Noise effects, emittance control, and luminosity issues in laser wakefield accelerators

To reach the new high energy frontiers (higher than a TeV center of mass energy) new acceleration methods seem to be needed. Plasma based wakefield accelerator is one possible candidate which can provide an ultra high gradient acceleration and thus make the total acceleration distance reasonable. However, the final energy is not the only requirement. The accelerator should maintain an excellent beam quality to meet the luminosity requirements at the Inter action Point (IP). One of the most important figures of merit which describe the quality of the beam is its emittance. We study the particle dynamics in laser pulse-driven wakefields over multi-stages in a several TeV range center of mass energy e+e − collider. The approach is based on a map of phase space dynamics over a stage of wakefield acceleration induced by a laser pulse (or electron beam). The entire system of the collider is generated with a product of multiple maps of wakefields, drifts, and magnets, etc. This systems map may include offsets of various elements of the accelerator, representing noise and errors arising from the operation of such a complex device. We find that an unmitigated strong focusing of the wakefield coupled with the alignment errors of the position (or laser beam aiming) of each Wakefield stage and the unavoidable dispersion in individual particle betatron frequencies leads to a phase space mixing and causes a transverse emittance degradation. The rate of the emittance increase in the limit of constant energy is proportional to the number of stages, the energy of the particles, the betatron frequency, the square of the misalignment amplitude, and the square of the betatron phase shift over a single stage. The accelerator with a weakened focusing force in a channel can, therefore, largely suppress the emittance degradation. To improve the emittance we introduce several methods: a mitigated wakefield focusing by working with a plasma channel, an approximately synchronous acceleration in a superunit setup, the “horn” model based on exactly synchronous acceleration achieved through plasma density variation and lastly an algorithm based on minimization of the final beam emittance to actively control the stage displacement of such an accelerator.; We analyze the IP Physics luminosity and background issues in a high beamstrahlung parameter regime using the Yokoya's Monte Carlo code “CAIN”. The possibility for delivering polarized electron and positron beams at the collision point as an additional leverage to control the complicated background situation is also investigated. We prove that the initial beam polarization is not degraded significantly by the beam transport and acceleration in the plasma based Wakefield accelerator and by the beamstrahlung at IP.; Finally, we propose a beam driven acceleration scheme which can provide and ultra high acceleration gradient (greater than 100 GeV/m). This scheme is based on a reasonable modification of current SLAC beam parameters.