Optical Visualization of Beam-driven Plasma Wakefield Accelerators

这个研究项目旨在创造和诊断由电离气体或“等离子体”构成的柔韧性微观结构，这种结构适合于加速、塑造和聚集电子束及其反物质孪生体“正电子”，比传统加速器要紧凑和便宜得多。世界上几乎所有用于工业、医院和科学发现的3万个传统粒子加速器，都在加速电子和质子等普通物质粒子。只有少数几个大型设施可以加速平行物质/反物质粒子束——比如电子和正电子，或者质子和反质子。当这样的双光束相互碰撞时，会产生新粒子的宝库，这些新粒子有着奇异的名字，比如“W”、“Z”和“希格斯”玻色子，这些玻色子已经获得了几项诺贝尔奖。不幸的是，过去这些突破背后的技术已经变得过于庞大和昂贵，无法支持新的突破。在这个项目中，一个充满能量的电子束在一个充满锂气体的管道中奔跑，产生并激活了只有1码长、比人的头发还细的短寿命等离子体细丝，就像一艘船在池塘里奔跑时产生的尾流一样。这种“尾流”可以将电子和正电子加速到与一英里长、18轮车宽的传统加速器相同的能量。计算机模拟表明，在电子“船”后面1/1000英寸的等离子体尾迹部分是一个很好的电子加速器，而在电子“船”后面1英寸的部分是一个很好的正电子加速器。这些尾迹部分的闪光全息图测试了模拟。此外，仔细计时的电子和正电子束可以在尾流上冲浪，以直接测试其作为加速器的可行性。成功不仅可以为未来的粒子物理学发现带来紧凑、负担得起的技术，还可以利用加速器产生的正电子光束的独特强度、狭窄度和能量均匀性，为正电子成像带来新的医疗和工业应用。这个为期3年的项目在第二代SLAC先进加速器科学和实验测试设施（FACET-II）进行了批准的实验E-324“光束驱动等离子体尾流场加速器的光学可视化”。FACET-II的10 GeV电子束在密度为10^17个电子/ cm^3的一米长的锂等离子体中驱动强烈的非线性等离子体尾流。一个100秒、近红外、近共传播的光学探针脉冲，与电子束同步，在0到50 ps的时间延迟范围内以掠掠角撞击在电子束的路径上，并从尾迹中衍射。下游探测器记录探针的衍射图样，由此可以重建尾流的电子密度分布图，并与计算机模拟的预测结果进行比较。科学目标是：(1)观察到计算机模拟预测在~ 50ps时形成的一个尖锐的轴上离子密度峰值；(2)用二级电子束激发和诊断该离子密度结构中的电子尾迹，并测试其对稳定正电子加速的预测适用性；(3)首次观测到初级电子束后延时小于1ps的气泡状电子尾迹，这非常适合于电子的加速。本论文的智力优势在于首次观察到粒子束驱动的等离子体尾迹，创造并确定了适合于电子和正电子加速的互补等离子体结构，从而为基于双等离子体的电子-正电子加速器铺平了道路。更广泛的影响包括开发紧凑、负担得起的加速器技术，以及培训下一代劳动力，包括具有不同背景的研究生和本科生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This research project aims to create and diagnose pliable, microscopic structures made of ionized gas, or "plasma", that are suitable for accelerating, shaping and bunching beams of electrons and their antimatter twins called "positrons" far more compactly and cheaply than conventional accelerators. Nearly all of the world's 30,000 conventional particle accelerators, which serve industry, hospitals and discovery science, accelerate ordinary matter particles, such as electrons and protons. A mere handful of mega-facilities accelerate parallel matter/antimatter particle beams - such as electrons and positrons, or protons and anti-protons. When collided with each other, such twin beams yield a treasure trove of new particles, with exotic names like "W", "Z" and "Higgs" bosons, which have reaped several Nobel prizes. Unfortunately, the technology underlying such past breakthroughs has become too large and expensive to support new breakthroughs. In this project, an energetic electron bunch racing through a pipe filled with lithium gas creates and energizes short-lived plasma filaments only a yard long, and thinner than a human hair, just as a boat racing across a pond creates a wake. This "wake" can accelerate both electrons and positrons to the same energy as a mile-long conventional accelerator as wide as an 18-wheeler. Computer simulations show that the part of the plasma wake 1/1000 inch behind the electron “boat” is a good electron accelerator, whereas the part one inch behind is a good positron accelerator. Flash holograms of these parts of the wake test the simulations. Moreover, carefully timed electron and positron bunches can be surfed on the wake, to test its viability as an accelerator directly. Success could lead not only to compact, affordable technology for future particle physics discoveries, but to new medical and industrial applications of positron imaging that take advantage of the unique intensity, narrowness and energy uniformity of positron beams from accelerators.The 3-year project carries out approved experiment E-324 "Optical visualization of beam-driven plasma wakefield accelerators" at the 2nd generation SLAC Facility for Advanced Accelerator Science and Experimental Tests (FACET-II). FACET-II's 10 GeV electron bunches drive strongly nonlinear plasma wakes in a meter-long lithium plasma of density 10^17 electrons per cm^3. A 100 fs, near-infrared, near-co-propagating optical probe pulse, synchronized with the electron bunch, impinges on the bunch's path at grazing angle at time delays ranging from 0 to 50 ps, and diffract from the wake. Downstream detectors record the probe's diffraction pattern, from which the wake's evolving electron density profile can be reconstructed and compared to predictions of computer simulations. Scientific goals are: (1) to observe a sharp on-axis ion density peak that computer simulations predict to form at ~50 ps; (2) to excite and diagnose an electron wake in this ion density structure with a secondary electron bunch, and to test its predicted suitability for stable positron acceleration; (3) to observe the bubble-shaped electron wake at delays below 1 ps behind the primary electron bunch, which is well suited for accelerating electrons, for the first time. The intellectual merit consists in observing particle-beam-driven plasma wakes for the first time, and in creating and identifying complementary plasma structures suitable for electron and positron acceleration, thereby paving the way for a dual plasma-based electron-positron accelerator. The broader impacts include development of compact, affordable accelerator technology and training of the next generation workforce, including graduate and undergraduate students with diverse backgrounds.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.