Collective instabilities and electron clouds in circular colliders

6 problems

• What sets the beam-beam tune-shift limit in electron-positron colliders

  Open

  Colliding beams focus each other; above a certain strength they blow up, and that ceiling is known from experience rather than derived.

• Release mechanism of dust grains causing fast LHC beam losses

  Partially resolved

  Micron-sized dust grains fall from the LHC vacuum chamber walls into the beam and cause sudden losses that can quench magnets; what makes them detach is unclear.

• Instabilities that appear minutes after a beam is predicted stable

  Partially resolved

  In the LHC, beams calculated to be stabilized by octupole magnets sometimes become unstable after minutes, suggesting slow erosion of the stabilizing frequency spread.

• Why identical LHC arcs show very different electron-cloud heat loads

  Partially resolved

  Stray electrons multiplied by the beam heat the cold beam pipes of the LHC, and some sections heat about twice as much as nominally identical ones.

• Mechanism of sudden beam losses at SuperKEKB

  Partially resolved

  The SuperKEKB collider in Japan repeatedly loses much of a stored beam within a few turns, together with pressure bursts in the vacuum pipe.

• Effect of space charge on the transverse mode-coupling instability threshold

  Contested

  The beam's own repulsion shifts its oscillation frequencies, and theories disagree on whether this raises or lowers the intensity at which bunches start to shake apart.