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Graduate Student Seminar - 05/04/2012

"Studies of Spectral Modification and Limitations of the Modified Paraxial Equations in Laser Wakefield Simulations"

by Wenxi Zhu

Friday, May 4, 2012 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisor:  Professor Thomas Antonsen

Laser pulses propagating through plasma undergo spectral broadening through local energy exchange with driven plasma waves. For propagation distances on the order of the energy depletion length, spectral shifts can be comparable to the laser central frequency and wavenumber, a result of approximate action conservation. Here we examine the local spectral shift, energy depletion, and action conservation of nonlinear laser pulses using the modified paraxial simulation code WAKE. Breakdown of the modified paraxial equation (MPE), which is based on the assumption of slow temporal variation of the pulse envelope, is monitored via consideration of the wave action. Although action is theoretically conserved for the continuous MPE, we observe that for large red shifts, action decays for the slowing down of the group velocity and the discrete implementation. Numerical analysis of the propagation algorithm verified the observed behavior. Increasing resolution and modifications to the WAKE numerical algorithm improves action conservation up to a time, which is identified to be the validity limit for the MPE.

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