Understanding particle acceleration in collisionless shocks using kinetic simulations

无碰撞冲击发生在由静电相互作用介导的碰撞的平均自由路径较大的脆弱等离子体中，如在超新星遗迹、星系团、相对论性喷流和脉冲星风中发现的。发生这种情况的条件，以及加速带电粒子的效率，还没有得到很好的理解。本项目将进行先进的数值模拟，并利用它们来构建一个自洽的冲击加速度理论。它将解决等离子体不稳定性、粒子散射、磁场产生和放大等复杂的微物理问题，并对理论进行校准，以便将其应用于天体物理对象。这些结果将有助于解释高能天体物理学和宇宙学来源的新研究，包括下一代实验室实验，并且物理见解甚至将适用于我们太阳系的冲击。这项工作将训练研究生和本科生的数值建模和多尺度系统的可视化，指导博士后，并支持开源仿真软件及其分析工具。无碰撞冲击跨越不同的速度和磁场强度，并加速同步加速器发射的非热电子和宇宙射线中常见的质子。主要过程被认为是扩散激波加速，其中粒子在磁波动中散射，但这种费米机制的条件及其效率还不能从第一原理中理解。该项目将进行大规模多维粒子单元（PIC）和天体物理无碰撞冲击的混合模拟，以考虑：1)加速度作为激波速度、马赫数和磁倾角的函数的相对效率是什么？2)在准平行和准垂直激波中，哪些波介导电子加速？3)下游电子与离子的温度之比是多少，能量传递的物理原理是什么？4)当离子加速有效时，激波的非线性结构是什么？通过PIC模拟测量的微物理参数化，新软件将结合背景等离子体的磁流体动力学描述和动力学粒子来代表宇宙射线及其在流动中的反反应。这将使建模宏观天体物理对象，如超新星遗迹或星系外流，包括宇宙射线及其加速度的影响。这些动力学模拟解决了非线性激波加速度是如何工作的，并将在有效时约束流的参数。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。

Collisionless shocks occur in tenuous plasmas where the mean free path for collisions mediated by electrostatic interaction is large, as found in supernova remnants, clusters of galaxies, relativistic jets, and pulsar winds. The conditions under which this occurs, and the efficiency of accelerating charged particles, are not well understood. This project will carry out advanced numerical simulations and use them to construct a self-consistent theory of shock acceleration. It will resolve the complex microphysics of plasma instabilities, particle scattering, and magnetic field generation and amplification, and calibrate the theory for its application to astrophysical objects. The results will help to interpret new studies of high-energy astrophysical and cosmological sources, including the next generation of laboratory experiments, and the physical insights will even apply to shocks in our solar system. The work will train graduate and undergraduate students in numerical modeling and the visualization of multiscale systems, mentor a postdoc, and support open source simulation software and its analysis tools.Collisionless shocks span different speeds and strengths of magnetic field, and accelerate both nonthermal electrons which energize synchrotron emission, and the protons commonly found in cosmic rays. The main process is thought to be diffusive shock acceleration, in which particles scatter on magnetic fluctuations, but the conditions for this Fermi mechanism and its efficiency are not understood from first principles. This project will carry out large-scale multidimensional particle-in-cell (PIC) and hybrid simulations of astrophysical collisionless shocks to consider: 1) what is the relative efficiency of acceleration as a function of shock speed, Mach number, and magnetic inclination? 2) what waves mediate electron acceleration in quasi-parallel and quasi-perpendicular shocks? 3) what is the ratio of electron to ion temperature downstream, and what is the physics of energy transfer? 4) what is the nonlinear structure of the shock when ion acceleration is efficient? With microphysical parameterizations measured with PIC simulations, new software will combine a magneto-hydrodynamic description of the background plasma with kinetic particles to represent cosmic rays and their back-reaction on the flow. This will enable modeling of macroscopic astrophysical objects such as supernova remnants or galactic outflows, including the effects of cosmic rays and their acceleration. These kinetic simulations address how nonlinear shock acceleration works and will constrain the parameters of the flow when it is efficient.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.