[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Dec 2015 · Physical Review Special Topics - Accelerators and Beams
[Show abstract][Hide abstract] ABSTRACT: 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 laser
[Show abstract][Hide abstract] ABSTRACT: Linac 4 presently under construction at CERN is designed to replace the existing 50 MeV Linac 2 in the LHC injector chain and will accelerate the beam of high current negative hydrogen ions to 160 MeV. During the commissioning a laserwire emittance scanner has been installed allowing non-invasive measuring of the emittance at 3 MeV and 12 MeV setups. A relatively low power infrared fibre coupled laser was focused in the interaction region down to ∼150 µm and collided with the ion beam neutralising negative ions. At each transverse laser position with respect to the ion beam the angular distribution of the neutral particle beamlets was recorded by scanning a diamond detector across the beamlet at a certain distance from the interaction point while the main beam of the H − ions was deflected using the dipole magnet installed upstream the detector. Measuring the profile of the beamlet by scanning the laser across the beam allows to directly measure the transverse phase-space distribution and reconstruct the transverse beam emittance. In this report we will describe the analysis of the data collected during the 3 MeV and 12 MeV operation of the Linac 4. We will discuss the hardware status and future plans.
[Show abstract][Hide abstract] ABSTRACT: This archive summarizes a working paper and conference proceedings related to
laser wire scanner development for the Future Linear Collider (FLC) in the
years 2001 to 2006. In particular the design, setup and data taking for the
laser wire experiments at PETRA II and CT2 are described. The material is
focused on the activities undertaken by Royal Holloway University of London
[Show abstract][Hide abstract] ABSTRACT: LINAC4 has started its staged commissioning at CERN. After completion it will accelerate high brightness H − beams to 160 MeV. To measure the transverse profile and emittance of the beam, a non-destructive method based on electron photo-detachment is proposed, using a pulsed, fibre-coupled laser to strip electrons from the H − ions. The laser can be focused and scanned through the H − beam, acting like a conventional slit. A downstream dipole separates the neutral H 0 beamlet, created by the laser interaction, from the main H − beam, so that it can be measured by a diamond strip-detector. Combining the H 0 beamlet profiles with the laser position allows the transverse emittance to be reconstructed. A prototype of this instrument was tested while commission-ing the LINAC4 at 3 and 12 MeV. In this paper we shall describe the experimental setup, challenges and results of the measurements, and also address the characteristics and performance of the diamond strip-detector subsystem. In addition, the proposal for a permanent system at 160 MeV, in-cluding an electron detector for a direct profile measurement, will be presented.
[Show abstract][Hide abstract] ABSTRACT: The CERN LINAC4 beam commissioning at 3 MeV was completed in early 2014 and the one at 12 MeV is ongoing. A novel system for measuring the transverse beam profile and emittance, based on low power laser stripping and H0 de-tection using a diamond detector, was successfully tested at these two energies. The measurement results agree with the operational slit-grid method within a few percent in terms of both transverse profile and emittance. After describing the general system setup, this remarkable achievement is dis-cussed in detail together with the present limitations, which will be addressed in order to design a laser based emittance monitor for the LINAC4 top energy of 160 MeV.
[Show abstract][Hide abstract] ABSTRACT: The new LINAC4 will accelerate H − ions to 160 MeV and ultimately replace the existing 50 MeV LINAC2 in the injector chain for the LHC upgrade. During commission-ing in 2013, a laserwire scanner and diamond strip detector were installed for non-invasive emittance measurements of the 3 MeV H − beam. Synergy with the 3 MeV H − Front End Test Stand at RAL, has stimulated collaborative de-velopment of a novel laserwire system. A low peak power (8 kW) pulsed laser is fibre-coupled for remote installation and alignment free operation. Motorized focusing optics enable remote control of the thickness and position of the laserwire delivered to the vacuum chamber, in which the laser light neutralises a small fraction of H − ions. Unde-flected by a dipole magnet, these H atoms drift downstream, where their spatial profile is recorded by a highly sensitive diamond strip detector with ns-time resolution. We present first tests of the laserwire emittance scanner, including mea-surements of the photo detachment signal with respect to the background from residual gas interactions. The first laser-wire transverse beam profile and emittance measurements are compared with conventional slit-grid diagnostics.
[Show abstract][Hide abstract] ABSTRACT: Laserwires were originally developed to measure micron-sized electron beams via Compton scattering, where traditional wire scanners are at the limit of their resolution. Laserwires have since been applied to larger beam-size, high power H- ion beams, where the non-invasive method can probe beam densities that would damage traditional diagnostics. While photo-detachment of H- ions is now routine to measure beam profiles, extending the technique to transverse and longitudinal emittance measurements is a key aim of the laserwire emittance scanner under construction at the Front End Test Stand (FETS) at the RAL. A pulsed, 30 kHz, 8 kW peak power laser is fibre-coupled to motorized collimating optics, which controls the position and thickness of the laserwire delivered to the H-interaction chamber. The laserwire slices out a beamlet of neutralized particles, which propagate to a downstream scintillator and camera. The emittance is reconstructed from 2D images as the laserwire position is scanned. Results from the delivery optics, scintillator tests and particle tracking simulations of the full system are reviewed. Plans to deploy the FETS laser system at the LINAC4 at CERN are outlined.
[Show abstract][Hide abstract] ABSTRACT: A search is made for massive long-lived highly ionising particles with the ATLAS experiment at the Large Hadron Collider, using 3.1 pb-1 of pp collision data taken at sqrt(s)=7 TeV. The signature of energy loss in the ATLAS inner detector and electromagnetic calorimeter is used. No such particles are found and limits on the production cross section for electric charges 6e <= |q| <= 17e and masses 200 GeV <= m <= 1000 GeV are set in the range 1-12 pb for different hypotheses on the production mechanism.
