Wakefield generation and GeV acceleration in tapered plasma channels - art. no. 056405

To achieve multi-GeV electron energies in the laser wakefield accelerator ( LWFA), it is necessary to propagate an intense laser pulse long distances i n a plasma without-disruption. One of the purposes of this paper is to eval uate the stability properties of intense laser pulses;propagating extended distances (many tens of Rayleigh ranges) in plasma channels. A three-dimens ional envelope equation for the laser field is derived that includes nonpar axial effects such as group velocity dispersion, as well as wakefield and r elativistic nonlinearities. It is shown that in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modu lation instabilities cancel. This cancellation can result in pulse propagat ion over extended distances, limited only by dispersion. Since relativistic focusing is not effective for short pulses, the plasma channel provides th e guiding necessary fbr long distance propagation Long pulses (greater than several plasma wavelengths), on the other hand, experience substantial mod ification due to Raman and modulation instabilities. For both short and lon g pulses the seed for instability growth is inherently determined by the pu lse shape and not by background noise. These results would indicate that th e self-modulated LWFA is not the optimal configuration for achieving high e nergies. The standard LWFA, although having smaller accelerating fields, ca n provide acceleration for longer distances. It is shown that by increasing the plasma density as a function of distance, the phase velocity of the ac celerating field behind the laser pulse can be made equal to the speed of l ight. Thus electron dephasing in the accelerating wakefield can be avoided and energy gain increased by spatially tapering the plasma channel. Dependi ng on the tapering gradient, this luminous wakefield phase velocity is obta ined several plasma wavelengths behind the laser pulse. Simulations of lase r pulses propagating in a tapered plasma channel are presented. Experimenta l techniques for generating a tapered density in a capillary discharge are described and an example of a GeV channel guided standard LWFA is presented .