Ewald Effinger

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 monitoring system's data acquisition card

The beam loss monitoring (BLM) system [1] of the LHC is one of the most critical elements for the protection of the LHC. It must prevent the super conducting magnets from quenches and the machine components from damages, caused by beam losses. Ionization chambers and secondary emission based beam loss detectors are used on several locations around the ring. The sensors are producing a signal current, which is related to the losses. This current will be measured by a tunnel electronic, which acquires, digitizes and transmits the data via an optical link to the surface electronic. The so called threshold comparator (TC) [2] collects, analyzes and compares the data with threshold table. It also gives a dump signal through the combiner card to the beam inter lock system (BIC). The usage of the system, for protection and tuning of the LHC and the scale of the LHC, imposed exceptional specification of the dynamic range and radiation tolerance. The input current dynamic range should allow measurements between 10pA and 1mA and it should also be protected to very high pulse of 1.5kV and its corresponding current. To cover this range, a current to frequency converter (CFC) is used in the tunnel card, which produces an output frequency of 0.05Hz at 10pA, and 5MHz at 1mA. In addition to the output frequency, the integrator output voltage is measured with a 12bit ADC to improve the resolution. The location of the CFC card next to the detector imposes the placement of the card in the LHC tunnel, exposing the card to radiation. The radiation tolerance was defined by assuming a 20 year operation period corresponding to 400Gy. A mixture of radiation tolerant Asics from the microelectronic group at CERN, and standard component was chosen to cope with these requirements.

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.

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.

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).

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.