A comparison of a direct electron detector and a high-speed video camera for a scanning precession electron diffraction phase and orientation mapping

A scanning precession electron diffraction system has been integrated with a direct electron detector to allow the collection of improved quality diffraction patterns. This has been used on a two-phase α–β titanium alloy (Timetal® 575) for phase and orientation mapping using an existing pattern-matching algorithm and has been compared to the commonly used detector system, which consisted of a high-speed video-camera imaging the small phosphor focusing screen. Noise is appreciably lower with the direct electron detector, and this is especially noticeable further from the diffraction pattern center where the real electron scattering is reduced and both diffraction spots and inelastic scattering between spots are weaker. The results for orientation mapping are a significant improvement in phase and orientation indexing reliability, especially of fine nanoscale laths of α-Ti, where the weak diffracted signal is rather lost in the noise for the optically coupled camera. This was done at a dose of ~19 e−/Å2, and there is clearly a prospect for reducing the current further while still producing indexable patterns. This opens the way for precession diffraction phase and orientation mapping of radiation-sensitive crystalline materials

Electron energy reconstruction in the ATLAS electromagnetic End-Cap calorimeter using calibration hits

The Large Hadron Collider, the most powerfull proton-proton collider existing so far, produced its first collisions during the months of November and December of 2009 and it is currently starting its first year of running. The high center of mass energy, high luminosity and several physic channels present at the LHC put a set of requirements in the expected performance of the ATLAS detector

Challenges of arbitrary waveform signal detection by Silicon Photomultipliers as readout for Cherenkov fibre based beam loss monitoring systems

Silicon Photomultipliers (SiPMs) are well recognised as very competitive photodetectors due to their exceptional photon number and time resolution, room-temperature low-voltage operation, insensitivity to magnetic fields, compactness, and robustness. Detection of weak light pulses of nanosecond time scale appears to be the best area for SiPM applications because in this case most of the SiPM drawbacks have a rather limited effect on its performance. In contrast to the more typical scintillation and Cherenkov detection applications, which demand information on the number of photons and/or the arrival time of the light pulse only, beam loss monitoring (BLM) systems utilising Cherenkov fibres with photodetector readout have to precisely reconstruct the temporal profile of the light pulse. This is a rather challenging task for any photon detector especially taking into account the high dynamic range of incident signals (100K – 1M) from a few photons to a few percents of destructive losses in a beam line and presumably an arbitrary temporal distribution of photons (localisation of losses). Nevertheless, a number of advantages and ongoing improvements of SiPM technology are considered to be a reasonable ground for this feasibility study of SiPM application in BLM systems. Transient SiPM responses to light pulses over a wide range of intensities have been measured and an analytical model has been applied to describe the results. Non-linearity of SiPMs due to the limited number of pixels and non-instant pixel recovery time is found to be a source of transient and history-dependent distortions of output signals

An Optical Fibre BLM System at the Australian Synchrotron Light Source

Increasing demands on high energy accelerators are triggering R&D into improved beam loss monitors with a high sensitivity and dynamic range and the potential to efficiently protect the machine over its entire length. Optical fibre beam loss monitors (OBLMs) are based on the detection of Cherenkov radiation from high energy charged particles. Bearing the advantage of covering more than 100m of an accelerator with only one detector and being insensitive to X-rays, OBLMs are ideal for electron machines. The Australian Synchrotron comprises an 100 MeV 15m long linac, an 130m circumference booster synchrotron and a 3 GeV, 216m circumference electron storage ring. The entire facility was successfully covered with four OBLMs. This contribution summarises a variety of measurements performed with OBLMs at the Australian Synchrotron, including beam loss measurements during the full booster and measurements of steady-state losses in the storage ring. Different photosensors, namely Silicon Photo Multipliers (SiPM) and fast Photo Multiplier Tubes (PMTs) have been used and their respective performance limits are discussed

A Versatile Beam Loss Monitoring System for CLIC

The design of a potential CLIC beam loss monitoring (BLM) system presents multiple challenges. To successfully cover the 48 km of beamline, ionisation chambers and optical fibre BLMs are under investigation. The former fulfils all CLIC requirements but would need more than 40000 monitors to protect the whole facility. For the latter, the capability of reconstructing the original loss position with a multi-bunch beam pulse and multiple loss locations still needs to be quantified. Two main sources of background for beam loss measurements are identified for CLIC. The two-beam accelerator scheme introduces so-called crosstalk, i.e. detection of losses originating in one beam line by the monitors protecting the other. Moreover, electrons emitted from the inner surface of RF cavities and boosted by the high RF gradients may produce signals in neighbouring BLMs, limiting their ability to detect real beam losses. This contribution presents the results of dedicated experiments performed in the CLIC Test Facility to quantify the position resolution of optical fibre BLMs in a multi-bunch, multi-loss scenario as well as the sensitivity limitations due to crosstalk and electron field emission

