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FCC-ee Lessons from SuperKEKB

Frank Zimmermann

TL;DR

The paper analyzes SuperKEKB performance to extract lessons for FCC-ee, showing that SuperKEKB surpassed KEKB in specific luminosity and validated three FCC-ee design elements: ultralow $β_y^{*}$, the virtual crab waist, and currents above 1.3 A. It also identifies challenges unique to SuperKEKB—such as sudden beam losses, large vertical emittance blow-up, and an intricate interaction-region with alignment and magnet-coil errors—that limit its luminosity but are not fundamental obstacles for FCC-ee. Through detailed comparisons and simulations, the authors argue that FCC-ee luminosity design is robust against these SuperKEKB-specific issues, thanks to a simpler IR, higher tolerances, and engineering choices (e.g., a straight booster, fixed BPMs, and no MO-type flanges). They propose a concrete path to replicated or improved performance for FCC-ee, including enhanced diagnostics, improved coupling control, mitigated space-charge in the LER, and full nonlinear-lattice-based luminosity studies to validate the target $L=10^{35} ext{ cm}^{-2} ext{s}^{-1}$. Overall, the work reinforces confidence that FCC-ee can achieve its design luminosity while highlighting practical, SuperKEKB-specific factors to address in the accelerator design and operation plan.

Abstract

SuperKEKB has achieved significantly higher specific luminosity than its predecessor KEKB, and it has proven a much more sustainable machine. It has successfully demonstrated several key design elements of FCC-ee. The design luminosity has not yet been reached, however. This observation is often (mistakenly) used to put into question the reliability of the FCC-ee design luminosity. In this note we review the accomplishments, challenges and obstacles of SuperKEKB, and compare these with the FCC-ee design.

FCC-ee Lessons from SuperKEKB

TL;DR

The paper analyzes SuperKEKB performance to extract lessons for FCC-ee, showing that SuperKEKB surpassed KEKB in specific luminosity and validated three FCC-ee design elements: ultralow , the virtual crab waist, and currents above 1.3 A. It also identifies challenges unique to SuperKEKB—such as sudden beam losses, large vertical emittance blow-up, and an intricate interaction-region with alignment and magnet-coil errors—that limit its luminosity but are not fundamental obstacles for FCC-ee. Through detailed comparisons and simulations, the authors argue that FCC-ee luminosity design is robust against these SuperKEKB-specific issues, thanks to a simpler IR, higher tolerances, and engineering choices (e.g., a straight booster, fixed BPMs, and no MO-type flanges). They propose a concrete path to replicated or improved performance for FCC-ee, including enhanced diagnostics, improved coupling control, mitigated space-charge in the LER, and full nonlinear-lattice-based luminosity studies to validate the target . Overall, the work reinforces confidence that FCC-ee can achieve its design luminosity while highlighting practical, SuperKEKB-specific factors to address in the accelerator design and operation plan.

Abstract

SuperKEKB has achieved significantly higher specific luminosity than its predecessor KEKB, and it has proven a much more sustainable machine. It has successfully demonstrated several key design elements of FCC-ee. The design luminosity has not yet been reached, however. This observation is often (mistakenly) used to put into question the reliability of the FCC-ee design luminosity. In this note we review the accomplishments, challenges and obstacles of SuperKEKB, and compare these with the FCC-ee design.

Paper Structure

This paper contains 15 sections, 25 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Achievements and Goals
  3. Sudden Beam Loss
  4. Stored Beam Emittance
  5. Injected Beam Emittance
  6. Injection Efficiency and Beam Current
  7. Interaction Region
  8. Vertical Beta Function at the Collision Point
  9. Specific Luminosity
  10. Summary and Conclusions
  11. Other Issues
  12. Insufficient beam diagnostics
  13. Tunnel floor deformation
  14. Limited IR aperture
  15. Incomplete beam dynamics

Figures (25)

  • Figure 1: Run history of SuperKEKB: beam currents in HER (e$^-$) and LER (e$^+$), along with instantaneous and integrated luminosity as a function of time from 2019 through 2024, as reported by Y. Ohnishi ohnishi25.
  • Figure 2: An example sudden beam loss event, presented by H. Ikeda ikeda25. The revolution period is 10 $\mu$s. The intensity of two long bunch trains can be seen, turn by turn, till the moment of beam abort.
  • Figure 3: A SuperKEKB collimator jaw damaged by an SBL event, from H. Ikeda ikeda25.
  • Figure 4: Vertical blow during a sudden beam loss event (left), from H. Ikeda ikeda25, and black stain next to an "MO"-type flange (right), shown by K. Shibata shibata25.
  • Figure 5: Vertical emittance in the HER as a function of the vertical betatron tune $Q_y$ in November 2024 after two successive optics corrections (red and orange curves) compared with 2022 (blue curve), as reported by Y. Ohnishi ohnishi25. The right picture shows the trace of this tune scan in the $Q_y$-versus-$Q_x$ tune diagram (red) along with low-order resonances (black lines).
  • ...and 20 more figures