Acceleration of particles by an asymmetric Hermite-Gaussian laser beam

The application of a focused laser beam to accelerate particles using the longitudinal electric-field component is investigated one step beyond the paraxial-ray approximation. Vacuum acceleration to high energies along the axis of an asymmetric Hermite-Gaussian beam is in principle possible, but the interaction distance is short (one Rayleigh length on each side of focus). The use of a gas can increase the energy gain per focal passage by a factor of 2.4, while permitting arbitrary spacing of drift tubes and lenses of a lens waveguide. Drift tubes are therefore not needed. A beam loaded with a graded-index gas, in which phase and particle velocities are equal over the interaction trajectory, permits three-dimensionally stable particle trajectories. This property is explained by the anisotropy of the medium in a comoving reference frame. The functions of acceleration, focusing, and bunching of particles can thus be performed simultaneously by a single optical beam that is guided in a lens waveguide.