Holographic Imaging of Evolving Laser-Plasma Structures

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This award is made in response to a proposal submitted to and reviewed under the NSF/DOE Partnership in Basic Plasma Science and Engineering joint solicitation NSF 09-596. The award provides funds to support undergraduate participation in the overall research effort, which is being funded separately by the DOE under contract to University of Texas (Grant DE-FG02-07ER54945). When intense femtosecond laser pulses or electron bunches propagate through an ionized gas, or plasma, they displace plasma electrons from within their envelopes, much like a boat displaces water as it propagates through a lake. As a result, they produce electron density structures that propagate at the speed of light and evolve in shape as they propagate, like the wake behind a boat. In recent years, scientists have developed miniature particle accelerators that work by surfing charged particles on these electron density waves. However, the performance of the accelerators is difficult to optimize because the wave structures are invisible, except indirectly through intensive computer simulations, and challenging to control. This project will develop methods for visualizing and controlling these light-velocity structures. To visualize them, the first of three tasks aims to develop a Frequency Domain Tomography (FDT) system that will take multi-frame "movies" of evolving plasma structures created by a single laser shot. The FDT system will multiplex an existing Frequency Domain Holography system, developed in prior work, that takes slightly blurred "snapshots" using a single probe pulse that co-propagates with and overlaps the evolving structure. The new FDT system adds probes propagating at oblique angles to the plasma structure. Tomographic algorithms similar to those used in medical CAT scans will then reconstruct multiple "frames" depicting the plasma structure at different stages of its evolution. In the second of three tasks, the visualization techniques will be extended to electron-bunch-driven plasma wakes in an experiment at Brookhaven National Laboratory's Advanced Test Facility (ATF) that was recently approved by the ATF Program Advisory Committee after external review. The temporal and radial structure of the plasma wakes will be visualized as the format of the drive bunch train and the plasma density vary. To control evolution of the drive laser pulse and plasma structures it creates, the third of three tasks introduces a secondary drive laser pulse that co-propagates with the main laser pulse and differs in frequency by approximately the electron plasma frequency. Theory developed previously by the co-PI of this project showed that such a secondary pulse can control the intense drive laser pulse's propagation, while a two-color terawatt laser system developed by the lead PI produces temporally synchronized sideband pulses needed to implement such experiments. The project introduces unique approaches to visualizing and controlling rapidly evolving relativistic laser-plasma interactions that the co-PIs pioneered. It will provide the first laboratory visualization of evolving laser- and e-beam driven plasma wakes, and will complement, benchmark and validate computer simulations of these structures. Two-color laser-plasma interactions provide a new approach to controlling plasma wave propagation that is ripe for experimental demonstration.The ability to combat or enhance relativistic self-focusing developed here can impact fast ignition of laser fusion, and lead to more reliable plasma-based accelerators, useful in turn as compact x-ray sources, injectors for conventional accelerators, and medical accelerators. Proposed investigations of two-color laser plasma interactions can lead to a new generation of plasma-based amplifiers and compressors for ultra-intense laser pulses that are free of material damage limits. Finally the proposed research will train a postdoc, three Ph.D. students, and an undergraduate from UT-San Antonio, which ranks 4th in the nation in number of undergraduate degrees awarded to Hispanics.This project is jointly funded by the NSF and the DOE.The NSF support of undergraduate participation adds a broader educational impact through the early-year training of students by introducing them to scientific research as a possible career path.