Collimator Setup (injection energy)

  • Initial remarks:
    The setup of the collimators should be done after a reference orbit has been established for the injection plateau and for the ramp (as in exit conditions of Phase A.4). The setup procedure has to be repeated if the orbit and/or the optics change significantly.
    The number of collimators needed and their settings have be defined as a function of beam intensity. The settings also depend on the global machine aperture for the reference closed orbit (see RA's talk).
    Positioning tolerances of collimator jaws have also to be defined accordingly.
    If already commissioned and operational, the (local) orbir feedback in the insertions should be ON.

  • Beam conditions:
    Setup done with low or intermediate intensity pilot beams (single bunch). Nominal emittance should be used (blow it up at the SPS or after injection into the LHC if needed).
    After setup with single bunch: perform consistency checks with larger beam intensities to see if the beam sizes at the collimator is the same.
    Remark: the collimator setup should be done at largest beam intensity that is considered SAFE.

  • Required beam instrumentation

    • BLM system:
      1) Acquisition of the dedicated monitors for each collimator (build-in within the collimator control application!)
      2) Ring BLM: Needed also to measure the global beam loss pattern around the machine. Continuous monitoring of losses around the ring at facq > 1 Hz. Possibility of recording reference loss maps?

    • Orbit measurement:
      Start orbit monitoring at 1Hz.

    • Emittance / beam size measurements:
      Systematically measure the emittance of each injected beam, both (1) at the SPS before extraction and (2) in the LHC ring. Repeat measurements regularly during the coast. Repeat measurements after every major beam scraping with the collimators.
      Available measurements: wire scanners (priority 1), synchrotron radiation monitors (2), IPM (3).

    • BCT measurement and corresponding data saving must be available.

    • Logging of relevant collimator parameters (jaw positions and gaps; temperatures).

  • Required software:

    • Single collimator control units for discrete trims.
      (function driven trims are not yet needed for the cleaning insertions. Could be needed to automatically move out the injection protection collimators after the single bunch is injected).

    • Possibility of storing reference settings and beam-based parameters into the appropriated databases.

    • Operational MCS interface to authorize authenticated users to update the critical beam based parameters.

    • Generation/trim of local orbit bumps. Automatic scans?

  • General procedure for system setup:

    • For each beam, the setup of the injection cleaning should start with IR7. Injection protection and TCQD use similar procedures but are not discussed here.
      Parallel setup of IR7 for one beam and IR3 for the other has to be considered.

    • Number of collimators to use and their settings must be defined (based on the measured global machine aperture, see RA's talk).
      Nominal IR7 settings for ultimate performance at injection: (TCP, TCS, TCLS) = (5.7, 6.7, 10) sigmas.

    • Beam-based alignment: find the local orbit position at each collimator for the collimator jaw centring
      Procedure: Create a sharp beam edge with one jaw and move the other until you see a spike in the BLM signal. Procedure tested in the SPS beam testes (see link).
      The beam-based orbit position at each collimator must be saved in the LSA database with MCS.

    • Beam size measurements at each collimator: Once the local orbit position is found, the beam size at each collimator has to be determined. Procedures exist to calculate the local beam size in case the optics information on the beta functions is not good enough (See link).
      Procedure for beam size measurements based on full beam scraping (destructive, needs inject-scrape mode of low intensity beams):
      - move one jaw "close" to the beam: 4-5 sigmas (do not see any losses on the BCT)
      - measure the beam emittance with wires and, immediately after,
      - scraper the beam with one jaw. Record simultaneously BCT, jaw position, BLM, orbit at the highest possible frequency
      - If dedicated software is available, calculate the beam size with appropriated Guassian fits (page 9 of the paper). Otherwise, do that offline
      - If time permits, repeat with the other jaw and compare to previous results
      The beam-based local beam sizes must be saved in the LSA database with MCS.

    • Setting generation for all the collimators - generate setting for the injection plateau (ready for higher intensities) and functions for the energy ramp (ready for higher stored beam energies).
      We propose that the collimator setup should start the setup with the collimators that have the largest normalized apertures. Then, one should proceed with decreasing normalized apertures (e.g. in IR7 setup first the TCLA's, then the TCS's and last the TCP's). This allows one to make sure that the relative retractions are respected (e.g. after setting up a TCLA, move the TCS until you see a spike on the BLM: this says when the TCS becomes closer to the beam than the TCLA. One can then calculate the retraction based on the local beam size).

    • Remark on angle adjustment: The measurements of the angle of the beam envelope at each collimator can require a long time. For the initial runs at low intensity one could rely on the optics model of the cleaning insertions based on the beam size measurements: (1) fit the beam based measurements with an optics model that reproduces the beam-based beta measurements at the collimators ; (2) calculate the angles [in unti sigma']; (3) setup the collimator angles at the desired normalized amplitude.

    • Sanity checks of settings: A method to measure the relative beta functions at three collimators is discussed in RA's talk at Chamonix 2006.

    • When the full system is in place, record reference global loss patterns with the collimation system in place with different beam parameters (lifetime, emittance, more bunches, ... ) (Detailed procedure to be defined).

  • Some open questions:
    - Put TCT's at coarse settings for MQX protection in view of higher intensities at 450GeV?
    - Reasonable time estimate for the collimator setup? (centring can be fairly fast but the determination of the beam sizes can take much longer).
    - Can we do IR7 and IR3 setups in parallel for two beams? Can we do the setup of one insertion in parallel for the two beams or the cross talk is too much?
    - Define procedures to verify the overall system settings (sanity checks) and the achieved performance.