Channeling of intense optical fields in plasmas is a rapidly developing scientific area, with a number of possible applications including x-ray generation, harmonic conversion and electron acceleration. In the context of laser plasma accelerators, intense, ultrashort pulses of laser light are injected into a plasma and create a wake field that can be used to accelerate particles. The major challenge is to produce a wake which is strong enough and coherent over a sufficiently large distance to accelerate particles to high energy.

Many nonlinear physical processes can be expected to affect the propagation of these intense pulses, and their study requires a synthesis of nonlinear optics and basic plasma physics. In particular, intense laser pulses are subject to self-channeling and Raman instabilities. Self-channeling occurs because the oscillating motion of plasma electrons in the presence of the laser field is relativistic for intense pulses. The increase in the effective mass of the electrons which accompanies their relativistic oscillations results in a change in the index of refraction of the plasma and consequently leads to self-guiding of the laser light. In addition, the laser light can also act to expel electrons from regions of high intensity further enhancing the channeling process. Raman instabilities occur when an intense pulse decays by creating a plasma wave and a lower frequency light wave. This usually leads to a breakup of the laser pulse. The interplay of these effects determines the distance over which laser pulses can propagate through plasmas. The parameters correspond to those of a proposed experiment to be conducted in Prof. Milchberg's laboratory. Displayed are two dimensional surface plots of the laser intensity after propagation through channels with four different parameters. Visible on the trailing edge of the laser pulses are modulations corresponding to excitation of Raman instabilities. Clearly, the parameters corresponding to channel a provide for the most stable propagation.

Results of a numerical simulation of laser light propagation through a preformed plasma channel.

This research is supported by the National Science Foundation.