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BE-BI-PM Section

BSRL: Longitudinal Density Monitor


The longitudinal density monitor of LHC (BSRL/LDM) is based on the detection, with precise time stamping, of single synchrotron light photons emitted by the LHC beams.


At present, only hybrid detectors are used. The three BSRL-detectors are denoted:

  • B1.SYS_A
    • Located in RA47.
    • Hybrid output signal goes to a constant-fraction discriminator in RA47, which generates a NIM pulse that goes to the TDC in UA47.
  • B1.SYS_B
    • Light is collected with a collimator in RA47, and guided through a single-mode fibre to UA47, where the detector is located.
    • Note: the detector output goes directly to the TDC, without passing through a discriminator.
  • B2.SYS_A
    • Placed in UA43
    • Hybrid output signal goes to a constant-fraction discriminator in RA43, which generates a NIM pulse that goes to the TDC in UA43.

The detectors are all PMA Hybrids (cathode -06) from PicoQuant, which has the lowest dark count rate. The spectral sensitivity peaks at about 350 nm. The light yield at injection in this wavelength reigon is limited, but fully sufficient given the large sensitive are of the detectors (6 mm diameter).


The hybrid detectors are sensitive to over-illumination. At a countrate of 10 Mcps, a shutter closes to prevent damage, but with a latency in the ms-range. The shutter remains closed for 3 seconds, and then opens again. Short (ns), but intense light pulses may also damage the detector, but will not visible as an increased countrate: a large number of simultaneous photons will only trigger a single count (i.e. an insufficiently filtered SR-profile from a single nominal bunch at flat top). It is therefore crucial to monitor both the average hybrid countrate, and the average number of counts per bunch. The average number of counts in one bucket must be kept well below 1.

BSRA LED Calibration

With the present configuration, B1.SYS_A and B2.SYS_A will receive a significant amount of light from the BSRA LED during BSRA-calibration. The hybrid detectors should be protected from this light, either by:

  • Ensuring that the effective absorbance of the ND-filters in the boxes is at least ND2, and/or:
  • Closing the BSRL shutter


The following tables lists the FESA classes and instances that are (currently) related to the BSRL.

BSRL related FESA classes and instances
Class nameInstanceFECFESA versionComment
BLDMLHCBSRL.B1.SYS_Acfi-ua47-bsrl2TDC read-out for B1.SYS_A (Input=CH2)
BLDMLHCBSRL.B1.SYS_Bcfi-ua47-bsrl2TDC read-out for B1.SYS_B (Input=CH1)
BLDMLHCBSRL.B2.SYS_Acfi-ua43-bsrl2TDC read-out for B2.SYS_A (Input=CH1)
BLDMLHCBSRL.GD.B1cfi-ua47-bsrl2Global device for detectors on B1-TDC
BLDMLHCBSRL.GD.B2cfi-ua43-bsrl2Global device for detectors on B2-TDC
BFILTERBOXLHC.BSRL.FLT.B1.SYS_Acfv-us45-bsrtb13Filterbox, B1.SYS_A
BFILTERBOXLHC.BSRL.FLT.B1.SYS_Bcfv-us45-bsrtb23Filterbox, B1.SYS_B
BFILTERBOXLHC.BSRL.FLT.B2.SYS_Acfv-us45-bsrtb13Filterbox, B2.SYS_A
BFILTERBOXLHC.BBOX.BSRT.RELAYS.B1cfv-us45-bsrtb13Slot 1 (First) controls power to B1.SYS_A
BFILTERBOXLHC.BBOX.BSRT.RELAYS.B1cfv-us45-bsrtb13Slot 5 (Last) controls B1.SYS_A shutter
BFILTERBOXLHC.BBOX.BSRT.RELAYS.B2cfv-us45-bsrtb23Slot 1 (First) controls power to B2.SYS_A
BFILTERBOXLHC.BBOX.BSRT.RELAYS.B2cfv-us45-bsrtb23Slot 1 (Last) controls B2.SYS_A shutter
BOBRBOBR_cfv-us45-bsracfv-us45-bsra2Calibration laser via fan-out (BunchSel=1, bit=1=second)
BOBRBOBR_cfv-ua47-bsrcfv-ua47-bsra3Turn clock-output used as trigger for B1-TDC
BOBRBOBR_cfv-ua43-bsrcfv-ua43-bsra3Turn clock-output used as trigger for B2-TDC

Filters & Diffusers

All detectors are equipped with a filter box, containing 5 ND-filters of different strength that can be moved in and out individually. B1.SYS_A and B2.SYS_A are equipped with a tube at the aperture, that contains additional, fixed, elements.


  • Filterbox (Slot 1-5): [ND 4, ND 2, ND 1, ND 0.4, ND 0.2]
    • Fixed in tube: 1 ND3 + 1 ground glass diffuser.


