Laser wakefield accelerators (LWFA) provide a cheap, compact and accessible form of electron acceleration. These systems use high intensity laser pulses to drive a large amplitude plasma density wake. This wake is then used to trap and accelerate electrons to high energies. This particle acceleration technique still requires considerable research in order to produce the high quality particle bunch associated with conventional RF accelerators. Fine control of plasma parameters will allow some of the limitations of this method to be overcome or suppressed. Theoretical considerations show that there are several limiting factors including laser diffraction in the plasma, electron dephasing and pump depletion. Here, each of the limitations is considered and a method of reducing their effects proposed. The consequences of pulse length variation in producing ideally formed wakefield structures is first investigated for three different pulse lengths. Each of these pulse lengths was simulated for Gaussian and square pulse envelope profiles. A method to inject electrons into the accelerating region of the plasma density wave using varying concentrations of Nitrogen neutrals is introduced and finally a focussing beam method is developed and used to demonstrate the detrimental effects of a rapidly focussing pulse. Each of the systems were simulated using a particle in cell code(EPOCH). There existed a resonance condition at which a stable bubble was formed, L = p and the Gaussian profile formed the most complete blow out regime. The introduction of Nitrogen neutrals greatly increased the number of electrons trapped and accelerated. The highest energy achieved over a distance of 1mm was 265MeV in nN = 5 ⇥ 1016cm3. The rapidly focussing pulse demonstrated that trapping and blow out would form more quickly for the focussing pulse but after the focal length the electron dephasing and diffraction effects were more severe than for the collimated beam. The Nitrogen injection technique was found to work very well but does require further investigation in order to find the optimal injection and acceleration. This combined with the pulse length resonance condition would provide the ideal basis for a high energy accelerator. Using a slowly focussing drive pulse through the injection set up would produce a coherent high energy bunch of electrons after 1mm with an energy in the range of 100200MeV. This bunch could then be used to inject into a second high energy stage to accelerate the electrons to GeV energies.
|Date of Award||2017|
- Queen's University Belfast
|Supervisor||Gianluca Sarri (Supervisor)|