High Energy Particle Acceleration by a Laser Beatwave

Abstract

Parallel intense laser beams, ω_o/ k_o and ω₁, k₁ shone on an underdense plasma with frequency separation equal to the plasma frequency, ω_p, are capable of creating coherent large electrostatic fields and accelerating particles to high energies in large flux. The photon beatwave excites, through forward Raman scattering, a large amplitude plasma wave whose velocity is (ω_o - ω₁)/(k_o — k₁), close to c. The plasma wave attains electrostatic field strengths of E_z ≃ mcω_p/e. This intense logitudinal field may be used to accelerate either preaccelerated charged particles, or plasma electrons.

Extensive computer simulations have been carried out to pinpoint the physics. At present, results are limited to one-dimensional relativistic and electromagnetic models. These code runs address questions of laser intensity threshold for particle trapping, temperature and density gradient effects, Raman Forward Scattering (RFS) and Raman Backward Scattering (RBS), the optical mixing mechanisms, wave synchronism and streaming instabilities. The simulations show the Beatwave Accelerator to be a viable method for obtaining the predicted electrostatic accelerating fields. Wave synchronism can be maintained and streaming instabilities suppressed by the proper choice of laser beatwave intensity. Optical mixing can be used to lower the RFS instability threshold by two orders of magnitude. If two- and three-dimensional instabilities can be addressed successfully, acceleration of plasma electrons or preaccelerated charged particles to TeV energies in short distances appears feasible.