  • Filterbox (Slot 1-5): [ND 0.2, ND 0.5, ND 1, ND 2, ND 5]
    • Note: The filter configuration of this box is reversed compared to B1.SYS_A and B2.SYS_A, which have a descending filter order.


  • Filterbox (Slot 1-5): [ND 5, ND 2, ND 1, ND 0.5, ND 0.2]
  • Fixed in tube: 1 ND3 + 1 ground glass diffuser


Two Python GUIs are available to control or monitor the BSRL: one "Expert GUI" for monitoring and controlling one BSRL-monitor, and one "Monitoring GUI" limited to performing ND-filter feedback on all BSRL-monitors simulatenously


The BSRL GUI displays beam profiles and analysis of one or several BSRL detectors, and can be launched from CCM:

  • LHC Control -> LHC Beam Measurements -> BSRL


  • "sh /user/mpalm/bin/"
  • "sh /user/mpalm/bin/ B1.SYS_A B1.SYS_B" (for B1.SYS_A and B1.SYS_B only)

Python source files are located in /user/mpalm/bin/BSRLGui/, with Doxygen-documentation (html) in /user/mpalm/bin/BSRLGui/doc/html/index.html.

Histograms are acquired from the BSRL TDCs. The raw data is corrected and analyzed. The location of RF-Bucket #1 is automatically identified by comparing the bunch structure of the BSRL histogram with a reference device (FBCT or BQM).

Brief description of functionality:

  • "Live": Toggle FESA subscription with "Live"-button
  • "Force": Retrieve latest accumulated histogram
  • Detectors can be shown/hidded with the corresponding check-box
  • "Max bunch [%]": indicates maximum weight of a bunch in % (average number of counts in one bunch per turn)
  • "Sat & Ghost [%]": Satellite and ghost fraction per detector (total=no baseline correction, and bunched=with baseline correction)
  • "Profiles"-tab
    • Shows full profile of beam (50 ps resolution). Profiles have been corrected for deadtime and pile-up
    • Horizontal axis has units of RF-buckets: 0.0-1.0 should correspond to RF-Bucket #1.
      • (May be incorrect if the signal is weak or only one bunch is in the ring)
    • Vertical axis has units of [counts/bin/turn]
    • "Sliding average": apply a uniform filter of n bins to each profile.
  • "Analysis"-tab
    • Plot output of histogram analysis.
    • Check boxes select what to plot on horizontal and vertical axis, respectively.
      • "RF-bucket": RF-bucket number
      • "q_tot": Relative population per bucket (no baseline correction)
      • "q_bunched": With baseline correction
      • "FBCT_A": Population per bucket (read from FBCT_A)
      • "FBCT_B": Population per bucket (read from FBCT_B)
      • "BQM": Population per bucket (read from BQM)
      • "FBCT_A/q_tot": Ratio per bucket
      • "FBCT_B/q_tot": Ratio per bucket
      • "FBCT_A/q_bunched": Ratio per bucket
      • "FBCT_B/q_bunched": Ratio per bucket
      • "4*RMS [ns]": Bunch length, 4*rms in ns
      • "4*FWHM/2.35 [ns]": Bunch length (=4*sigma for a perfect Gaussian)
      • "Phase shift [deg]": Synchronous phase shift per bunch, in degrees
      • "Skew": Bunch skew
      • "GOF Gaussian": Goodness-of-fit, Gaussian
      • "GOF cos2": Goodness-of-fit, cos^2
      • "GOF Square": Goodness-of-fit, square
    • "Norm": Divides selected data by its mean value
    • SpinButton: select number of plots
    • "xDCCTA": scales q_tot and q_bunched with the number of particles from DCCTA (=> particles/bucket)
  • "Slots"-tab
    • Plot overlapping selected slots
    • The scrolled list should contain all buckets identified as filled by the BSRL
      1. Select filled buckets (if any)
      2. Select number of buckets before and after filled to plot
      3. Select filter width (sliding average)
      4. Select normalization
        • None
        • Area = normalize to sum of plot region
        • Peak = normalize to peak value in plot region
      5. "Selected": plot all selected
      6. "Average": plot average of selected
    • "Plot!": Plot it.
  • "Traces"-tab
    • Indicate the time-evolution of certain quantities vs. time (1 new data point per successful histogram analysis).
      • Q_sat_tot: Total fraction in satellite buckets
      • Q_ghost_tot: Total fraction in ghost buckets
      • Q_sat_bunched: Bunchedfraction in satellite buckets
      • Q_ghost_bunched: Bunched fraction in ghost buckets
      • N_sat_tot: Number of particles in satellite buckets (no baseline correction)
      • N_ghost_tot: Number of particles in ghost buckets (no baseline correction)
      • N_sat_bunched: Bunchedfraction in satellite buckets (with baseline correction)
      • N_ghost_bunched: Bunched fraction in ghost buckets (with baseline correction)
      • 4*RMS [ns]: Mean bunch length [ns]
      • 4*FWHM/2.35 [ns]: Mean bunch length from FWHM
      • Skew : Mean bunch skew
      • GoF Gaussian: Mean bunch goodness-of-fit, Guassian profile
      • GoF Square: Mean bunch goodness-of-fit, Square profile
      • GoF cos2: Mean bunch goodness-of-fit, cos^2
      • Bucket width [ns]: Bucket width (calculated from bunch locations)
      • Offset 1st bucket: Histogram offset to 1st RF-bucket
    • Number of plots per detector can be set with spin button
    • "Clear": clear detector traces
  • "Map"-tab
    • Color-plot of all valid and non-filled buckets.
    • Each stripe has a width of 400 buckets, and a height of 10 buckets. Buckets 1-10 are located at the left edge of the upper stripe, bucket 1 being in the top left corner.
    • Blue dots indicate filled or invalidated buckets (masked values).
    • Green circles indicate filled buckets
  • "Log"-tab
    • Logging messages.
    • Log-level is set under Logging->
  • Menus
    • File
      • Load Timber (Time)...
        • Load histogram from Timber: specify UTC timestamp.
      • Load Timber (Fill)...
        • Load histogram from Timber: specify Fill number and select from list
    • View
      • Settings...
        • Specify analysis parameters
        • Bucket width = bucket width in bins
        • Offset to 1st RF-bucket in bins
        • Bunch threshold [abs]: At least one bucket must exceed this value for the histogram to be considered valid
        • Bunch threshold [rel]: Buckets with a weight >= max_bunch*bunch_thresh_rel are considered filled
        • Ref. bunch struct: Reference device
        • Ref. beamlos struct: Reference device for beam loss estimates (should be from the opposite beam)
        • Int. width center: integration width at bucket centres (for calculating bunched charge)
        • Int. width edges: integration width at bucket edges (for baseline correction)
        • Set: Apply settings
      • Expert settings...
        • Allows changing settings of auxiliary FESA devices. Only enabled for trusted users.
        • BSRL.GD.B1/B2: Set integration and notification period and clear current histogram
        • TDC B1/2: Set CoarseDelay on BOBR Turn Clock output used for TDC trigger
        • BOBR_cdf-us45-bsra: Enable/disable calibration laser
          • First: slot location of first laser pulse
          • Distance: distance between laser pulses in slots (minimum 2)
          • # Pulses: Number of pulses (0 = off)
          • Set: Enable settings

