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

研究了波长尺度粒子与强激光等离子体相互作用的物理过程。虽然强场与原子、分子、小团簇和平面固体相互作用的性质已经被广泛研究了多年，但强激光脉冲如何与空间尺度与光相当的物体相互作用？的波长在很大程度上未被探索。这些相互作用可能表现出的性质是相当不同的强脉冲与单原子和分子的相互作用，一方面和大平面固体等离子体。这项工作填补了强场物理学的这一空白，精心设计的实验隔离了波长尺度目标所特有的效应。以前，我们的研究表明，强场相互作用可以通过适当选择的目标颗粒尺寸来增强。由粒子施加的边界条件在局部激光场中产生米氏增强，从而增加相互作用的非线性响应。目前的工作的具体重点是由一个重要的问题，从我们以前的实验和计算研究的动机：热电子在波长尺度等离子体周围的无碰撞吸收的性质不同于从一个简单的平面固体的无碰撞吸收？许多理论最近推测，这种吸收在这些尺度上是不同的，由所谓的多程随机加热主导，其中热电子可以通过来回多次穿过微米尺度等离子体从激光场吸收能量。这种新的加热机制已被认为是重要的，在一些实验和粒子在细胞模拟，但还没有系统的实验研究。目前的研究旨在解决这一缺陷。实验的目的是直接测量在无碰撞加热过程中产生的电子和离子分布，并通过研究明确定义的尺寸的颗粒中的特定标度，确定这种随机加热机制的重要性或存在。研究利用20 TW，高时间对比度激光在UT，电子和离子能量诊断，和一种新型的静电粒子注入器作为目标。这些实验的补充模拟使用代码在UT。这项工作是通过德克萨斯大学的一个学术研究实验室和哈维穆德学院（Harvey Mudd College，HMC）的一个本科院校之间的独特合作完成的。这些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紧凑型中子源。除了这些科学影响之外，这种合作工作还有望对研究生和本科生教育产生重大的、有点新颖的影响。除了它的科学价值，并在UT研究生研究的支持，这项研究显着增加了研究机会的范围和数量提供给物理本科生在HMC。这项工作代表了一个独特的情况下，一个本科组直接参与，并在一个关键的方式在更大规模的强场光学研究，这是一个领域，否则禁止资源密集型本科水平的研究。PI在合作和有意义地让本科生参与他们的合作方面有着出色的记录。本科生有机会在HMC做研究，在夏季前往UT参加使用他们开发的设备的研究，并前往会议，向科学界介绍他们的工作。这种合作将继续是一种有效的方式，以激励本科生寻求科学的高级学位。此外，研究所将继续通过在适当的同行评审期刊和国际会议上发表论文来传播他们的工作，就像他们在过去的资助期间所做的那样。该奖项的资金由NSF数学和物理科学理事会的物理部和能源部的聚变能源科学办公室在NSF/能源部基础等离子体科学和工程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.