{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"5c4e8e4bb9e273ee707349ec8fcfc368277c8be7c929e93d9c6cfa2393c1f8f5","created":"2026-10-03T07:17:56Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011","c7f3731a615018b75f1e6a39418abb3757e0b3f549578cfb9b41d7d935a3c429"],"salt":"541cb15c969cdbc24da3e6a8d589fd3b2850a360a804fd47cce766293ad01089","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"beams.beam-beam-limit.crab-waist-instability","kind":"phenomenon","literature_status":"open","n":"1","parents":[{"note":"Crab-waist X-Z instability is the large Piwinski angle part of beam-beam limit","parent_revision":"c7f3731a615018b75f1e6a39418abb3757e0b3f549578cfb9b41d7d935a3c429","relation":"special_case"}],"plain":"New colliders cross very flat beams at a large angle with a special focusing scheme, the crab waist, to reach high collision rates. Unexpected instabilities, some involving the beam's interaction with its surroundings, have kept performance below design.","posed_since":"","precise":"In crab-waist schemes with Piwinski angle $\\phi >> 1$, strong-strong simulations predict coherent X-Z instabilities and interplay with longitudinal and transverse impedance. Determine the stability boundary in $(\\xi_x,\\xi_y,\\nu_s,\\mathrm{impedance})$ and whether it explains the luminosity shortfall of SuperKEKB relative to its design value of $8e35\\ \\mathrm{cm}^{-2}\\ \\mathrm{s}^{-1}$ (later target about $6.5e35$).","problem_ref":null,"references":"","settled_by":"Strong-strong simulations including impedance that reproduce measured SuperKEKB blow-up and luminosity, and machine experiments that confirm the predicted cures.","status_note":"SuperKEKB set a world-record peak of about $5.1e34\\,\\mathrm{cm}^{-2}\\,\\mathrm{s}^{-1}$ in December 2024, roughly 6 to 8 percent of its design and later target values; vertical blow-up, a vertical instability linked to collimator impedance and sudden beam losses are the identified limits.","title":"Beam-beam instabilities in large Piwinski angle crab-waist 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