The Influence of Focusing Geometry on the Direct Laser Acceleration of Electrons

Direct Laser Acceleration (DLA) of electrons is a mechanism for superponderomotive energy gain during relativistically intense laser-plasma interactions. The presence of the plasma produces a channel with transverse electric and azimuthal magnetic fields that enable energy exchange from the laser to the electrons. We investigate DLA using experiments performed at the OMEGA EP laser facility and particle-in-cell simulations. The experimental variables include the plasma density and profile, the laser pulse focusing, energy and pulse duration. Matching the focal spot to the size of the oscillatons the electrons perform within the channel is most favorable for achieving the highest energies. Therefore an optimum focal spot size is found to be a function of density and laser pulse power. Applications of DLA are for bright directional sources of x-rays, or to create secondary interactions creating Bremstrahlung-photons or positrons.