The LHC Beam Presence Flag System

Before injecting any high intensity bunches into the LHC a circulating low intensity pilot bunch must be present to confirm the correct settings of the main machine parameters. For the 2010 LHC run the detection of this pilot beam was done with the beam current transformer system. To increase redundancy of this important safety function a dedicated beam presence flag system was designed, built and tested with beam to be used operationally in the 2011 run. In this system signals from four electrodes of a beam position monitor (BPM) are processed with separate channels, resulting in a quadruple system redundancy for either beam. Each system channel consists of an analogue frontend converting the BPM signals into two logic states, which are then transmitted optically to the machine protection and interlock systems. For safety reasons the system does not have any remote control or adjustable elements and its only inputs are the beam signals. This paper describes the new LHC beam presence flag system, in particular the analogue front-end based on diode peak detectors. Paper presented at DIPAC2011 Conference – Hamburg/DE from 16 to 18 May 2011 THE LHC BEAM PRESENCE FLAG SYSTEM M. Gasior, T. Bogey, CERN, Geneva, Switzerland Abstract Before injecting high intensity bunches into the LHC a circulating low intensity pilot bunch must be present to confirm the correct settings of the main machine parameters. For the 2010 LHC run the detection of this pilot beam was done with the beam current transformer system. To increase the redundancy of this important safety function a dedicated beam presence flag system was designed, built and tested with beam to be operationally used in the 2011 run. In this system signals from four electrodes of a beam position monitor (BPM) are processed with separate channels, resulting in a quadruple system redundancy for either LHC beam. Each system channel converts a BPM signal into two logic states, which are then transmitted optically to the machine protection and interlock systems. For reliability the system does not have any remote control or adjustable elements and its only inputs are the beam signals. This paper describes the new LHC beam presence flag system, in particular the analogue front-end based on diode peak detectors.Before injecting high intensity bunches into the LHC a circulating low intensity pilot bunch must be present to confirm the correct settings of the main machine parameters. For the 2010 LHC run the detection of this pilot beam was done with the beam current transformer system. To increase the redundancy of this important safety function a dedicated beam presence flag system was designed, built and tested with beam to be operationally used in the 2011 run. In this system signals from four electrodes of a beam position monitor (BPM) are processed with separate channels, resulting in a quadruple system redundancy for either LHC beam. Each system channel converts a BPM signal into two logic states, which are then transmitted optically to the machine protection and interlock systems. For reliability the system does not have any remote control or adjustable elements and its only inputs are the beam signals. This paper describes the new LHC beam presence flag system, in particular the analogue front-end based on diode peak detectors. INTRODUCTION The described beam presence flag (BPF) system is based on signals from two short-circuited stripline BPMs, one for each LHC beam (BPMC.8R4, electrode distance of 48 mm, length of 150 mm). Using a BPM with four electrodes allows quadruple redundancy of the signal source. However, the electrode signals change with the beam position and to make the BPF threshold levels more beam position independent, the signals from the opposing electrodes are combined and split, as shown in the system block diagram in Fig. 1. For increased robustness the BPM signals are attenuated, limiting the power on the combiners and the following electronics. The BPM signals for both LHC beams (B1 and B2) are converted into logic signals by two BPF front-end (FE) units, each having 4 identical channels. The resulting 8 logic signals, 4 per beam, are transmitted independently to 4 beam interlock user interfaces, CIBFs [1], with two channels each. The CIBF is a standard building block of the LHC interlock system [2], which receives 2 electrical interlock signals and transmits them through optical links to the interlock system. Only there the 4 independent BPF signals of each beam are combined to a single beam presence flag used in the LHC interlock system. The chosen distribution of the BPM signals, BPF FEs and CIBFs guaranties that a malfunctioning of one of the system modules will not enable high intensity injections for either beam even if the module gives the unsafe spurious state ‘BPF = true’ while there is no beam in the machine. Malfunctioning with the spurious state ‘BPF = false’ with some beam in the machine would only disable high intensity injections and is considered safe. BPF FRONT-END The BPF FE is designed for robustness and simplicity and therefore relies only on beam signals. In particular, it does not depend on any interfaces for remote control, calibration or testing. Therefore, it must give ‘BPF = true’ state for all possible LHC beam configurations with unique gain. This implies operation with a small signal for the pilot beam and potential overdrive on the input with the nominal intensity, requiring special protection for the first FE amplifier. The transition of the output logic states from ‘BPF = true’ to ‘BPF = false’ (T-F) must be generated no later than a few revolution periods after the beam intensity goes below the threshold considered as the limit for safe machine operation (about 10 charges). This is required to minimise the probability of injecting high intensity just after the pilot beam has been lost. The opposite transition from ‘false’ to ‘true’ (F-T) can take place even a second after the pilot bunch has been injected. The block diagram for one BPF FE channel is shown in Fig. 2, alongside with sketches of signal waveforms in the most important nodes of the circuit. First the BPM electrode signal, attenuated and combined as described before, is routed through the constant-impedance low pass filter (LPF). Its cut-off frequency of about 150 MHz was chosen to match the BPM signal to the bandwidth of the following stages, in particular of the insulation transformer. The transformer provides a galvanic insulation between the input coaxial cable shields at the LHC vacuum chamber ground and the FE ground, suppressing ground currents on the input cables. Its role is very important, as the system has large gain and even small interference at the input should be avoided. The transformer is followed by a 40 dB amplifier, built with two fast operational amplifiers (op-amps). For increased robustness, on its high CIBF #1 H1.B1 (ch. A) H2.B1 (ch. B) (H.B1) CIBF #3 V1.B1 (ch. A) V2.B1 (ch. B) (H.B1) CIBF #2 H1.B2 (ch. A) H2.B2 (ch. B) (H.B2) CIBF #4 V1.B2 (ch. A) V2.B2 (ch. B) (H.B2) = 20dB/5W attenuator