Collaborative Research: Collisionless Heating During Strong Field Irradiation of Wavelength-scale Particles

研究了波长尺度粒子辐照时强激光与等离子体相互作用的物理过程。虽然多年来人们已经广泛研究了强场与原子、分子、小星团和平面固体相互作用的本质，但强烈的激光脉冲如何与空间尺度上与光相当的物体相互作用？S波长在很大程度上还没有被探索。这些相互作用很可能表现出与强脉冲与单个原子和分子的相互作用以及与大的平面固体等离子体的相互作用非常不同的性质。这项工作填补了强场物理学中的这一空白，通过精心设计的实验，分离出波长尺度目标特有的影响。此前，我们的研究表明，适当选择靶粒子的大小可以增强强场相互作用。粒子施加的边界条件在局部激光场中产生Mie增强，从而增加了相互作用的非线性响应。当前工作的具体焦点是由我们之前的实验和计算研究中产生的一个重要问题引起的：在波长尺度的等离子体周围被热电子无碰撞吸收的性质是否与简单平面固体的无碰撞吸收不同？许多理论最近推测，这种吸收在这些尺度上是不同的，由所谓的多通道随机加热主导，在这种加热中，热电子可以通过在微米尺度的等离子体中来回多次传递来吸收来自激光场的能量。这一新的加热机制在许多实验和粒子模拟中被认为是重要的，但到目前为止还没有系统的实验研究。目前的研究旨在解决这一缺陷。这些实验旨在直接测量在无碰撞加热过程中产生的电子和离子的分布，并通过研究明确定义的尺寸的颗粒中的特定标度来确定这种随机加热机制的重要性或存在。这些研究利用了20TW、UT高时间对比度激光、电子和离子能量诊断以及新型静电粒子注入器作为目标。这些实验还得到了使用德克萨斯大学提供的代码进行模拟的补充。这项工作是通过德克萨斯大学的学术研究实验室和本科生机构哈维·穆德学院(HMC)的独特合作完成的。这类研究的应用可能有助于开发用于射线照相和时间分辨衍射的新型明亮的激光驱动X射线源，或用于其他科学应用成像的致密中子源。除了这些科学影响之外，这项合作工作有望对研究生和本科生教育产生重大的、有些新颖的影响。除了它的科学价值和对德克萨斯大学研究生研究的支持外，这项研究还显著增加了HMC物理本科生可获得的研究机会的范围和数量。这项工作代表了一种独特的情况，在这种情况下，本科生小组直接并以关键的方式参与到更大规模的强场光学研究中，这一领域原本只对本科生水平的研究来说资源密集得令人望而却步。PI在合作方面有很好的记录，并有意义地让本科生参与他们的合作。本科生有机会在HMC做研究，在夏天前往德克萨斯大学使用他们开发的设备参与研究，并参加会议向科学界介绍他们的工作。这种合作将继续成为激励本科生攻读理科高级学位的有效途径。此外，PIS将继续通过在适当的同行评议期刊和国际会议上发表文章来传播他们的工作，就像他们在过去的资助期间所做的那样。这一奖项的资金由NSF数学和物理科学局内的物理部和能源部聚变能源科学办公室在NSF/DOE在基础等离子体科学和工程方面的合作伙伴关系的背景下提供。

The physics of the interaction of intense laser light with plasma when wavelength-scale particles are irradiated is investigated. While the nature of strong field interactions with atoms, molecules, small clusters and planar solids has been studied extensively for many years, how intense laser pulses interact with objects that are of a spatial scale comparable to the light?s wavelength is largely unexplored. These interactions are likely to exhibit properties which are quite distinct from the interactions of intense pulses with single atoms and molecules on one hand and large planar solid plasmas on the other. This work fills this gap in strong field physics with careful, well-designed experiments that isolate the effects which are peculiar to wavelength-scale targets. Previously, our investigations showed that strong field interactions could be enhanced by an appropriate choice of target particle size. Boundary conditions imposed by the particles create Mie enhancements in the local laser field and thereby increase the nonlinear response of the interaction. The specific focus of the current work is motivated by an important question that arose from our previous experimental and computation studies: is the nature of collisionless absorption by hot electrons around wavelength-scale plasmas different from collisionless absorption from a simple planar solid? A number of theories have recently surmised that such absorption is different at these scales, dominated by what has been termed multi-pass stochastic heating, in which hot electrons can absorb energy from the laser field by passing back and forth multiple times through the micron-scale plasma. This novel heating mechanism has been suggested as being important in a number of experiments and particle-in-cell simulations but there has as yet been no systematic experimental investigation. The current studies are designed to address this shortcoming. The experiments are designed to directly measure the electron and ion distributions generated in the collisionless heating process, and, by studying specific scalings in particles of well defined size, ascertain the importance or existence of this stochastic heating mechanism. The studies utilize a 20 TW, high temporal contrast laser at UT, electron and ion energy diagnostics, and a novel electrostatic particle injector as a target. These experiments are complemented by simulations using codes available at UT. The work is done through a unique collaboration between an academic research lab at the University of Texas and at Harvey Mudd College (HMC), an undergraduate institution.Application of these kinds of studies may aid in the development of novel bright, laser-driven x-ray sources for radiography and time resolved diffraction, or compact neutron sources for imaging of other scientific applications. In addition to these scientific impacts, this collaborative work promises to make a significant and somewhat novel impactin graduate and undergraduate education. In addition to its scientific merit, and support of graduate student research at UT, this research significantly adds to the scope and number of research opportunities available to physics undergraduate students at HMC. This work represents a unique situation in which an undergraduate group is involved directly and in a critical way in larger scale strong field optics research, a field which is otherwise prohibitively resource intensive for undergraduate-level research alone. The PIs have an excellent record of working together and meaningfully involving undergraduates in their collaborations. Undergraduates have the opportunity to do research at HMC, travel to UT in the summers to participate in research using equipment they have developed, and travel to conferences to present their work to the scientific community. This collaboration will continue to be an effective way to motivate undergraduates to seeking advanced degrees in science. Furthermore the PIs will continue to disseminate their work through publication in appropriate peer-reviewed journals and international conference as they have done actively during the past funding period.Funds for this award are provided by the Physics Division within the NSF's Mathematics and Physical Sciences Directorate and the Office of Fusion Energy Sciences of the DoE within the context of the NSF/DOE Partnership in Basic Plasma Science and Engineering.