What are the beam charge compensation and the crab-waist collision scheme?

Beam charge compensation is an old idea to fight the beam-beam interaction by trying to neutralize each of the beam at the collision point. An extra e+ beam is merged at the collision point with the e- beam (moving the same direction), and extra e- beam is merged with the e+ beam. The idea is that a particle in one of the beams sees a net '0' electromagnetic field from the counter-rotating beam.

This idea was tried in an acceleration a long time ago, but the performance fell short by a factor 100 or so.

There are complicated issues associated with this scheme:

  • 2 additional beams have to be produced (an issue for the e+ beam in particular),
  • 4 beams have to be merged at the IP - integration.

Finally no numerical simulations have ever demonstrated that this concept really works.

A similar idea is used for high intensity proton machine where an electron beam is brought parallel to the proton beam is a section of the machine to compensate the beam-beam effects of the collisions with the other proton beam (see RHIC) - called the electron lens. The electron beam is at very low energy (keV) and can be easily coupled in and out of the main ring thanks to the large energy difference (keV versus GeV-TeV). This has been proven to work and will be used at RHIC next year (2015).

Crab-waist is a special optics scheme using sextupoles places at an adhoc distance to the IP. The waist of off-energy (head-tail of the bunches) particles at the IP is shifted longitudinally to counteract the modulation of the beam-beam effect with the longitudinal position. This is very important when the bunch is signifcantly longer than ß* (hourglass effect significant). In simulation it is clearly observed that so-called synchro-betatron resonances (due to coupling between the transverse and longitudinal plane) driven by the beam-beam effect in such conditions are strongly suppressed. This leads to the potential to achive higher beam-beam parameters and therefore also high luminosity.