Abstract
The longitudinal density monitor (LDM) is primarily intended for the measurement of the particle population in nominally empty rf buckets. These so-called satellite or ghost bunches can cause problems for machine protection as well as influencing the luminosity calibration of the LHC. The high dynamic range of the system allows measurement of ghost bunches with as little as 0.01% of the main bunch population at the same time as characterization of the main bunches. The LDM is a single-photon counting system using visible synchrotron light. The photon detector is a silicon avalanche photodiode operated in Geiger mode, which allows the longitudinal distribution of the LHC beams to be measured with a resolution of 90 ps. Results from the LDM are presented, including a proposed method for constructing a 3-dimensional beam density map by scanning the LDM sensor in the transverse plane. In addition, we present a scheme to improve the sensitivity of the system by using an optical switching technique.
Highlights
The LHC is the world’s largest particle accelerator and is designed to accelerate and collide protons or heavy ions [1]
Like any Geigermode avalanche photodiode (APD), the photon detection module (PDM) has a dead time after each count, during which the detector is blind to further photons
These ghost bunches are created in the LHC by the modulation of the rf voltage during injection to optimize capture for newly injected bunches, which led some particles from previously injected bunches to become debunched
Summary
The LHC is the world’s largest particle accelerator and is designed to accelerate and collide protons or heavy ions [1]. This debunched beam can later be recaptured to form ghost bunches, which spread throughout the LHC ring. These satellite and ghost bunches can collide at the interaction points and create background noise for the experiments. They cause problems in the calibration of other instruments, principally for the measurement of absolute current and luminosity. Synchrotron radiation (SR) is an excellent tool for particle beam diagnostics as it is nondisruptive and carries information on both the transverse and longitudinal particle distribution It is widely used in electron storage rings [5] where the SR intensities are very high. SPC compares favorably in this respect with electromagnetic measurements of the beam, such as wall current monitors or beam current transformers (BCT), where the dynamic range is usually limited due to noise and small mismatches in the impedance of the acquisition chain
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