E. Bravin

Longitudinal beam profile measurements at CTF3 using a streak camera

The proposed Compact Linear Collider (CLIC) is a multi-TeV electron-positron collider for particle physics based on an innovative two-beam acceleration concept. A high-intensity drive beam powers the main beam of a high-frequency (30 GHz) linac with a gradient of 150 MV/m, by means of transfer structure sections. The aim of the CLIC Test Facility (CTF3) is to make exhaustive tests of the main CLIC parameters and to prove the technical feasibility. One of the points of particular interest is the demonstration of bunch train compression and combination in the Delay Loop and in the Combiner Ring. Thus, detailed knowledge about the longitudinal beam structure is of utmost importance and puts high demands on the diagnostic equipment. Among others, measurements with a streak camera have been performed on the linac part of the CTF3 as well as on the newly installed Delay Loop. This allowed e.g. monitoring of the longitudinal structure of individual bunches, the RF combination of the beam, the behavior during phase shifts and the influence of the installed wiggler. This article first gives an overview of the CTF3 facility, then describes in detail the layout of the long optical lines required for observation of either optical transition radiation or synchrotron radiation, and finally shows first results obtained during the last machine run this year.

Collision rate monitors for LHC

Collision rate monitors are essential in bringing particle beams into collision and optimizing the performances of a collider. In the case of LHC the relative luminosity will be monitored by measuring the flux of small angle neutral particles produced in the collisions. Due to the very different luminosity levels at the four interaction regions (IR) of LHC two different types of monitors have been developed. At the high luminosity IR (ATLAS and CMS) fast ionization chambers will be installed while at the other two (ALICE and LHC-b) solid state polycrystalline Cadmium Telluride (CdTe) detectors will be used. The ionization chambers are being developed by LBNL while the CdTe monitors are being developed by CERN and CEA-LETI.

A beam halo monitor based on adaptive optics

In future high intensity, high energy accelerators, beam losses have to be minimized to maximize performance and reduce activation of accelerator components. 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 high resolution measurements of the transverse beam profile. An imaging system based on a beam core-suppression technique, in which the core of the beam is deflected by means of a micro mirror array, to allow for direct observation of the halo has been developed. In this contribution, a possible layout of a novel diagnostic system based on adaptive optics is presented and the results of first tests carried out in our optical lab are summarized.

Transverse polarization beyond the Z energy at LEP

Experimental results on transverse polarization obtained at LEP at a beam energy of 50 GeV are shown. The application of the refined orbit correction procedure known as harmonic spin matching, implemented to compensate for depolarizing effects originating from orbit errors and misalignments of the machine elements, is described. Prospects and plans to improve the transverse polarization level at higher energies to extend the range of application of the direct and precise calibration with resonant depolarization are reported.

A modified two-slit interferometer for characterizing the asymmetric lateral coherence of undulator radiation

Recently we have shown that the asymmetric lateral coherence of betatron radiation is characterized by peculiar properties that are evidenced with the analysis of the coherence factor of radiation. We extend such results to radiation emitted by ultra-relativistic proton beams in a 2-periods undulator. Results of a 2-dimensional simulation show that the real analysis of the asymmetric lateral coherence substantially improves the resolving power, thus transverse beam non-uniformities and the beam size can be measured at large distance.

Demonstration of a laserwire emittance scanner for hydrogen ion beams at CERN

A non-invasive, compact laserwire system has been developed to measure the transverse emittance of an H- beam and has been demonstrated at the new LINAC4 injector for the LHC at CERN. Light from a low power, pulsed laser source is conveyed via fibre to collide with the H- beam, a fraction of which is neutralized and then intercepted by a downstream diamond detector. Scanning the focused laser across the H- beam and measuring the distribution of the photo-neutralized particles enables the transverse emittance to be reconstructed. The vertical phase-space distribution of a 3 MeV beam during LINAC4 commissioning has been measured by the laserwire and verified with a conventional slit and grid method.

Measuring the bunch frequency multiplication at the 3rd CLIC Test Facility

The CLIC Test Facility 3 (CTF3) is being built and commissioned by an international collaboration to test the feasibility of the proposed Compact Linear Collider (CLIC) drive beam generation scheme. Central to this scheme is the use of RF deflectors to inject bunches into a delay loop and a combiner ring, in order to transform the initial bunch frequency of 1.5 GHz from the linac to a final bunch frequency of 12 GHz. To do so, the machines transverse optics must be tuned to ensure beam isochronicity and each rings length can finally be adjusted with wiggler magnets to a sub millimeter path length accuracy. Diagnostics based on optical streak camera and RF power measurements, in particular frequency bands, have been designed to measure the longitudinal behaviour of the beam during the combination. This paper presents the diagnostics and recent commissioning measurements.

High Speed Measurements of the LHC Luminosity Monitor

The LHC luminosity monitor is a gas ionization chamber designed to operate in the high radiation environment present in the TAN neutral absorbers at the LHC. One of the challenges is to measure the luminosity of individual bunch crossings with a minimum separation of 25 nsec. To test the time response and other aspects of a prototype chamber, we have performed a test using an x‐ray beam of 40–60 keV with pulse spacing of 26 nsec as an ionizing beam. The tests were made at BL 8.3.2 at the Advanced Light Source (ALS). This work was supported by the Director, Office of Science, Office of High Energy Physics, of the U.S. Department of Energy under Contract No. DE‐AC02‐05CH11231.

CTF3 Injector Profile Monitor

The electron gun of the CLIC Test Facility 3 (CTF3) produces 15.6‐μs‐long pulses with an energy of 140 keV and a current that can be as high as 9 A. For the nominal beam operation, a 5.4‐A beam current is generated and sent downstream into the bunching system and the rest of the accelerator. The corresponding beam charge will induce a thermal load that most of the materials, considered as radiators, would not withstand. With this problem in mind, we have built a beam imaging system, installed just after the gun and equipped with two screens. The first one is a phosphorescent screen which yields a high signal and can be for low beam currents. The second screen, a thin graphite foil, is used as a forward OTR radiator and can stand the full beam intensity. Moreover, the time resolution of OTR is very good, in the femtosecond range. This allows for observation of the evolution in time of the beam size during the pulse by using a gated camera. We present in this paper the first results obtained using this system.

The BRAN luminosity detectors for the LHC

This paper describes the several phases which led, from the conceptual design, prototyping, construction and tests with beam, to the installation and operation of the BRAN (Beam RAte of Neutrals) relative luminosity monitors for the LHC. The detectors have been operating since 2009 to contribute, optimize and maintain the accelerator performance in the two high luminosity interaction regions (IR), the IR1 (ATLAS) and the IR5 (CMS). The devices are gas ionization chambers installed inside a neutral particle absorber 140 m away from the Interaction Points in IR1 and IR5 and monitor the energy deposited by electromagnetic showers produced by high-energy neutral particles from the collisions. The detectors have the capability to resolve the bunch-by-bunch luminosity at the 40 MHz bunch rate, as well as to survive the extreme level of radiation during the nominal LHC operation. The devices have operated since the early commissioning phase of the accelerator over a broad range of luminosities reaching 1.4×1034 cm−2 s−1 with a peak pileup of 45 events per bunch crossing. Even though the nominal design luminosity of the LHC has been exceeded, the BRAN is operating well. After describing how the BRAN can be used to monitor the luminosity of the collider, we discuss the technical choices that led to its construction and the different tests performed prior to the installation in two IRs of the LHC. Performance simulations are presented together with operational results obtained during p-p operations, including runs at 40 MHz bunch rate, Pb-Pb operations and p-Pb operations. (Less)