Vacuum beat wave acceleration

Abstract

A vacuum beat wave accelerator (VBWA), in which two focused laser beams of differing wavelengths generate a beat wave that can impart a net acceleration to particles, is analyzed and simulated. The mechanism relies on the ponderomotive (v×B) force, thus circumventing the so-called Lawson-Woodward theorem. No gas, plasma, or other proximate material medium is required to achieve a net energy gain. The single-stage energy gain of the VBWA is limited by diffraction of the laser beams, particle slippage, and radial walkoff. In the simulations the particles are synchronous with the beat wave for a short interval of time and the energy gain has the nature of an impulse delivered near the focal region. Simulations show that the problem of radial walkoff may be ameliorated by using a converging beam of particles, as naturally occurs for injection of a finite-emittance beam. For terawatt-level laser beams, with wavelengths 1 μm and 0.5 μm, and a 4.5 MeV finite-emittance electron beam, the energy can be increased to ∼12.5 MeV in a nonsynchronous interaction over a distance of under 4 mm, with a peak acceleration gradient ∼15 GeV/m and an estimated trapping fraction of ∼1%. The simulated energy gain is compared with analytical predictions. Scaling is illustrated by increasing the injection energy to 50 MeV.