Isosurfaces and centroids of the electric field of two lasers (red and blue) show the laser braiding. The projections of the laser centroids are also shown on the walls. This simulation shows that in a plasma, one light beam can influence another beam's propagation by affecting the properties of the medium. The attractive force originates from relativistic mass increase of the plasma electrons in a strong laser field. This braiding effect might be useful in optical steering applications and might occur in nature when intense photon fluxes filament as they emanate from supernovas and powerful celestial gamma ray sources

Research Objectives
This research attempts to test the feasibility of various plasma-based accelerator concepts, to model full-scale plasma-based accelerator experiments, and to help develop new advanced accelerator concepts. The main objectives are to support the beat wave experiments at the Neptune Lab at UCLA, to support the E-157 experiment at SLAC (a collaborative experiment between SLAC/UCLA/USC), and to study basic physics in intense particle and laser plasma interactions.

Computational Approach
We apply particle-based models including fully explicit particle-in-cell (PIC) codes, ponderomotive guiding center PIC codes, and new photon kinetic codes. We are integrating all these algorithms into an object-oriented framework that supports massively parallel processing.

Accomplishments
Full-scale 2D and 3D modeling of the E-157 experiment has allowed us to study the role of the focusing and the acceleration wakefields. These simulations have been instrumental in the proper interpretation of this plasma wakefield experiment and have been a guide for finding new physics in GeV beam plasma interactions.

We have begun to understand hosing of short pulse plasma wakefield drivers. We have also performed the first simulations of the wakes generated by positron drivers. And we have begun to study the feasibility of adding a 100 GeV afterburner wakefield stage at the end of E-157. This involves understanding both electron and positron drivers, beam loading and hosing.

We have verified in simulations our prediction that there is a mutual attraction between two laser beams in a plasma. The simulations showed the beams actually form a braided pattern. We have also performed simulations of a new asymmetric spot size self-modulation instability of intense lasers in plasmas. In addition, we have developed and tested a new ponderomotive guiding center parallel PIC code which will allow us to carry out full-scale 2D simulations of the Neptune beat wave experiments.

Significance
In plasma-based acceleration, electrons "surf" on relativistic space charge plasma waves. In such waves electrons can be accelerated with gradients orders of magnitude larger than current technology. If plasma-based accelerator technology is successfully developed, then multi-GeV stages could be miniaturized to fit on a tabletop. Tabletop accelerators could have impacts in fields as diverse as high-energy physics, synchrotron radiation sources, medicine, and biology. If the simulations indicate that an afterburner concept is found to be viable, then this work could lead to much larger and broader R&D effort.

Publications
C. Ren, R. G. Hemker, R. A. Fonseca, B. J. Duda, and W. B. Mori, "Mutual attraction of laser beams in plasmas: Braided light," Phys. Rev. Lett. 85, 2124 (2000).

R. G. Hemker, W. B. Mori, S. Lee, and T. Katsouleas, "Dynamic effects in plasma wakefield excitation," Phys. Rev. ST Accel. Beams 3, 61301 (2000).

M. J. Hogan, R. Assmann, F.-J. Decker, et al., "E-157: A 1.4-m-long plasma wake field acceleration experiment using a 30 GeV electron beam from the Stanford Linear Accelerator Center Linac," Phys. Plasmas 7, 2241 (2000).

http://www.ee.ucla.edu/labs/laser-plasma/