I Plasma Suppression of Beam - Beam in Circular ColUders

A possibility to suppress beam-beam interaction in a circular collider by means of introducing a plasma at the interaction point of the colliding beams is considered. It is shown that for TeV proton and muon colliders, the overdense plasma can easily suppress the beam-beam tune-shift parameter several times without degrading the beam lifetimes. In this paper we study a possibility to overcome a major obstacle in future colliders – limitation of the luminosity caused by the beam-beam interaction [1]. Electromagnetic interaction of the intense colliding beams can result in strong perturbation of particle motion which, in extreme situation, makes the motion of the beam particles unstable. The conventional measure of the beam-beam interaction is given by the so called beam-beam tune shift parameter ~ [2], which, for round beams, is equal to ~ = NrCl,,,/(2~~), where N is the number of particles in the bunch, Y is the relativistic factor, ~1,,, refers to the classical radius of the particles comprising the beam (rCl,,, = 1.6x 10-16 for protons, and ~lu,= 1.4x 10-15 for muons), and E. is the normalized emittance of the beam. In the r design and operation of modern colliders, the parameter~ is usually set below 0.05 for electron machines and less then 0.005 for hadron colliders in order to avoid the diminishing of the dynamic aperture. Relatively small values of ~ result in the limitation of the luminosity of the collider because, for a given number of particles in the bunch and its dimensions at the interaction point, the luminosity is proportional to ~. The general tendency in high energy physics looks for dramatic increase in collider luminosity, which inevitably push ~ to higher values. Recently, it was found [3] that in the muon collider currently under study [4], the beam emittance incremes substantially even after a single beam crossing. It is therefore highly desirable to find a means to ameliorate the long standing problem of beam-beam interaction. In order to suppress the effect of beam-beam collisions on particle dynamics, we propose to intercept the colliding beams at the interaction region with a plasma. If the plasma density is larger than the particle density of the colliding bunches, the electric field of the beams will be suppressed by repelling (in case of negatively charged bunches) or attracting (in case of bunches of positive charges) plasma electrons. However, the suppression of electric field only is not sufficient; it …