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

The Procedure in Short

We use K-modulation to determine the \(\beta\)-functions at the interaction points and, with better accuracy than classical measurements, at the BPMs closest to the IPs. The quadrupole circuits closest to the IP are modulated and the consequent tune changes are measured. See this page for details on the K-modulation method.

Please keep in mind the general checks for measurements, and see the K-Mod guide for how to perform the tasks listed below.

Procedure¶

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. Pay attention to potential resonance crossings.
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.
Start the K-modulation GUI

See the K-Mod pages.
Ensure the orbit feedback is ON

In case of any (design) orbit excursion in the quadrupoles, enable orbit feedback to avoid a change of the CO around the ring. The status is visible in the top right corner of the GUI.
Ensure the tune feedback is OFF

Otherwise modulation and feedback would work against each other. The status is visible in the top right corner of the GUI.
Run K-Modulation

See the following page for how to perform a modulation of the desired circuit. Check from the modulation graph for the quality of the modulation and data.
Start the Analysis

Analysis is now launched directly from the Pykmod app. See this page for details on how to analyze and export the data.
Import Results Into the Beta-Beat GUI

The results are useful to compute optics corrections. See the Beta-Beat GUI page for how to import the results.