Jonathan Emery

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.

An FPGA Based Implementation for Real-Time Processing of the LHC Beam Loss Monitoring System's Data

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 to be triggered. The processing involves a proper analysis of the loss pattern in time and for the decision the energy of the beam needs to be accounted. This complexity needs to be minimized by all means to maximize the reliability of the BLM system and allow a feasible implementation. In this paper, a field programmable gate array (FPGA) based implementation is explored for the real-time processing of the LHC BLM data. It gives emphasis on the highly efficient successive running sums (SRS) technique used that allows many and long integration periods to be maintained for each detectors data with relatively small length shift registers that can be built around the embedded memory blocks.

Design of an actuator for the fast and high accuracy Wire Scanner

This paper presents the design of the actuator for the fast and high accuracy Wire Scanner system. The actuator consists of a rotary brush-less synchronous motor with the permanent magnet rotor installed inside of the vacuum chamber and the stator installed outside. Fork, permanent magnet rotor and two angular position sensors are mounted on the same axis and located inside the beam vacuum chamber. The system has to resist a bake-out temperature of 200°C and ionizing radiation up to tenths of kGy/years. The requirements imply a maximum angular speed of 210 rad.s-1 an required acceleration of 20 000 rad.s-1 and a angular position measurement accuracy of 5 arc seconds. The system must deal with extremely low vibration and low electromagnetic interference. A digital feedback controller will allow maximum flexibility for the loop parameters and feeds the 3 phases linear power driver. The performances of the presented design are investigated through simulations and experimental tests.

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.

Secondary particle acquisition system for the CERN beam wire scanners upgrade

The increasing requirements of CERN experiments make essential the upgrade of beam instrumentation in general, and high accuracy beam profile monitors in particular. The CERN Beam Instrumentation Group has been working during the last years on the Wire Scan- ners upgrade. These systems cross a thin wire through a circulating beam, the resulting secondary particles produced from beam/wire interaction are detected to reconstruct the beam profile. For the new secondary shower acquisition system, it is necessary to perform very low noise measurements with high dynamic range coverage. The aim is to design a system without tuneable parameters and compatible for any beam wire scanner location at the CERN complex. Polycrystalline chemical vapour deposition diamond detectors (pCVD) are proposed as new detectors for this application because of their radiation hardness, fast response and linearity over a high dynamic range. For the detector readout, the acquisition electronics must be designed to exploit the detector capabilities and perform bunch by bunch measurements at 40MHz. This paper describes the design challenges of such a system, analysing different acquisition possibilities from the signal integrity point of view. The proposed system architecture is shown in detail and the development status presented.

A fast and accurate wire scanner instrument for the CERN accelerators to cope with severe environmental constraints and an increased demand for availability

The Beam Instrumentation group of the European Organization for Nuclear Research (CERN) has been developing an instrument called the Beam Wire Scanner (BWS) for the past few years. This system is used to measure the size of proton beams in the Large Hadron Collider (LHC) and its injector chain. An electro-mechanical system moves a very thin wire of 30 μm through the particle beam and measures the induced radiation losses generated by this interaction. The actuator, based on a Permanent Magnet Synchronous Motor (PMSM), a solid rotor resolver and an in-house designed high precision optical encoder are located in underground installations and have to cope with large irradiation levels. Another difference with respect to its predecessors is the placement of all moving parts in the vacuum. The control electronics is situated far away from the beam tunnels to minimize the destructive impact of ionizing particles. Challenges arise from the long distance between these two parts, up to 250 meters, and the high scanning speed of the wire of up to 20 ms1, with a target position accuracy as low as 5 μm rms. This paper describes the challenges of the BWS design, details the current status and introduces the philosophy of its conception to the IEEE control application community.

The LHC beam loss monitoring system’s data contribution to other systems

The strategy for machine protection and quench prevention of the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) is presently based on the Beam Loss Monitoring (BLM) system. At each turn the BLM system is able to acquire and process in real-time data from approximately 4000 detectors in order to decide if the beams should be permitted to continue circulating or their safe extraction is necessary. At the same time in the system, by making full use of its VME based processing cards, data is continuously recorded from both the acquisition, the processing results as well as the status of the electronics which later will be provided to various systems in the LHC. Part of the recorded data will be used to drive an on-line event display and write an extensive logging database at a refresh rate of 1 Hz. Other parts of the same processing units, initiated by external triggers, will provide fast updates of the loss pattern seen in the last 84 ms by 2.54 ms integrals, necessary for the automated collimator adjustments, 100 ms worth of data for every beam injection and scheduled dump to verify the correctness of those procedures, and the last 1.7 s by 40 us integrals to be used for post-mortem analysis in the event of an unforeseen dump as well as FFT analysis studies. The paper discusses the realization of each of those recording functions and their verification with beam measurements.

Functional and Linearity Test System for the LHC Beam Loss Monitoring Data Acquisition Card

In the frame of the design and development of the Beam Loss Monitoring (BLM) system for the Large Hadron Collider (LHC) a flexible test system has been developed to qualify and verify during design and production the BLM LHC data acquisition card. It permits to test completely the functionalities of the board as well as realizing analog input signal generation to the acquisition card. The system utilize two optical receivers, a Field Programmable Gate Array (FPGA), eights flexible current sources and a Universal Serial Bus (USB) to link it to a PC where a software written in LabWindows/CVI© (National Instruments) runs. It includes an important part of the measurement processing developed for the BLM in the future LHC accelerator. It is called Beam Loss Electronic Current to Frequency Tester (BLECFT).