BUNCH BY BUNCH MEASUREMENTS AT LHC - CERN BY BUNCH MEASUREMENTS AT LHC S. Bart-Pedersen, ... a...
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BUNCH BY BUNCH MEASUREMENTS AT LHC
S. Bart-Pedersen, D. Belohrad, A. Boccardi, E. Bravin, S. Burger, E. Calvo, B. Dehning, E. Effinger, J. Emery, M. Gasior, J.J. Gras, J. Gonzalez, E. Griesmayer, A. Guerrero, A. Jeff, L. Jensen,
T. Lefevre, M. Ludwig, G. Papotti, A. Rabiller, F. Roncarolo, J.J Savioz and R. Steinhagen
CERN, Geneva, Switzerland
Abstract Most of the beam instrumentation developed for LHC
has been designed to allow bunch-by-bunch measurements: Beam Position Monitors, Beam Current Transformers, Wall Current Monitor, Wire Scanners, Synchrotron Light Monitors, Schottky Monitors, Longitudinal Density Monitors and Luminosity Monitors. The current status of all these devices is presented highlighting their already achieved performances in 2010 and their known limitations (hardware or software). The plans for upgrades in 2011 will finally be discussed.
INTRODUCTION LHC will be colliding 2808x2808 proton bunches
when reaching its nominal performance, the commissioning of the machine has started with a single bunches per ring of reduced intensity. The number of bunches and the intensity per bunch was increased in steps for safety reasons. After six months of operation, trains of nominal intensity bunches were injected in the LHC and collisions with up to 368 bunches per ring were routinely performed by the end of the year. While increasing the number of bunches, beam-beam effects  and coupled bunch instabilities from impedance  inducing emittance growth, head-tail oscillations and beam losses were observed. Moreover when the bunch spacing was finally reduced from 150 to 75 and finally 50ns, electron cloud effects  became clearly visible with strong vacuum pressure rise causing beam instabilities and emittance growth along the train. Many collective effects were observed in 2010 and bunch-by-bunch measurements are becoming important in order to understand the behaviour of the beams. This paper presents the status of the bunch-by-bunch measurements developed for the LHC.
BEAM POSITION MONITOR The LHC BPM front-end electronic works by design
in bunch-by-bunch mode  and can in certain acquisition modes provide the position for each bunch. Orbits and trajectories are then calculated at the firmware and software level. Several synchronous modes of operation are already implemented. The Post-Mortem mode, available whenever a beam dump happens, gives the average position over all bunches for the last 1024 turns. The Synchronous orbit, being commissioned at the moment, provides the average horizontal and vertical positions (1 value per plane) and bunch positions (3564 values per plane) over 225 turns
at a nominal update rate of 0.1Hz. Finally the capture mode has the flexibility to store N (bunch) x T (turns) samples. The current digital acquisition board limits the number of values to 128k samples but during operation, a strong limitation comes from the LSA concentrator, which cannot handle more than 2000 values per plane.
To overcome this limitation, it has been proposed to calculate in the BPM front-ends turn by turn data averaged over all bunches and to return these values as a new field to be used for Injection Quality Checks (IQC) and Beta-beat measurements. Some dedicated BPMs, with higher memory cards (512k) could be upgraded and would allow retrieving the bunch-by-bunch values for coupled-bunch studies..
HEAD-TAIL MONITOR In point 4, two strip-line BPMs (one per beam) are
used as head-tail monitors. A hybrid converts the four strip-line output signals into sigma and delta signals. These signals are digitalized with a 3GHz 10Gsa/s oscilloscope, which can either be used to look at turns, trains or bunches by adjusting the frame length. The main limitation comes from the memory of the oscilloscope, capable of recording for example 100us x 10 turns or 500ns x 5000 turns. Typical signals, measured during a high intensity fills are displayed in Figure 1. In this particular case the beam was instable because of electron clouds.
Figure 1: Variation of the beam horizontal position in time as seen by the Head-Tail monitors: looking at a train of consecutives bunches (a) or inside a bunch (b)
FAST BEAM CURRENT TRANSFORMER
The LHC Fast BCTs  were designed to provide bunch-by-bunch measurements as illustrated in Figure 2. The output signal of the transformer is split in several channels with low or high bandwidth and different sensitivities. There are two high bandwidth channels with a 20MHz high cut-off frequency and sensitivity ranges for pilot and nominal bunch intensities. Typical
resolutions are 1.5 106 and 2.2 107 protons respectively for high and the low gain channels. Bunch intensities (3564 slots) are averaged over 1 s and stored in the logging database every minute.