Full-text · Article · Dec 2013 · Physics Letters B
[Show abstract][Hide abstract] ABSTRACT: The Accelerator Test Facility 2 (ATF2) is a scaled demonstrator system for
final focus beam lines of linear high energy colliders. This paper describes
the high resolution cavity beam position monitor (BPM) system, which is a part
of the ATF2 diagnostics. Two types of cavity BPMs are used, C-band operating at
6.423 GHz, and S-band at 2.888 GHz with an increased beam aperture. The
cavities, electronics, and digital processing are described. The resolution of
the C-band system with attenuators was determined to be approximately 250 nm
and 1 m for the S-band system. Without attenuation the best recorded C-band
cavity resolution was 27 nm.
Full-text · Article · Jan 2013 · Review of Modern Physics
[Show abstract][Hide abstract] ABSTRACT: This Letter describes a model-independent search for the production of new resonant states in photon+jet events in 2.11 fb(-1) of proton-proton collisions at √s=7 TeV. We compare the photon+jet mass distribution to a background model derived from data and find consistency with the background-only hypothesis. Given the lack of evidence for a signal, we set 95% credibility level limits on generic Gaussian-shaped signals and on a benchmark excited-quark (q*) model, excluding 2 TeV Gaussian resonances with cross section times branching fraction times acceptance times efficiency near 5 fb and excluding q* masses below 2.46 TeV, respectively.
[Show abstract][Hide abstract] ABSTRACT: A search for the decay of a light Higgs boson (120-140 GeV) to a pair of weakly interacting, long-lived particles in 1.94 fb(-1) of proton-proton collisions at sqrt[s] = 7 TeV recorded in 2011 by the ATLAS detector is presented. The search strategy requires that both long-lived particles decay inside the muon spectrometer. No excess of events is observed above the expected background and limits on the Higgs boson production times branching ratio to weakly interacting, long-lived particles are derived as a function of the particle proper decay length.
[Show abstract][Hide abstract] ABSTRACT: The results of a search for pair production of the scalar partners of bottom quarks in 2.05 fb(-1) of pp collisions at sqrt[s]=7 TeV using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an R-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the b(1) - χ(1)(0) mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded.
Full-text · Article · May 2012 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Diamond Light source is a third generation synchrotron facility dedicated to producing radiation of outstanding brightness, ranging from infra-red to x-rays. The short electron bunches that are accelerated around the storage ring are susceptible to the phenomenon of microbunching instabilities when the bunch charge exceeds a threshold. The primary feature of the microbunch instabilities is the onset of bursts of radiation in the THz range. The high frequencies involved in the emissions make detection and analysis challenging. A 60-90 GHz Schottky Barrier Diode detector was installed to investigate turn by turn evolution of the instabilities.
No preview · Article · May 2012 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: A Coherent Diffraction Radiation (CDR) experiment was built at the CLIC Test Facility 3 at CERN. Two silicon wafers are positioned on one side of the beam and the CDR radiation originating from them is translated towards the Michelson Interferometer. We have demonstrated that the first target cuts off the backgrounds, generated by the beam from upstream of the experimental setup. The results of the theoretical and experimental investigations of the CDR from the two targets and a work towards longitudinal diagnostics of sub-picosecond electron bunches is presented. The radiation spatial distribution measurements were performed and compared with the theory. A capability to obtain interferometric measurements of CDR at the experimental setup was proven. Bunch shape instabilities at CTF3 were investigated, as well.
No preview · Article · May 2012 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: A measurement of the production cross-section for top quark pairs
(tt¯) in pp collisions at s=7 TeV is presented using data
recorded with the ATLAS detector at the Large Hadron Collider. Events
are selected in the single lepton topology by requiring an electron or
muon, large missing transverse momentum and at least three jets. With a
data sample of 35 pb-1, two different multivariate
methods, one of which uses b-quark jet identification while the other
does not, use kinematic variables to obtain cross-section measurements
respectively. The two measurements are in agreement with each other and
with QCD calculations. The first measurement has a better a priori
sensitivity and constitutes the main result of this Letter.
Full-text · Article · Apr 2012 · Physics Letters B
[Show abstract][Hide abstract] ABSTRACT: The ATLAS experiment at the LHC has measured the centrality dependence
of charged particle pseudorapidity distributions over |η|<2 in
lead-lead collisions at a nucleon-nucleon centre-of-mass
energy of sNN=2.76 TeV. In order to include particles with
transverse momentum as low as 30 MeV, the data were recorded with the
central solenoid magnet off. Charged particles were reconstructed with
two algorithms (2-point "tracklets" and full tracks) using information
from the pixel detector only. The lead-lead collision centrality
was characterized by the total transverse energy in the forward
calorimeter in the range 3.2<|η|<4.9. Measurements are
presented of the per-event charged particle pseudorapidity distribution,
dNch/dη, and the average charged particle multiplicity in
the pseudorapidity interval |η|<0.5 in several intervals of
collision centrality. The results are compared to previous mid-rapidity
measurements at the LHC and RHIC. The variation of the mid-rapidity
charged particle yield per colliding nucleon pair with the number of
participants is consistent with lower sNN results. The shape
of the dNch/dη distribution is found to be independent of
centrality within the systematic uncertainties of the measurement.
Full-text · Article · Mar 2012 · Physics Letters B