Transverse Beam Profiles

The performance and safe operation of a particle accelerator is closely connected to the transverse emittance of the beams it produces. For this reason many techniques have been developed over the years for monitoring the transverse distribution of particles along accelerator chains or over machine cycles. The definition of beam profiles is explained and the different techniques available for the detection of the particle distributions are explored. Examples of concrete applications of these techniques are given.Comment: 37 pages, 53 figure

Instrumentation2: Other instruments, ghost/satellite bunch monitoring, halo, emittance, new developments

In order to estimate in absolute terms the luminosity of LHC certain beam parameters have to be measured very accurately. In particular the total beam current and the relative distribution of the charges around the ring, the transverse size of the beams at the interaction points and the relative position of the beams at the interaction point. The experiments can themselves measure several of these parameters very accurately thanks to the versatility of their detectors, other parameters need however to be measured using the monitors installed on the machine. The beam instrumentation is usually built for the purpose of aiding the operation team in setting up and optimizing the beams, often this only requires precise relative measurements and therefore the absolute scale is usually not very precisely calibrated. The luminosity calibration requires several machine-side instruments to be pushed beyond their initial scope.Comment: 6 pages, 9 figures, presented at the LHC Lumi Days: LHC Workshop on LHC Luminosity Calibration, 13-14 January 2011, CERN, Geneva, Switzerland; CERN-Proceedings-2011-001, pp. 102-10

Energy calibration at LEP using Nuclear Magnetic Resonance probes

The accurate Standard Model investigations carried out at LEP require knowledge of the beam energies of the order of a few 10-5. The resonant depolarisation method, used for absolute calibration in de dicated experiments, cannot be used to monitor continuously the beam energy during the physics runs. Moreover appreciable polarisation of the beams has not been measured above energies of 55 GeV. A me thod for continuous energy monitoring based on Nuclear Magnetic Resonance (NMR) probes mounted in tunnel magnets has been in use at LEP since 1995. The average field of the dipole magnets is sampled v ia 24 NMR probes mounted in the gap of the C-shaped yokes on top of the vacuum chamber. The probes are distributed over the 27 km of the accelerator. The probes are used for the continuous monitoring of the field during LEP operation and to determine the absolute field value

Model of Dipole Field Variations in the LEP Bending Magnets

The determination of the Z mass at LEP requires a knowledge of the relative beam energy in the order of 10 ppm, therefore it is essential to understand the dipole field variations to the same level of accuracy. In LEP the bending magnet field shows a relative increase of the order of 100 ppm over 10 hours, which was found to be caused by leakage currents from railways flowing along the vacuum cham ber and temperature variations. A LEP dipole test bench was set up for systematic investigations. Field variations were monitored with NMR probes while the cooling water temperature of both coil and vacuum chamber was kept under control. The results lead to a parametrisation of the magnetic field variation as a function of the vacuum chamber current and temperature

N2_2 and Xe Gas Scintillation Cross-Section, Spectrum, and Lifetime Measurements from 50 MeV to 26 GeV at the CERN PS and Booster

Beam parameters in CERN's Proton Synchrotron (PS) accelerator must be controlled (and measured) with tighter precision than ever before to meet the stringent requirements of the Large Hadron Collider (LHC) programme. A non-destructive beam profile measurement system would be a valuable diagnostic tool. To this end, we measured N2 and Xe gas scintillation absolute cross-sections and lifetimes for proton beam energies from 1.4 to 25 GeV, which should prove valuable in the design and construction of a gas scintillation profile measurement system. We also measured relative cross-sections for proton beam energies between 0.05 and 1.4 GeV

