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.
In the 1870s, English photographer Eadweard Muybridge captured motion pictures within one cycle of a horse’s gallop, which settled a hotly debated question of his time by showing that the horse became temporarily airborne. In the 1940s, Manhattan project photographer Berlin Brixner captured a nuclear blast at a million frames per second, and resolved a dispute about the explosion’s shape and speed. In this project, we developed methods to capture detailed motion pictures of evolving, light-velocity objects created by a laser pulse propagating through matter. These objects include electron density waves used to accelerate charged particles, laser-induced refractive index changes used for micromachining, and ionization tracks used for atmospheric chemical analysis, guide star creation and ranging. Our "movies", like Muybridge’s and Brixner’s, are obtained in one shot, since the laser-created objects of interest are insufficiently repeatable for accurate stroboscopic imaging. Our high-speed photographs have begun to resolve controversies about how laser-created objects form and evolve, questions that previously could be addressed only by intensive computer simulations based on estimated initial conditions. Resolving such questions helps develop better tabletop particle accelerators, atmospheric ranging devices and many other applications of laser-matter interactions. Our photographic methods all begin by splitting one or more "probe" pulses from the laser pulse that creates the light-speed object. A probe illuminates the object and obtains information about its structure without altering it. We developed three single-shot visualization methods that differ in how the probes interact with the object of interest or are recorded. (1) Frequency-Domain Holography (FDH). In FDH, there are 2 probes, like "object" and "reference" beams in conventional holography. Our "object" probe surrounds the light-speed object, like a fleas swarming around a sprinting animal. The object modifies the probe, imprinting information about its structure. Meanwhile, our "reference" probe co-propagates ahead of the object, free of its influence. After the interaction, object and reference combine to record a hologram. For technical reasons, our recording device is a spectrometer (a frequency-measuring device), hence the name "frequency-domain" holography. We read the hologram electronically to obtain a "snapshot" of the object’s average structure as it transits the medium. The figure (left) shows an example: an electron density wave ("laser wake") in ionized gas ("plasma"), analogous to a water wake behind a boat. Such waves are the basis of tabletop particle accelerators, in which charged particles surf on the light-speed wave, gaining energy. Comparing our snapshots to computer simulations (right) deepens understanding of laser wakes. The object in the figure is quasi-static --- i.e. like Muybridge’s horse standing still on a treadmill. If the object changes shape, FDH images blur, as when a subject moves while a camera shutter is open. Many laser-generated objects of interest do evolve as they propagate. To overcome this limit of FDH, we developed .... (2) Frequency-Domain Tomography (FDT). In FDT, 5 to 10 probe pulses are fired simultaneously across the object’s path at different angles, like a crossfire of bullets. The object imprints a "streaked" record of its evolution on each probe, which we record as in FDH, then recover a multi-frame "movie" of the object’s evolving structure using algorithms of computerized tomography. When propagation distance exceeds a few millimeters, reconstructed FDT images distort. This is because the lenses that image probes to detector have limited depth of field, like cameras that cannot focus simultaneously on both nearby and distant objects. But some laser-generated objects of interest propagate over meters. For these applications we developed … (3) Multi-Object-Plane Phase-Contrast Imaging (MOP-PCI). In MOP-PCI, we image FDT-like probes to the detector from multiple "object planes" --- like recording an event simultaneously with several cameras, some focused on nearby, others on distant, objects. To increase sensitivity, we exploit a phase-contrast imaging technique developed by Dutch Nobel laureate Fritz Zernike in the 1930s. Using MOP-PCI we recorded single-shot movies of laser pulse tracks through more than 10 cm of air. We plan to record images of meter-long tracks of electron bunches propagating through plasma in an experiment at the Stanford Linear Accelerator Center (SLAC). This will help SLAC scientists understand, optimize and scale small plasma-based particle accelerators that have applications in medicine, industry, materials science and high-energy physics. The Department of Energy provided 98.5% of the funding for this work; NSF provided the remaining 1.5%.
