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Measuring \(\beta^{*}\) Using K-Modulation in the LHC

Note

Please keep in mind the general checks for measurements.

The Procedure in Short 
The important stuff.
This decides over luminosity and lives in the experiments.

Measurement

  • Adjust Working Point

    The tunes should be moved to a working point with a large tune separation, such as \(Q_x = 0.28 / Q_y = 0.31\), to allow for maximum modulation amplitude.

  • Check Coupling

    Perform quick check for \(|C^{-}|\) to avoid influence from a possible closest tune approach. Also check for any unwanted local coupling bumps around the modulated quadrupole.

  • Check feedbacks

    • Turn on orbit feedback

      In case of any (design) orbit excursion in the quadrupoles, enable orbit feedback to avoid a change of the CO around the ring. Caveat: for the determination of the crossing angles, orbit feedback should be off.

    • Turn off tune feedback

      Otherwise modulation and feedback would work against each other.

  • Run K-Modulation

    Fire up the K-Mod application. There two options are available:

    • IP Modulation : Runs a modulation on both quadrupoles closest to the selected IP.

    • Single circuit modulation : Runs a modulation on a selected quadrupole circuit (used for measuring the beta-functions in IR4, where BSRT is located).

  • Chose Modulation Amplitude

    Choose a modulation current such that the change in tune is roughly 0.01. This can either be done by looking up old shifts with similar optics or by increasing the amplitude until satisfactory tune change is observed. Modulation frequency is chosen by the system, with higher modulation amplitude resulting in lower modulation frequency.

  • Document Measurement

    As no automatic logging of the modulation is implemented for now, parameters should be logged in the logbook. Parameters to log are: Starttime, Endtime, Modulation current, IP, other comments such as \(\beta^{*}\), status of the OFB, is significant tunejitter/-jump observed.

Analysis

  • Extract Data from Timber

    After the analysis, a window should open to allow for extraction of the data from Timber. Alternatively, Extract previous trim can be used. Saving in a separate directory with a descriptive name is recommended (e.g. Kmod_IPX_beta_beforeCorrection_starttime) and should be added to the modulation logbook entry.

  • Start the Analysis

    Run the python codes on the extracted Timber data to get the \(\beta\) you need. As of now, only the Kmod analysis from Beta-Beat.src can be called from the K-Modulation GUI for the case of an analysis of an IP-Modulation. Codes and some documentation may be found for Python2 and for Python3.

  • Check Results

    The results of the analysis should be located in the previously specified working directory and can be checked by eye using a text editor of choice.

    • Use in Beta-Beat GUI for Correction

      Using this script, the results can be brought in a form which is readable for the BBGUI and can then be used to calculate a correction.

    • Publish Results

      If results are satisfactory, both Python2 and Python3 should create a file called lsa_results.tfs, which can be uploaded using the LSA optics uploader for other users to access data.