BEAMS In the literature: open

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 $8e35\ \mathrm{cm}^{-2}\ \mathrm{s}^{-1}$ (later target about $6.5e35$).

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

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.