Instability and Beam-Beam Study for Multi-TeV PWFA e+e− and γγ Linear Colliders

Beam-driven plasma wakefield acceleration (PWFA) is considered as one of the most promising novel acceleration technologies capable of drastically shrinking the footprints of high-energy particle colliders. There are however several challenges that have to be addressed before PWFA can be deployed for high-energy, high-efficiency, high beam brightness particle colliders. Transverse instabilities due to interactions between particle beams and accelerating structures are known to restrict the maximum beam charge in conventional accelerators, and are considered as one of the most important challenges for PWFA accelerators. This thesis focuses on transverse instabilities induced by transverse wakefields and the corresponding implications for multi-TeV PWFA e+e− and γγ colliders. First, a simplified model describing the PWFA transverse instability in the form of a wake function parameterised only with an effective cavity aperture radius a is benchmarked against PIC (Particle In Cell) simulations. This wake function implies a 1/a4-scaling of the transverse wakefields, which indicates transverse intra-beam wakefields typically several orders of magnitude larger than in conventional acceleration structures. Furthermore, the wakefield formalism is utilised to perform a parameter study for a 1.5 TeV plasma wakefield accelerator, where the constraint on drive beam to main beam efficiency imposed by transverse wakefields is taken into account. Ultimately, an electron accelerator parameter set with promising properties in terms of energy spread, stability and luminosity per power is derived, and is the basis for subsequent PWFA e+e− and γγ linear collider parameter studies. The thesis then presents a beam-beam parameter study for a TeV-scale PWFA e+e− linear collider using GUINEA-PIG (Generator of Unwanted Interactions for Numerical Experiment Analysis - Program Interfaced to GEANT) simulations based on the derived accelerator parameters. The beam-beam parameter study shows that the total luminosity follows the 1/√σz-scaling predicted by beamstrahlung theory, where σz is the rms beam length, which is advantageous for PWFA, as short beam lengths are preferred. We also derive a parameter set for a 3 TeV PWFA linear collider with main beam parameters optimised for luminosity and luminosity spread introduced by beamstrahlung. Due to the considerable challenges associated with positron acceleration in plasma, this thesis also compares the performance for scenarios with reduced positron beam charge at 3 TeV and 14 TeV with CLIC (Compact Linear Collider) parameters. We also perform a similar beam-beam parameter study for a 3 TeV γγ collider again using the derived PWFA main beam parameters. The parameter study involved using electron beams with short beam lengths in the range 2 µm ≤ σz ≤ 10 µm to scatter the laser photons. The results for such short examined electron beam lengths indicate that at 3 TeV, the total luminosity, as well as the sharpness of the luminosity spectrum for a γγ collider are independent of the beam length of the electron beams, given an adequate final focus system and that the hourglass effect is avoided. The total luminosity can consequently be maximised by minimising the horizontal and vertical beta functions β∗x,y at the interaction point. Furthermore, we perform background studies for both collider types using GUINEA-PIG. Simulation results indicate that our proposed parameter set for a 3 TeV PWFA γγ collider is able to deliver a total luminosity significantly higher than a γγ collider based on CLIC parameters, but gives rise to more background particles. The examined γγ collider parameter sets are able to deliver significantly larger total luminosities than the e+e− collider parameter sets considered in this thesis, but result in comparatively larger luminosity spreads. The examined parameter sets for 3 TeV and 14 TeV e+e− colliders offer interesting improvements over the CLIC parameter set in terms of luminosity and beam power, but also suffer from strong backgrounds.