BSRL Monitor

The BSRL monitor can be launched on nfs via: sh /user/mpalm/bin/ By default, all BSRL detectors are monitored. Alternatively, monitoring of e.g. only the B1-monitors is launched with their names as argument (no dash): sh /user/mpalm/bin/ B1.SYS_A B1.SYS_B.

This application regulates the detector countrate and number of counts/turn/bunch by inserting/removing ND-filter. An upper countrate limit, and a nominal range of the maximum permissible number of counts/turn/bucket are used as input.

In addition, two minimum filter configurations (corresponding to a minimum absorbance) at injection energy and above injection energy, can be specified. At a given beam energy, filter configurations with a lower absorbance are prohibited.

The GUI:

  • Left panel
    • Current beam mode
    • Beam particle species
    • Settings
      • Minimum filter setting at injection and above injection (protons)
      • Minimum filter setting at injection and above injection (ions)
      • Countrate limits (left=soft=filter feedback threshold, right=hard=ILK threshold. If exceeded, detector shall go to a safe state)
      • Nom. bucket range [min/max]: If max_bunch is outside this span, no feedback is applied.
      • Bucket ILK: If max bucket exceeds this value, the detector shall go to a safe state.
      • Max Abs. Step: Maximum absorbance step in one feedback iteration
      • Soft CR timeous [s]: If the soft countrate limit is exceeded, the new filter setting must not be deceeded for this period.
      • Set: Apply settings.
    • Detector panel
      • Countrate: current countrate in counts/second
      • Max bucket: maximum weight of a single bucket (counts/turn/bucket)
      • The filter panel indicates filter strenghts, selected position (in=green, out=grey) and status (in=green, out=grey, yellow=unknown, red=error). Clicking "Set" applies the filter configuration specified by the check-boxes. The spin-button can be used to toggle the check-boxes in absorbance order.
    • Feedback: enable/disable. When enabled, clicking "SAFE MODE" will insert all filter and (if applicable) close the shutter.
    • Shutter: Shutter control (open/closed)
    • Power: Power control (ON/OFF)

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Page last modified on April 25, 2016, at 05:03 PM EST