Particle acceleration in magnetised shocks produced by laser and pulsed power facilities

我们提出了一个雄心勃勃的多机构实验计划，以研究天体物理学中最大的谜团之一：导致高能宇宙射线产生的加速机制。宇宙中存在高能粒子是一个公认的事实，对宇宙射线（CR）光谱的测量延伸到惊人的1e20 eV。尽管如此，导致如此高能粒子的确切机制仍然存在争议。本提案的中心主题是开展一项有关CR加速机制的地面实验和理论研究计划，以解决这一长期存在的问题。尽管许多不同的过程可能导致CR加速，但目前的理解是，激波和湍流在为星际介质中的电子和离子提供能量方面起着至关重要的作用。我们将对在磁化等离子体中形成的强冲击中电子的加速进行相关的实验和数值研究。冲击将由高强度激光和兆安级脉冲电流产生的超音速等离子体流形成。第一组实验将研究电子的初始加速度，这将允许形成能量大大超过其初始热能的电子群。这是由于冲击前等离子体中被激波反射的离子激发的等离子体波湍流所导致的，但这种机制从未被实验验证过。我们将描述湍流的发展，并使用我们以前开发的最先进的诊断技术测量加速电子的参数。在第二组实验中，我们将研究所谓的扩散激波加速机制，这被认为是宇宙射线加速最合理的机制。这将通过向激波中注入足够能量的电子来实现，以这种方式，这些电子将对激波前和激波后区域进行采样，根据该机制有效运行所需的要求，通过激波前沿进行多次通道。使用磁谱仪可以直接测量加速电子的能量，这将与理论预测进行比较。作为该项目的一部分，我们还将使用最先进的混合mhd和PIC代码进行数值模拟，并将结果与我们的实验数据进行交叉比较。该项目的计算和理论组成部分将使我们能够在实验、天体物理模型和观测之间建立牢固的联系。拟议的研究位于等离子体物理学和天体物理学之间的边界，并将推进实验室天体物理学的新研究领域的发展，该领域旨在通过规模实验室实验，结合计算机建模，加强对直接控制天体物理对象行为的物理学的理解。由于高能激光器和快速上升时间大电流脉冲功率设备的出现，在实验室中创建天体物理环境大规模重建所需的极端等离子体条件最近才成为可能。在关键的无量纲参数（如马赫数）方面，实验室与自然之间的相似性足够接近，使此类实验具有高度相关性。国际上对这一领域日益增长的兴趣也强调了这一建议的及时性，美国（罗切斯特，利弗莫尔- NIF）和欧洲（波尔多- LaserMegajoule）做出了重大努力。该提案作者的专业知识和来自天体物理学界的国际合作者的参与将使我们能够为实验室天体物理学研究创造和利用前所未有的能力，并为该计划提供广度和深度。

We propose an ambitious multi-institution experimental programme to investigate one of the greatest mysteries in astrophysics: the acceleration mechanism that leads to generation of high energy cosmic rays. The presence of energetic particles in the Universe is a well established fact, with measurements of the cosmic ray (CR) spectrum extending up to astonishing 1e20 eV. In spite of this, the exact mechanism that leads to such high energy particles still remains controversial. The central theme of this proposal is to conduct a programme of linked earth-based experimental and theoretical investigations into CR acceleration mechanisms to address this long running problem. Although many different processes may result in CR acceleration, the present day understanding is that shock waves and turbulence play an essential role in energizing both the electrons and ions present in the interstellar medium. We will perform linked experimental and numerical studies of the acceleration of electrons in strong shocks formed in magnetised plasmas. The shocks will be formed by supersonic plasma flows created by high intensity lasers and Mega-Ampere-level pulsed currents. The first set of experiments will investigate the initial acceleration of electrons, which should allow the formation of electron population with energies significantly exceeding their initial thermal energy. This is expected to occur due to plasma wave turbulence which is excited in the pre-shock plasma by the ions reflected from the shock front, but this mechanism has never been tested by experiment. We will characterise the development of the turbulence and measure the parameters of the accelerated electrons using state-of-the-art diagnostic techniques previously developed by us. In the second set of experiments, we will investigate the so-called diffusive shock acceleration mechanism, which is considered as the most plausible mechanism of cosmic ray acceleration. This will be achieved by injecting sufficiently energetic electrons into the shock, in such a way that these electrons will then sample both the pre- and post-shock regions, performing multiple passages through the shock front as required for this mechanism to operate efficiently. Use of a magnetic spectrometer will allow direct measurements of the energy of the accelerated electrons which will be compared with theoretical predictions. As part of this project we will also perform numerical simulations using state of the art hybrid-MHD and PIC codes and cross-compare the results with our experimental data. The computational and theoretical components of the project will allow us to forge a strong connection between experiment, astrophysical models and observations.The proposed research lies at the border between Plasma Physics and Astrophysics, and will advance the development of the novel research area of Laboratory Astrophysics, which seeks to enhance the understanding of the physics governing the behaviour of astrophysical objects directly via scaled laboratory experiments, combined with computer modelling. Creating the extreme plasma conditions required for scaled reconstruction of astrophysical environments in the laboratory, became possible only recently thanks to the advent of high energy lasers and fast rise-time high-current pulsed power facilities. The similarity between the lab and nature in terms of key dimensionless parameters (e.g. Mach number) is sufficiently close to make such experiments highly relevant. The timeliness of this proposal is also underlined by the growing interest in this field internationally with major efforts in USA (Rochester, Livermore - NIF) and Europe (Bordeaux - LaserMegajoule). The combined expertise of the authors of this proposal and the involvement of international collaborators from Astrophysics community will allow us to create and exploit an unprecedented capability for the Laboratory Astrophysics research and provide both breadth and depth to the programme.