Intense Sub-Femtosecond Optical Radiation from Relativistic Plasmas

这项工作将探索有史以来产生的最短持续时间的光学闪光，它是通过将静止的电子加速到几乎光速而产生的，距离不到人类头发厚度的百分之一。加速的带电粒子是由低能量(但非常高强度)的激光脉冲与密集的氢气射流相互作用而产生的。氢气的所有电子都被激光脉冲剥离，形成等离子体，激光脉冲的辐射压力然后将自由电子推到一边。这会产生一种静电波，它在等离子体中以波的形式移动(所谓的等离子体波)，它的强度如此之大，以至于它可以将自己的电子从静止状态加速到接近光速的定向光束的形式。这些加速的电子束对医学和科学成像很有用。它们的一个有趣、令人兴奋和潜在有用的副产品是辐射闪光发射，它发生在辐射发射本身的半个周期内，使其持续时间为亚飞秒(不到1/1000000000000000秒长)。这样的光源，除了其迷人的内部相对论动力学之外，还可以作为有史以来最快的光学频闪源来捕捉图像，例如，在原子中飞行中的电子轨道。这个项目将开发和表征一种在光学范围内产生强烈、相干的亚飞秒脉冲(或闪光)的新来源。这些脉冲直接来自相对论等离子体波的破波，并且仅由泵浦激光场间接驱动。辐射闪光似乎纯粹是由暴露在相对论等离子体波内部的巨大电场和磁场中的带电粒子的经典加速引起的。它们的光谱和相干特性受到等离子体密度的强烈影响，等离子体密度决定了等离子体波的群速度、辐射束的空间尺度以及启动激光脉冲的相对论自聚焦所需的激光能量。这样的闪光与电子从静止到接近光速的单向加速是一致的。该项目将包括对闪光相干特性、时间脉冲结构的详细测量，以及对这种辐射的定时控制的探索。伴随着实验的将是广泛的2D和3D细胞内粒子模拟。一种新的1千赫、10毫焦耳的激光系统将使实验能够以更高的重复率进行，改善每张照片的重复性，并允许更高的数据采集率，这两者都能带来更好的信噪比和更精确的测量。

This work will explore the shortest duration optical flashes ever generated, produced by accelerating electrons from rest to almost the speed of light over a distance less than one-hundredth the thickness of a human hair. The accelerating charged particles are produced by the interaction of low energy (but very high intensity) laser pulses with dense jets of hydrogen gas. The hydrogen gas has all of its electrons stripped off by the laser pulse, forming a plasma, and radiation pressure from the laser pulse then pushes the free electrons out of the way. This produces an electrostatic disturbance that moves as a wave in the plasma (a 'plasma wave') and is so strong that it can accelerate its own electrons from rest to nearly the speed of light in the form of directed beams. These beams of accelerated electrons are useful for medical and scientific imaging. One of their intriguing, exciting, and potentially useful byproducts is radiation flash emission, which takes place over just one half cycle of the radiation emission itself, making it of sub-femtosecond duration (less than 1/1000000000000000 second long). Such a light source, in addition to its fascinating internal relativistic dynamics, can act as the fastest-ever optical strobe source for capturing images, for example, of electrons in mid-flight in their orbits in an atom.This project will develop and characterize a new source of intense, coherent, sub-femtosecond pulses (or flashes) in the optical range. These pulses arise directly from wave breaking of relativistic plasma waves and are only indirectly driven by a pump laser field. The radiation flash appears to arise purely from classical acceleration of charged particles exposed to the enormous electric and magnetic fields inside relativistic plasma waves. Their spectral and coherence characteristics are strongly affected by the plasma density, which sets the group velocity of the plasma waves, the spatial scale of the radiating bunches, and the laser energy required for relativistic self-focusing of the initiating laser pulse. Such flashes are consistent with unidirectional acceleration of electrons from rest to nearly the speed of light. The project will consist of detailed measurements of flash coherence properties, temporal pulse structure, and exploration of timing control of this radiation. Accompanying the experiments will be extensive 2D and 3D particle-in-cell simulations. A new 1 kHz, 10 mJ laser system will enable experiments at a much higher repetition rate, improving shot-to-shot reproducibility and allowing high data collection rates, both of which lead to better signal-to-noise and more precise measurements.