Performance study of little ionization chambers at the Large Hadron Collider

The main detector type for beam loss monitoring of the LHC is a parallel plate Ionization Chamber (IC). In the locations where the beam losses could saturate the read-out electronics of the ICs, two other monitor types, Little Ionization Chambers (LIC) and Secondary Emission Monitors, have been installed to extend the dynamic range of the ICs. The LICs have the same gas composition and pressure as the ICs, but the active volume is 30 times smaller. This reduction in geometrical acceptance reduces the collected dose and holds the LICs under the saturation limit in high loss events, such as during injection failures. In total there are 108 LICs installed in the LHC. In this document the performance of the LICs and their use in the LHC is discussed

BLM Crosstalk Studies on the CLIC Two-Beam Module

The Compact Linear Collider (CLIC) is a proposal for a future linear e⁺-e⁻ accelerator that can reach 3 TeV centre of mass energy. It is based on a two-beam acceleration scheme, with two accelerators operating in parallel. One of the main CLIC elements is a 2 m long two-beam module where power from a high intensity, low energy drive beam is extracted through Power Extraction and Transfer Structures (PETS) and transferred as RF power for the acceleration of the low intensity, high energy main beam. One of the main potential limitations for a Beam Loss Monitoring (BLM) system in a two-beam accelerator is so-called 'crosstalk', i.e. signals generated by losses in one beam, but detected by a monitor protecting the other beam. This contribution presents results from comprehensive studies into crosstalk that have been performed at a two-beam module at the CLIC Test Facility (CTF3) at CERN. The capability of estimating the origin of losses for different scenarios is also discussed

Position Resolution of Optical Fibre-Based Beam Loss Monitors Using Long Electron Pulses

Beam loss monitoring systems based on optical fibres (oBLM), have been under consideration for future colliders for several years. To distinguish losses between consecutive quadrupoles, a position resolution of less than 1 m is required. A resolution of better than 0.5 m has been achieved in machines with single, nanosecond long pulses. For longer beam pulses, such as the ~150 ns CLIC pulse, the longitudinal length of signals in the fibre is close to the duration of the beam pulse itself which makes loss reconstruction very challenging. In this contribution, results from experiments into the position resolution of an oBLM based on long beam pulses are presented. These measurements have been performed at the CLIC Test Facility (CTF3) and the Australian Synchrotron Light Source (ASLS). In CTF3, controlled beam losses were created at different quadrupoles in the 28 m long decelerating Test Beam Line (TBL) LINAC by altering the current supplied or misaligning them. In ASLS the flexibility of the facility allowed the location of beam losses generated by single bunches to be studied as well as losses for longer bunch trains up to 600 ns in duration

Beam Loss Monitoring for LHC Machine Protection

The energy stored in the nominal LHC beams is two times 362 MJ, 100 times the energy of the Tevatron. As little as 1 mJ/cm 3 deposited energy quenches a magnet at 7 TeV and 1 J/cm 3 causes magnet damage. The beam dumps are the only places to safely dispose of this beam. One of the key systems for machine protection is the beam loss monitoring (BLM) system. About 3600 ionization chambers are installed at likely or critical loss locations around the LHC ring. The losses are integrated in 12 time intervals ranging from 40 μs to 84 s and compared to threshold values defined in 32 energy ranges. A beam abort is requested when potentially dangerous losses are detected or when any of the numerous internal system validation tests fails. In addition, loss data are used for machine set-up and operational verifications. The collimation system for example uses the loss data for set-up and regular performance verification. Commissioning and operational experience of the BLM are presented: The machine protection functionality of the BLM system has been fully reliable; the LHC availability has not been compromised by false beam aborts

Beam Loss Monitoring for Run 2 of the LHC

The Beam Loss Monitoring (BLM) system of the LHC consists of over 3600 ionization chambers. The main task of the system is to prevent the superconducting magnets from quenching and protect the machine components from damage, as a result of critical beam losses. The BLM system therefore requests a beam abort when the measured dose in the chambers exceeds a threshold value. During Long Shutdown 1 (LS1) a series of modifications were made to the system. Based on the experience from Run 1 and from improved simulation models, all the threshold settings were revised, and modified where required. This was done to improve the machine safety at 7 TeV, and to reduce beam abort requests when neither a magnet quench or damage to machine components is expected. In addition to the updates of the threshold values, about 800 monitors were relocated. This improves the response to unforeseen beam losses in the millisecond time scale due to micron size dust particles present in the vacuum chamber. This contribution will discuss all the changes made to the BLM system, with the reasoning behind them