Holographic Imaging of Evolving Laser-Plasma Structures

该奖项是根据NSF/DOE基础等离子体科学与工程合作伙伴关系联合招标NSF 09-596提交并审查的提案作出的。该奖项提供资金支持本科生参与整体研究工作，由美国能源部根据与德克萨斯大学的合同单独资助（Grant DE-FG02-07ER54945）。当强烈的飞秒激光脉冲或电子束在电离气体或等离子体中传播时，它们会将等离子体电子从其外壳中移走，就像船在湖中传播时将水移走一样。结果，它们产生了以光速传播的电子密度结构，并在传播过程中改变形状，就像船后面的尾流一样。近年来，科学家们已经开发出微型粒子加速器，通过在这些电子密度波上冲浪带电粒子来工作。然而，加速器的性能很难优化，因为波的结构是不可见的，除非通过密集的计算机模拟，并且很难控制。该项目将开发可视化和控制这些光速结构的方法。为了使它们可视化，三个任务中的第一个目标是开发一个频域断层扫描（FDT）系统，该系统将拍摄由单次激光射击产生的不断发展的等离子体结构的多帧“电影”。FDT系统将复用现有的频域全息系统，该系统在先前的工作中开发，使用单个探针脉冲与不断发展的结构共传播并重叠，以拍摄稍微模糊的“快照”。新的FDT系统在等离子体结构上增加了以斜角传播的探针。然后，类似于医学CAT扫描中使用的断层扫描算法将重建多个“帧”，描绘等离子体在不同发展阶段的结构。在三项任务中的第二项任务中，可视化技术将在布鲁克海文国家实验室先进测试设施（ATF）的一个实验中扩展到电子束驱动等离子体尾流，该实验最近在外部审查后得到了ATF项目咨询委员会的批准。等离子体尾迹的时间和径向结构将随着驱动束列的格式和等离子体密度的变化而可视化。为了控制驱动激光脉冲及其产生的等离子体结构的演变，三个任务中的第三个任务引入了一个辅助驱动激光脉冲，该脉冲与主激光脉冲共传播，并且频率大约与电子等离子体频率不同。由该项目的共同PI先前开发的理论表明，这种二次脉冲可以控制强驱动激光脉冲的传播，而由牵头PI开发的双色太瓦激光系统产生实现此类实验所需的临时同步边带脉冲。该项目引入了独特的方法来可视化和控制快速发展的相对论性激光等离子体相互作用，这是co- pi首创的。它将提供激光和电子束驱动等离子体尾迹演变的第一个实验室可视化，并将补充、基准和验证这些结构的计算机模拟。双色激光-等离子体相互作用为控制等离子体波传播提供了一种新的方法，这种方法已经成熟，可以进行实验验证。这里开发的对抗或增强相对论自聚焦的能力可以影响激光聚变的快速点火，并导致更可靠的基于等离子体的加速器，这些加速器反过来又可用作紧凑的x射线源、传统加速器的注入器和医疗加速器。提出的双色激光等离子体相互作用的研究可以导致新一代基于等离子体的放大器和压缩器，用于无材料损伤限制的超强激光脉冲。最后，这项拟议中的研究将培养一名博士后、三名博士生和一名UT-San Antonio大学的本科生。UT-San Antonio大学授予西班牙裔美国人的本科学位数量在全美排名第四。该项目由美国国家科学基金会和美国能源部共同资助。美国国家科学基金会对本科生参与的支持，通过将学生引入科学研究作为一种可能的职业道路，对他们进行早期培训，从而增加了更广泛的教育影响。

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.