Analysis of Gaussian beam and Bessel beam driven laser accelerators.

Abstract

This paper presents a comparison of Gaussian and Bessel beam driven laser accelerators. The emphasis is on the vacuum beat wave accelerator (VBWA), employing two laser beams of differing wavelengths to impart a net acceleration to particles. Generation of Bessel beams by means of circular slits, holographic optical elements, and axicons is outlined and the image space fields are determined by making use of Huygens' principle. Bessel beams-like Gaussian beams-experience a Guoy phase shift in the vicinity of a focal region, resulting in a phase velocity that exceeds c, the speed of light in vacuo. In the VBWA, by appropriate choice of parameters, the Guoy phases of the laser beams cancel out and the beat wave phase velocity equals c. The particle energy gain and beam quality are determined by making use of an analytical model as well as simulations. The analytical model--including the v x B interaction--predicts that for equal laser powers Gaussian and Bessel beams lead to identical energy gains. However, three-dimensional, finite-emittance simulations, allowing for detuning, transverse displacements, and including all the electromagnetic field components, show that the energy gain of a Gaussian beam driven VBWA exceeds that of a Bessel beam driven VBWA by a factor of 2-3. The particle beam emerging from the interaction is azimuthally symmetric and collimated, with a relatively small angular divergence. A table summarizing the ratios of final energies, acceleration lengths, and gradients for a number of acceleration mechanisms is given.