G. Ferioli

Beam loss monitoring system for the LHC

One of the most critical elements for the protection of CERNs Large Hadron Collider (LHC) is its beam loss monitoring (BLM) system. It must prevent the superconducting magnets from quenching and protect the machine components from damages, as a result of critical beam losses. By measuring the loss pattern, the BLM system helps to identify the loss mechanism. Special monitors will be used for the setup and control of the collimators. The specification for the BLM system includes a very high reliability (tolerable failure rate of 10/sup -7/ per hour) and a high dynamic range of 10/sup 8/ (10/sup 13/ at certain locations) of the particle fluencies to be measured. In addition, a wide range of integration times (40 /spl mu/s to 84 s) and a fast (one turn) trigger generation for the dump signal are required. This paper describes the complete design of the BLM system, including the monitor types (ionization chambers and secondary emission monitors), the design of the analogue and digital readout electronics as well as the data links and the trigger decision logic.

The LHC beam loss measurement system

An unprecedented amount of energy will be stored in the circulating beams of LHC. The loss of even a very small fraction of a beam may induce a quench in the su- perconducting magnets or cause physical damage to machine components. A fast (one turn) loss of 3 . 10 -9 and a constant loss of 3 . 10 -12 times the nominal beam intensity can quench a dipole magnet. A fast loss of 3 . 10 -6 times nominal beam intensity can damage a magnet. The stored energy in the LHC beam is a factor of 200 (or more) higher than in existing hadron machines with superconducting magnets (HERA, TEVATRON, RHIC), while the quench levels of the LHC magnets are a factor of about 5 to 20 lower than the quench levels of these machines. To comply with these requirements the detectors, ionisation chambers and secondary emission monitors are designed very reliable with a large operational range. Several stages of the acquisition chain are doubled and frequent functionality tests are automatically executed. The failure probabilities of single components were identified and optimised. First measurements show the large dynamic range of the system.

The LHC Beam Loss Monitoring System's Surface Building Installation

The strategy for machine protection and quench prevention of the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) is mainly based on the Beam Loss Monitoring (BLM) system. At each turn, there will be several thousands of data to record and process in order to decide if the beams should be permitted to continue circulating or their safe extraction is necessary. The BLM system can be sub-divided geographically to the tunnel and the surface building installations. In this paper the surface installation is explored, focusing not only to the parts used for the processing of the BLM data and the generation of the beam abort triggers, but also to the interconnections made with various other systems in order to provide the needed functionality.

Wire scanners at LEP

Two sets of wire scanners, each for measuring the horizontal and vertical profile, are installed in LEP in a straight section where the dispersion in both planes is zero. The authors present the design and discuss some limitations of the instrument. A carbon fiber with a diameter of 36 mu m moves through the beam with a speed of about 0.5 m/s. The bremsstrahlung photons generated by the passage of the wire through the beam are detected in scintillators located 80-m downstream. During the first months of LEP operation, the fibers were destroyed occasionally. The various causes, tests and remedies are discussed. At injection energy a significant blowup of the beam results from the wire scan and has to be taken into account for the estimation of the genuine emittance. A model of this blowup is proposed, where the effect is renormalized on the actual measured data. This provides an effective data treatment to obtain the unperturbed beam size.<<ETX>>

Electron cloud studies and analyses at SPS for LHC-type beams

A summary of the main results obtained so far from the electron cloud studies using strip detectors, pick-ups, COLDEX and a 100 MHz coaxial resonator will be presented. The spatial and energy distributions of the electrons in the cloud measured by the strip detectors will be detailed and compared to the results obtained with a conventional retarding field detector. The evidence of the scrubbing effect and of the NEG coatings as remedies to reduce the electron cloud activity will also be shown. In a second part, the improved hardware of the experiments will be presented together with the program of measurements foreseen for the 2003 SPS run.

LHC beam loss monitor system design

At the LHC a beam loss system will be installed for continuous surveillance of particle losses. The system is designed to prevent hardware destructions, to avoid magnet coil quenches and to provide quantitative loss values. Over 3000 ionization chambers will be used to initiate the beam abort if the loss rates exceed the quench levels. The time and beam energy dependent quench levels require the acquisition of chamber currents in the range from 50 pA to 0.5 mA and an update of the values every 89 μs. The acquisition and control electronics will consist of a front end electronics near (< 400 m) to the ionization chambers and a threshold controller in the surface buildings. The front end will include a charge balance converter, a counter and multiplexer part. The charge balance converter is most suiteable to cover the large dynamic range. The introduced error is smaller than few % in the required dynamic range. Six channels will be transmitted over one cable of up to 3 km length. The threshold controller wi...

Wire scanner news from the CERN-SPS

Since electrons and positrons are accelerated in the SPS (Super Proton Synchrotron) for LEP (Large Electron Positron collider) injection, the detection system of the wire scanner was modified and the instrument was used to measure lepton bunch profiles. For the measurement of beam profiles of the high-intensity proton beam in fixed target physics, a limit was found for the beam intensity which the fiber can support without heat damage. This allows use of the wire scanners to measure the profiles at maximum beam energy and intensity without destroying the fiber. The development of high-precision wire scanners for the SPS is accompanied by the need for a better data acquisition system. The new scanners generate more data (up to 32 K per scan) than the old wire scanners and are driven from electronics residing in a VME system. A prototype data acquisition system housed in a VME crate and connected to the operator consoles via a token ring network is described.<<ETX>>

LHC beam loss detector design: Simulation and measurements

The beam loss monitoring (BLM) system is integrated in the active equipment protection system of the LHC. It determines the number of particles lost from the primary hadron beam by measuring the radiation field of the shower particles outside of the vacuum chamber. The LHC BLM system will use ionization chambers as its standard detectors but in the areas where very high dose rates are expected, the secondary emission monitor (SEM) chambers will be additionally employed because of their high linearity, low sensitivity and fast response. The sensitivity of the SEM was modeled in Geant4 via the Photo-Absorption Ionization module together with custom parameterization of the very low energy secondary electron production. The prototypes were calibrated by proton beams. For the calibration of the BLM system the signal response of the ionization chamber is simulated in Geant4 for all relevant particle types and energies (keV to TeV range). The results are validated by comparing the simulations to measurements using protons, neutrons, photons and mixed radiation fields at various energies and intensities.

High sensitivity beam intensity and profile monitors for the SPS extracted beams

Secondary emission monitors using caesium iodide coated thin aluminium foils have been installed in the SPS transfer channels to monitor the intensity of the extracted heavy ion beams. Tests have shown an increase by a factor twenty of their sensitivity with respect to bare aluminium foils. Luminescent screens viewed with TV cameras are used to monitor the position and the profiles of the extracted beams. Various luminescent screen materials have been tested. Results on chromium doped alumina, thallium doped caesium iodide and quartz are reported. A dynamic range of 10/sup 3/ in beam intensities can be achieved by using these three materials in turn in the usual three screen tanks. Intensifiers used together with CCD cameras and video frame grabbers with incorporated projection calculations are used in conjunction with these screens. Results with heavy ions in the transfer channels and with protons extracted from circulating beams in the SPS are given. Detection sensitivities down to a few tens of protons per video frame have been observed.<<ETX>>