Dynamics of relativistic leptonic jets in low-density plasmas

天体物理喷流是迄今为止在宇宙中发现的最令人印象深刻、最有趣的现象之一。它们被观察到是从自然界中发现的一些最具能量的现象中喷射出来的，比如黑洞和脉冲星。这些喷流可以以一种非常准直的方式传播，距离可以达到千秒差距（1秒差距= 3.09 x 10^13公里，即30亿亿公里）数量级。研究这些喷流对于深入了解这些超大质量物体的物理特性至关重要，并可能有助于理解宇宙射线和超高亮度伽马射线爆发。尽管这些结构激发了人们的基本兴趣，但它们的关键特性（如组成、密度和能量）仍然缺乏透彻的理解。这是因为，尽管进行了广泛的理论建模和观察，但显然不可能清楚地获得现场的相关物理量。关于它们只有一些有根据的猜测：例如，人们普遍认为它们中的大多数应该主要由电子、正电子（电子的反粒子）和伽马射线光子组成，尽管它们的典型相对百分比和光谱尚未完全确定。推断它们特征的一种间接方法是监测它们与星系间空间的相互作用。尽管星系间空间是自然界中观测到的最接近纯真空的空间（它的平均密度约为每立方厘米一个粒子），但它的密度仍然不完全为零；据观察，在如此巨大的距离上，即使存在如此低密度的介质，也会影响天体物理射流的动力学，引起细丝、不连续传播和弯曲。通过了解在什么条件下可以触发这些不稳定性，就有可能推断出这些喷流的特征。因此，深入研究电子-正电子射流在低密度气体中的传播将在理解这些现象中发挥核心作用。幸运的是，这些令人印象深刻的扩展和能量现象是可扩展的：换句话说，通过采用合适的实验参数，有可能产生更小规模的复制品（小到几毫米大小），它们将以类似的方式表现。这表明，在实验室中利用可控的、小规模的复制品来研究天体物理喷流是可能的。我们的研究小组最近展示了使用紧凑的激光驱动装置产生受控电子-正电子射流的可能性，其特征与天体物理学中的同类产品相似。此外，我们还演示了通过简单的设置改变，电子束中电子和正电子的相对百分比从纯电子束（最高电荷，因此最高磁场）到中性电子-正电子束（几乎没有电荷，因此没有磁场）的可能性。拟议的研究项目被认为是这些有希望的结果的自然延伸。我们的目的是探测这些激光驱动的电子-正电子喷流在不同密度的背景等离子体中的传播。我们的目的是研究作为气体密度（即比电子-正电子喷射更密集、更稀薄）和电子束中电子和正电子的相对百分比的函数所触发的不同不稳定性。这将使我们能够通过实验表征这些喷流的传播特性，并通过将我们的实验室结果与天体物理喷流的观测结果进行比较，为理解这些神秘的天体物理现象提供一组有用的数据。这些结果不仅会引起天体物理学界的兴趣，也会引起等离子体物理和粒子物理学界的兴趣

Astrophysical jets represent some of the most impressive and intriguing phenomena ever detected in the Universe. They are observed to being ejected from some of the most energetic phenomena ever identified in Nature, such as black holes and pulsars. These jets can propagate, in an extremely collimated way, for enormous distances of the order of kiloparsecs (1 parsec = 3.09 x 10^13 km, i.e. 3 millions of billions of km). Studying these jets is crucial for a thorough insight into the physics of these ultra-massive objects and might contribute towards the understanding of cosmic rays and ultra-high luminosity bursts of gamma-rays. Despite the fundamental interest that these structures excite, their key properties (such as composition, density, and energy) are still lacking a thorough understanding. This is due to the fact that, despite extensive theoretical modelling and observation, clear access to the in-situ relevant physical quantities is obviously impossible. There are only educated guesses around them: for instance, it is widely accepted that most of them should be predominantly constituted of electrons, positrons (the anti-particle of the electron), and gamma-ray photons even though their typical relative percentage and spectrum have not been fully determined yet. An indirect way of inferring their characteristics is by monitoring their interaction with the intergalactic space. Even though the intergalactic space represents the best approximation to a pure vacuum that has been ever observed in Nature (it has an average density of approximately one particle per cubic centimetre), its density is still not exactly zero; it has been observed that, over such enormous distances, even the presence of such a low density medium affects the dynamics of an astrophysical jet inducing filaments, discontinuous propagation and bending. By knowing under what conditions these instabilities can be triggered, it is thus possible to infer the characteristics of these jets.It is thus clear that an in-depth study of the propagation of electron-positron jets in low density gases will play a central role in the understanding of these phenomena. Fortunately, these impressively extended and energetic phenomena are scalable: in other words, by adopting the suitable experimental parameters, it is possible to produce much smaller scale replica (down to a few millimetre size) which will behave in a similar manner. This suggests that it is possible to study astrophysical jets exploiting controlled, smaller-scale reproductions in the laboratory.Our research group has recently demonstrated the possibility of generating controlled electron-positron jets, with characteristics similar to their astrophysical counterparts, using compact laser-driven setups. Moreover, we demonstrated the possibility of tuning, by simple changes in the setup, the relative percentage of electrons and positrons in the beam going from a purely electronic beam (highest charge and, therefore, highest magnetic field) to a neutral electron-positron beam (virtually no charge and, therefore, no magnetic field).The proposed research project is then thought as the natural extension of these promising results. We aim at probing the propagation of these laser-driven electron-positron jets through background plasmas of different density. We aim at studying the different instabilities triggered as a function of the density of the gas (i.e. denser, comparable to and more rarefied than the electron-positron jet) and the relative percentage of electrons and positrons in the beam. This will allow us to experimentally characterise the propagation properties of these jets and, by comparing our laboratory results with observation of astrophysical jets, to provide a set of data useful for understanding these enigmatic astrophysical phenomena.Not only these results will be of interest to the astrophysical community, but also to the plasma physics and particle physics community