Operational experience with a CID camera system

In future high intensity, high energy accelerators particle losses must be minimized as activation of the vacuum chambers or other components makes maintenance and upgrade work time consuming and costly. It is imperative to have a clear understanding of the mechanisms that can lead to halo formation, and to have the possibility to test available theoretical models with an adequate experimental setup. Measurements based on optical transition radiation (OTR) provide an interesting opportunity for analyzing the transverse beam profile due to the fast time response and very good linearity of the signal with respect to the beam intensity. On the other hand, the dynamic range of typical acquisition systems as they are used in the CLIC test facility (CTF3) is typically limited and must be improved before these systems can be applied to halo measurements. One possibility for high dynamic range measurements is an innovative camera system based on charge injection device (CID) technology. With possible future measurements in CTF3 in mind, comparative measurements performed with this innovative camera system, a standard CCD camera and a step-by-step measurement technique based on a small photomultiplier are summarized with emphasi

Luminosity measurements at LEP

Fast luminosity measurements are vital for the optimisation of the machine conditions needed for physics. At LEP this has been achieved since the startup by means of 16 small tungsen-silicon calorimeters measuring the rate of Bhabba scattering events. To increase the counting rate the detectors are placed close to the beams and mounted on collimator jaws. The rate of Bhabba scattering is calculated using the rate of coincidental detections of e- and e+ at both sides of the interaction point. The correction term arising from accidental off-momentum particle coincidence is calculated from the background rates. This technique could be successfully used at beam energies around 45 GeV since the correction term was small.Starting in '95 however, the energy of LEP has been increased up to 91.5 GeV per beam. In these conditions the background event rate almost doubles while the Bhabba cross section adopted and presented in this paper consists of checking the collinearity in the vertical plane of the particle tracks. This is obtained by measuring the vertical centre position of the showers inside the calorimeters using silicon strip detectors

Experimental results of the laserwire emittance scanner for LINAC4 at CERN

Within the framework of the LHC Injector Upgrade (LIU), the new LINAC4 is currently being commissioned to replace the existing LINAC2 proton source at CERN. After the expected completion at the end of 2016, the LINAC4 will accelerate H- ions to 160 MeV. To measure the transverse emittance of the H- beam, a method based on photo-detachment is proposed. This system will operate using a pulsed laser with light delivered via an optical fibre and subsequently focused through a thin slice of the H- beam. The laser photons have sufficient energy to detach the outer electron and create H0/e- pairs. In a downstream dipole, the created H0 particles are separated from the unstripped H- ions and their distribution is measured with a dedicated detector. By scanning the focused laser across the H- beam, the transverse emittance of the H- beam can be reconstructed. This paper will first discuss the concept, design and simulations of the laserComment: Presented at the International Conference on Laser Applications at Accelerators, LA3NET 2015; Submitted to Nucl. Instr. and Meth. in Phys. Res. section

The Influence of Train Leakage Currents on the LEP Dipole Field

The determination of the mass and the width of the Z boson at CERN's LEP accelerator, an e+e- storage ring with a circumference of approximately 27 kilometres, imposes heavy demands on the knowledge of the LEP counter-rotating electron and positron beam energies. The precision required is of the order of 1 MeV or »20 ppm frequency. Due to its size the LEP collider is influenced by various macroscopic and regional factors such as the position of the moon or seasonal changes of the rainfall in the area, as reported earlier. A new and not less surprising effect of the LEP energy was observed in 1995: railroad trains in the Geneva region perturb the dipole field. A parasitic flow of electricity, originating from the trains, travels along the LEP ground cable and the vacuum chamber, interacting with the dipole field. An account of the phenomenon with its explanation substantiated by dedicated measurements is presented

Advanced Simulations of Optical Transition and Diraction Radiation

Charged particle beam diagnostics is a key task in modern and future accelerator installations. The diagnostic tools are practically the “eyes” of the operators. The precision and resolution of the diagnostic equipment are crucial to define the performance of the accelerator. Transition and diffraction radiation (TR and DR) are widely used for electron beam parameter monitoring. However, the precision and resolution of those devices are determined by how well the production, transport and detection of these radiation types are understood. This paper reports on simulations of TR and DR spatial-spectral characteristics using the physical optics propagation (POP) mode of the Zemax advanced optics simulation software. A good consistency with theory is demonstrated. Also, realistic optical system alignment issues are discussed