Terzani, Davide (2018) Numerical and theoretical modelling of the laser-plasma interaction at conditions relevant to plasma-based acceleration schemes. [Tesi di dottorato]

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Laser Plasma Accelerators (LPA) have been a technological breakthrough for the creation of compact accelerating machines. Due to its capability to support accelerating fields many orders of magnitude bigger than the ones implied in the conventional RF accelerators, plasma allows in principle to reach ultra-high energies in a very reduced space. The intrinsically strongly nonlinear dynamics of a plasma and of the coupled electromagnetic fields still requires a lot of experimental and theoretical efforts to be managed, so, nowadays, the bunch quality obtained by a plasma accelerator is still too poor to allow a direct application. However, the recent production by the Lawrence Berkeley National Laboratory of a 4.2 GeV, 6 % r.m.s energy spread, 6 pC charge and 0.3 mrad r.m.s divergence beam in a 9 cm capillary waveguide is an absolutely astonishing result that leads to other steps forward in order to overcome the conventional technology. In this work, we present the theoretical and computational modelling of the nonlinear laser-plasma interaction in regimes relevant to the acceleration process. In particular, due to the strongly limited computational speed that can be reached by a standard fully kinetic Particle-In-Cell code, we addressed the problem of developing some reduced numerical model. Our goal is in fact to boost the simulations without losing the most important kinematic details. For this reason, we implemented in the ALaDyn code an explicit integration of the so called laser envelope model in which, assuming a broad laser pulse, only the relevant long scales are retained while the short ones are averaged out, allowing to strongly reduce the resolution needed to evolve the system. Also, we implied this numerical technique to validate a novel and very promising acceleration scheme, based on the decoupling of the wakefield generation and of the particle ionization process. Due to the complexity of the model, a fully 3D kinetic simulation is unfeasible with the currently available computational resources, so we performed a stage-by-stage comparison making use of the reduced model implemented in ALaDyn and of the hybrid, cylindrical, quasi-static code QFluid, showing an excellent agreement.

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