Measurements of Electron Weibel Instability in a Laboratory Plasma Using Relativistic Electron Beams

在宇宙中人们可以观察到的大多数物质都是由带电粒子、电子和离子组成的等离子体状态。因此，为了解释星系的形成、伽马射线爆发和我们的太阳，了解等离子体的行为是至关重要的。通常，等离子体中的电子温度在一个方向上比在另一个方向上更热。这样的等离子体被认为是各向异性的。各向异性的等离子体最终会变成各向同性的--在空间的所有三维空间中达到相同的温度--通过一个被称为韦伯不稳定性的过程。 魏贝尔不稳定性在大约50年前就被魏贝尔预测到了，但是由于在实验室中不可能产生具有已知温度各向异性的等离子体，因此无法检验这种不稳定性增长的速度以及为什么它最终停止增长（饱和）的理论。 该项目旨在首次测量韦伯不稳定性的产生和饱和。 这项实验工作将由加州大学-洛杉矶研究小组在布鲁克海文国家实验室的加速器测试设施进行，并将涉及等离子体和加速器科学方面的一名研究生和一名博士后助理的培训。高度各向异性的等离子体可以通过使用圆偏振激光脉冲的He气体的光场电离产生，导致等离子体在横向上非常热，但在激光传播方向上相当冷。这样的等离子体经历了等离子体动力学不稳定性的层次，从双流不稳定性和电流双折射不稳定性开始，这些不稳定性在皮秒内将等离子体各向异性从~100降低到小于10。此后，电子韦贝尔不稳定性开始增长，种子的聚结电流与不稳定性。 纯韦伯不稳定性的特征在于具有宽波数谱的静磁场的快速增长。这个波数谱很快就缩小到一个相当明确的，最不稳定的模式，具有螺旋拓扑结构。 这项工作将首次测量该磁场的指数增长和饱和及其拓扑结构。 为了测量韦伯B场的动力学，将使用相对论电子束作为探针。在探测电子通过等离子体的过程中，它将被螺旋磁场的横向分量偏转。 这些偏转可以在离等离子体一定距离的屏幕上被可视化为探测电子密度结构。从不同时间拍摄的图像中，可以获得k谱的演变、其谱幅和B场的二维空间轮廓，从而可以将实验与韦伯不稳定性的动力学理论进行定量比较。该奖项反映了NSF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。

Most of the matter that one can observe in the universe is in a plasma state comprised of electrically charged particles, electrons and ions. It is therefore imperative to understand the behavior of plasmas in order to explain galactic formation, gamma-ray bursts, and our own Sun. Often, the temperature of electrons in a plasma is hotter in one direction than in the other. Such plasmas are said to be anisotropic. An anisotropic plasma eventually becomes isotropic -- reaching the same temperature in all three dimensions of space -- by a process known as the Weibel instability. The Weibel instability was predicted by Weibel almost 50 years ago, but it has been impossible to check the theory of how rapidly this instability grows and why it eventually stops growing (saturates) because it has not been possible to generate a plasma with known temperature anisotropy in the laboratory. This project aims to measure the generation and saturation of the Weibel instability for the first time. This experimental work will be performed by the University of California - Los Angeles team at the Accelerator Test Facility at Brookhaven National Laboratory, and will involve training of a graduate student and a postdoctoral associate in plasma and accelerator science.Highly anisotropic plasmas can be produced by optical field ionization of He gas using a circularly polarized laser pulse, resulting in a plasma that is very hot in the transverse direction but rather cold in the direction of propagation of the laser. Such a plasma undergoes a hierarchy of plasma kinetic instabilities, starting with the two stream instability and current filamentation instability that reduce the plasma anisotropy from ~100 to less than 10 in a pico-second. Thereafter, the electron Weibel instability begins to grow, seeded by coalescence of currents associated with the filamentation instability. The pure Weibel instability is characterized by rapid growth of a static magnetic field with a broad wavenumber spectrum. This wavenumber spectrum quickly narrows to a fairly well defined, most unstable mode that has helicoid topology. This work will measure the exponential growth and saturation of this magnetic field and its topology for the first time. To measure the dynamics of the Weibel B-field, a relativistic electron beam will be used as a probe. During its passage through the plasma, the probe electrons will be deflected by the transverse component of the helicoid magnetic field. These deflections can be visualized as probe electron density structures on a screen placed some distance away from the plasma. From images taken at different times, the evolution of the k spectrum, its spectral amplitude and 2D spatial profile of the B-field can be obtained, thereby allowing a quantitative comparison of experiment with kinetic theory of the Weibel instability.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.