice, however, small numbers of particles can occupy these supposedly empty buckets, causing problems for machine protection and for the absolute calibration of the LHC’s luminosity. The Longitudinal Density Monitor (LDM) is a new monitor, designed to measure the longitudinal distribution of particles in the LHC with a sufficiently high dynamic range to quantify the relative particle population in the supposedly empty buckets. A non-interceptive measurement is made possible by the use of synchrotron radiation (SR). Single photon counting with an avalanche photo-diode operating in Geiger mode allows a very high dynamic range to be achieved despite the low levels of light available. The imperfect response of the avalanche photo-diode is compensated using a specially designed correction algorithm which reduces noise and distortion to a minimum. This work presents the design, implementation and operation of the LDM. Signal correction methods are discussed with reference to the deadtime and afterpulsing of the avalanche photodiode, and the analysis of the LDM data for use in LHC luminosity calibration is explained. Experimental results with both proton and heavy ion beams are shown illustrating the LDM‘s exceptional performance, combining a high dynamic range of 105 with a 90 ps time resolution. Finally, a novel scheme to extend the dynamic range by several more orders of magnitude is presented.