Beam-beam instabilities in large Piwinski angle crab-waist collisions
In plain words
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.
Precise statement
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 $8 \times 10^{35}\ \mathrm{cm}^{-2}\ \mathrm{s}^{-1}$ (later target about $6.5 \times 10^{35}$).
What would settle it
Strong-strong simulations including impedance that reproduce measured SuperKEKB blow-up and luminosity, and machine experiments that confirm the predicted cures.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
SuperKEKB set a world-record peak of about $5.1 \times 10^{34}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ 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.
Parent problems
- Special case of What sets the beam-beam tune-shift limit in electron-positron colliders? · open · from revision 1 Crab-waist X-Z instability is the large Piwinski angle part of beam-beam limit The parent was revised since revision 1: it now states another question.
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