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John Salvesen (CERN) presented on advancing the SuperKEKB lattice in Xsuite, a relatively new framework developed at CERN for the FCC. Due to the clear synergy between FCC-ee and SuperKEKB, benchmarking the framework against an operational collider is essential. In collaboration with SuperKEKB/Belle II colleagues, CERN’s team is focusing on IP optics studies, beam collimation, beam-beam processes, impedance studies, and more. Despite the complexity of the SuperKEKB lattice, they successfully converted the SAD lattice into Xsuite, using it for the beam loss simulations reported by the next speaker. Current status and findings from the comparison of SuperKEKB lattice implementation in Xsuite and SAD included:
The developers are working to refine the framework to resolve remaining discrepancies, unify magnetic field implementation, and simplify lattice conversion. They welcome further contributions (particularly having greater clarity on the multipolar distribution of the sliced IR in SAD), especially from the SuperKEKB/Belle II team.
Giacomo Broggi (CERN) discussed recent progress in beam loss simulations with Xsuite. One of Xsuite’s features is its ability to interface with external tools, such as modeling particle interactions for collimators. By using a dedicated tool called collimasim, it is possible to call Geant4 libraries (via BDSim) during multi-turn particle tracking. The team is focused on improving the interface and reducing CPU time. The Xsuite-BDSim coupling has been benchmarked against existing tools and hadron machine measurements. However, further development and benchmarking are required for lepton machines like SuperKEKB and DAΦNE. Promising results have already emerged from SuperKEKB beam loss simulations. The team is working on refining the beam loss process physics implementation, improving collimator descriptions, and comparing Data/MC for Belle II data, with findings to be shared in future publications and conferences. They are keen to continue collaborating with the SuperKEKB/Belle II team to deepen their understanding of beam losses at SuperKEKB.
Andrii Natochii (BNL), on behalf of Qingyuan Liu (University of Hawaii), reported on collimator simulation studies at the LER. Based on previous run operations, large beam loss rates in the IR were partially traced to scattered particles at the D02V1 collimator, the nearest vertical collimator to the IR. Damage to its head, especially from sudden beam losses (SBLs), could negatively impact the detector background and machine stability. Qingyuan studied possible collimation system modifications and their impact on storage backgrounds and SBLs. One proposal, supported by Run Coordinators and the vacuum group, is to relocate the D06V2 collimator (~1 km upstream from the IP, largely unused in previous runs but effective in reducing injection beam losses in 2024c) to D03V4 (~400 m upstream from the IP). Simulations suggest that D03V4 could:
The high-Z material (e.g., Ta) for its head is critical to preventing multi-turn beam losses from tip-scattered particles.
Simulations mimicking an SBL event showed that D03V4 can "shadow" D02V1, reducing IR losses. Before relocation, it was suggested to study a hybrid collimator head (Ta + graphite) and check the transverse mode coupling instability (TMCI) threshold due to the collimator's narrow aperture.
Aurélien Martens (IJCLab) reviewed Touschek lifetime simulations in SAD, suggesting possible improvements. Currently, scattering centers are distributed among conventional quadrupoles in each ring, but other locations may contribute to losses. Aurélien estimated that the final focusing superconducting magnets, where betatron functions are large, might contribute to extra Touschek losses. He proposed extending the list of scattering centers to include other elements (e.g., QCS) and switching from Bruck's formula (non-relativistic horizontal momentum and flat beams) to the more generalized Piwinski's formula. The estimated average lifetime in the HER could increase by ~15%, though further development is needed for accuracy. Additionally, Aurélien discussed photon scattering off thermal photons in the vacuum beam pipe. Assuming a 330K beam pipe temperature, the estimated lifetime from this process in the HER is about 150 hours. Although preliminary, this process could contribute to overall beam lifetime and losses, especially in high-temperature regions due to synchrotron radiation. Further studies are planned, with small projects could be assigned as service tasks for students and postdocs.
Shijie Wang (University of Tokyo, IPNS) provided an update on the SVD beam-induced background analysis from the November 2024 single-beam studies. The absolute occupancy of layer-3 (innermost) detectors matched results from the April 2024 study. Despite some gas pressure differences between the study days, the overall background occupancy was the same for April and November. This finding is consistent with previous preliminary analysis based on archived PVs, where TOP and CDC BG rates were about twice as high in November compared to April, while SVD occupancy remained unchanged at the same beam currents. See the April summary at https://indico.belle2.org/event/12059/ and the November summary at https://indico.belle2.org/event/13674/.
Our next group meeting is scheduled for February 19, one week before the B2GM, to summarize 2024c operations and discuss plans for the 2025ab shutdown period.