Collaborative Research: Preformed Laser-driven Plasma Waveguides for Multi-GeV Laser-Plasma Electron Acceleration

这项研究项目旨在展示一种新型电子加速器背后的物理原理，这种加速器将比最好的现代加速器更小、更便宜。自20世纪30年代以来，科学家们建造了更大、更昂贵的机器，称为加速器，将电子加速到超过光速的99%，爱因斯坦发现光速是宇宙的速度极限。以如此巨大的速度，电子可以探测自然界最深的亚原子秘密，照射癌症肿瘤，并产生强大的X射线来测量生命所必需的分子结构。作为美国最大的电子加速器之一，一台名为“SLAC”的2英里长的机器(最初是“斯坦福直线加速器中心”的缩写)将电子加速到光速的99.99999999%。这样一个电子携带的能量为30千兆瓦电子伏特(GeV)，比蚊子拍动翅膀一次所用的能量还少，但对于一个电子来说，这是很大的能量。这项研究项目的目标是实现桌面电子加速器，这种加速器比SLAC小数千倍，价格也更低，但仍然可以将电子加速到与SLAC相同的能量。这项新技术是由等离子体或电离气体(人们在荧光灯和恒星中发现的相同状态的物质)制成的一条细长的管道。这个管子是一个“跑道”，最终目的是限制和引导电子，以及一个强大的激光脉冲，推动它们加速，直到它们达到30GeV。另一台强大的激光将被用来将液体状的等离子体塑造成一根管道。计算机计算将被用来理解等离子体管道是如何形成的，并将在实验室中演示管道的模型版本。这个为期两年的项目将阐明形成轴向电子密度在1到3倍10^17粒子/cm^3之间、半径~50µm的圆柱形等离子体波导的科学基础。这种波导最终可以以低阶模式引导来自德克萨斯拍瓦特(PW)激光器的100 J、150 fs驱动脉冲，其相对论强度高达泵浦耗尽极限，从而将单级2GeV激光-等离子体电子加速器的性能扩展到数十GeV水平。通道形成方法是基于20世纪90年代开发的物理原理，但正在扩展到~20倍的等离子体密度。使用德克萨斯大学奥斯汀分校实验室提供的2J、80-300ps的驱动脉冲，将演示在稀薄的He等离子体中形成短(~1厘米)通道的过程。这一原型装置将使沟道形成脉冲的持续时间、能量和焦点，以及在低等离子体密度下优化等离子体加热和沟道形成的预电离和掺杂条件被发现。科罗拉多大学博尔德分校的研究小组将对稀薄氦中的通道形成和相对论激光脉冲的通道传输进行模拟，以指导实验。智能的优点在于发现激光等离子体条件，以优化形成高质量的单模等离子体通道，其密度比以往任何时候都低。更广泛的影响包括开发等离子体波导技术，该技术可能最终将未来激光-等离子体加速器的单级能量增益扩展到SLAC的典型水平；促进来自历史上代表性不足群体的三名博士生的职业生涯；以及引入一名本科生从事专业研究。

This research project aims to demonstrate the physical principles underlying a new type of electron accelerator that would be thousands of times smaller and less expensive than the best modern accelerators. Since the 1930s, scientists have built ever bigger and more expensive machines, called accelerators, to accelerate electrons to more than 99% of the speed of light, which Einstein discovered to be the speed limit of the universe. At such enormous speeds, electrons can probe into nature's deepest subatomic secrets, irradiate cancerous tumors, and generate powerful x-rays that measure the structure of molecules essential to life. One of America's biggest electron accelerators, a 2-mile-long machine called "SLAC" (which originally stood for "Stanford Linear Accelerator Center") accelerates electrons to 99.99999999% of the speed of light. The energy carried by such an electron, at 30 giga-electronvolts (GeV), is less than a mosquito uses to flap its wings once, but it's a lot of energy for one electron. The goal of this research project is to enable tabletop electron accelerators that are thousands of times smaller and less expensive than SLAC, but which can nevertheless accelerate electrons to the same energy as SLAC does. The new technology is a long, narrow pipe made of plasma, or ionized gas (the same state of matter one finds inside fluorescent light bulbs and stars). This pipe is a "racetrack" that is intended ultimately to confine and guide electrons, and a powerful laser pulse that fuels their acceleration, until they reach 30 GeV. A separate powerful laser will be used to shape the fluid-like plasma into a pipe. Computer calculations will be used to understand how the plasma pipe forms, and a model version of the pipe will be demonstrated in the laboratory. This two-year project will elucidate the science underlying the formation of cylindrical plasma waveguides with axial electron density in the range between 1 and 3 times 10^17 particles per cm^3 and radius of ~50 µm. Such waveguides can ultimately guide 100 J, 150 fs drive pulses from the Texas Petawatt (PW) Laser in a low-order mode at relativistic intensity up to the pump depletion limit, in order to extend the performance of a single-stage 2 GeV laser-plasma electron accelerator to the tens-of-GeV level. The channel formation method is based upon physical principles developed during the 1990s, but is being extended to ~20x lower plasma density. Formation of short (~1 cm) channels in tenuous He plasma will be demonstrated, using 2J, 80-300 ps drive pulses available in the laboratory at the University of Texas at Austin. This prototype setup will enable the duration, energy and focus of channel-forming pulse, and the pre-ionization and doping conditions that optimize plasma heating and channel formation at low plasma density to be discovered. Simulations of channel formation in tenuous helium, and channeled propagation of relativistic laser pulses, by the research group at the University of Colorado Boulder will guide experiments. The intellectual merit lies in discovering laser-plasma conditions that optimize formation of high quality, single-mode plasma channels of lower density than ever previously demonstrated. The broader impacts include developing plasma waveguide technology that may ultimately extend single-stage energy gain of future laser-plasma accelerators to levels typical of SLAC; advancing the careers of three doctoral students from historically under-represented groups; and introducing an undergraduate student to professional research.