Figure 2: Schematic of the FBCT detection system
The Fast BCTs are operational since the very first days of beam operation since they only allowed measuring the low intensity pilots however some accuracy issues have been observed. The dependence on bunch length must be investigated and there are still some improvements to be done to provide an accurate calibration procedure.
TRANSVERSE PROFILE MONITORING
Wire Scanners A schematic presented on Figure 6 shows the working
principle and the hardware configuration of the LHC wire scanner . The shower of secondary particles generated by the interaction of a thin wire with the beam itself is measured by a detector consisting of a scintillator, a set of variable attenuators and a photomultiplier. The bunch-by-bunch acquisition mode is installed as an alternative for the normal acquisition chain and is using a pre-amplifier in the tunnel (200MHZ bandwidth), long high-quality cables and a 40 MHz integrator card (IBMS card) on a DAB module installed in the WS VME crate located in an adjacent service area (US45).
Figure 6: Schematic of the LHC Wire Scanners
The 40MHz mode was tested at the end of run and preliminary comparisons with the standard turn acquisition mode have agreed to within 10%. Few modifications are nevertheless planned to avoid saturating the pre-amplifier. The system should be operational for the coming run in 2011.
Synchrotron Light Monitors Synchrotron Radiation (SR) is used in LHC for
transverse and longitudinal profiles monitoring. A description of the system can be found in . The continuous monitoring of the transverse beam sizes relies on the use of intensified video cameras  (Proxicam HL4 S NIR with a red-enhanced S25
photocathode and an image intensifier). In normal operation the camera integrates over 20ms (all bunches over 224 turns), beam profiles are calculated and the data published every second.
In 2010 bunch-by-bunch images were also acquired with the same camera using a different set-up. The image intensifier was gated to 25ns exposure time using a trigger signal synchronized with the LHC revolution clock, by adjusting the delay any bunches in the machine could be measured independently. The camera sensitivity is sufficient to observe a pilot proton bunch at injection energy. Bunch-by-bunch measurements were for the moment only available on demand but this mode was used extensively during the commissioning of bunch trains. An example of bunch-by-bunch emittance measurement is depicted in Figure 7. The data refers to Beam 2 with the machine filled with 4 trains of 24 bunches spaced by 50ns, each train being spaced by 1.83us. Electron cloud build-up is clearly visible as an emittance blow-up along the trains.
Figure 7: bunch-by-bunch horizontal and vertical beam emittances measured using a gated camera. The horizontal axis is expressed in RF bucket (slot of 25ns)
The slow acquisition rate (1Hz) currently limits the speed at which the transverse profile of all bunches can be obtained. A fast-framing camera, capable of bunch-by-bunch and turn-by-turn acquisitions will be installed during the winter shutdown and will provide faster measurements in 2011.
LONGITUDINAL PROFILE MONITORING
Beam Quality Monitor (BQM) A Beam Quality Monitor, similar to the one
developed few years ago for the SPS  has been installed on LHC to provide bunch length estimate and the filling pattern of the machine. The system, presented in Figure 3, is based on a Wall Current Monitor connected to 8Gsa/s 10bits 100us ADC. The latter is triggered by a precise timing signal derived from the LHC Radio-Frequency system. An Acquisition (~ 1 turn) is performed every 5s and several beam parameters like FWHM bunch lengths, peak amplitudes and bucket numbers are calculated and logged.
Figure 3: Principle of operation of the Beam Quality Monitor
Injection Quality Checks verify that the bucket number corresponds to the one requested by the injection sequencer. The BQM has been used daily in 2010 for online bunch length measurements and has demonstrated its capability to follow changes during the fill and identify problems when they occur. An example of the evolution of the bunch length during a fill is shown in Figure 4. The bunch length shrinks at the beginning of the energy ramp, and then starts to increase as the beams starts to collide due to beam-beam interactions. In this example, the monitor captured an RF cavity trip, which is characterized by a sudden bunch lengthening, returning to the initial value when the cavity came back.
Figure 4: Bunch length evolution during a fill as measured by the BQM
Future improvements will focus on performing multiple turn acquisitions to study longitudinal oscillations.
Wall Current Monitor (WCM) Two other wall current monitors (one per beam) have
been installed in point 4 and provide complementary information of the longitudinal beam structure. The