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Beam profile evolution induced by rf crab cavity amplitude noise in the CERN SPS

A. Fornara* and R. B. Appleby

H. Bartosik, X. Buffat, R. Calaga, S. Kostoglou, G. Sterbini, G. Trad, and N. Triantafyllou

  • CERN, Meyrin, Switzerland

  • *Also at CERN, Meyrin, Switzerland.
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Phys. Rev. Accel. Beams 29, 061002 – Published 8 June, 2026

DOI: https://doi.org/10.1103/5wfr-3xhy

Abstract

In the context of the large hadron collider (LHC) luminosity upgrade, rf crab cavities (CCs) are key components for achieving the target integrated luminosity. Due to the increased bunch intensity required to achieve this goal, the collider will operate with a large crossing angle scheme to alleviate the long-range beam-beam effects. The CCs will counteract the geometric luminosity reduction factor resulting from the large crossing angle. Amplitude and phase noise injected from the CC low-level rf control are known to induce transverse bunch emittance growth. This study presents measurements performed in the CERN Super Proton Synchrotron (SPS), where two CCs were installed in 2023 for testing purposes. Only one cavity was used for this experiment. A particular focus is given to emittance growth studies driven by amplitude noise: the measured emittance growth was found to be dependent on the amplitude detuning induced by the SPS octupoles, although no dependence was predicted by the available theories and models. A new lattice model is presented that is able to reproduce both the previous measurements with phase noise and these new ones. The second part of the study is dedicated to the beam distribution evolution: the Beam Synchrotron Radiation Telescope (BSRT) measurements were able to capture the evolution of the tails of the bunch over time. Under the effect of amplitude noise, the tail contribution is reduced; in the absence of noise, it remains constant. To understand this effect, an analytical and numerical framework is used to quantify the mechanism of diffusion of the beam profile. The results of a full simulation on the SPS lattice with heavy tailed distribution evolution are then presented and discussed.

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Phys. Rev. Accel. Beams 29, 061002– Published 8 June, 2026
  • Received 6 December 2025
  • Accepted 14 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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