Dynamics of relativistic leptonic jets in low-density plasmas

天体物理喷流代表了宇宙中有史以来发现的一些最令人印象深刻和最有趣的现象。它们被观察到从自然界中发现的一些最具活力的现象中喷射出来，如黑洞和黑洞。这些喷流可以以极其准直的方式传播数千秒差距（1秒差距= 3.09 × 10^13公里，即3百万亿公里）的巨大距离。研究这些喷流对于深入了解这些超大质量天体的物理学至关重要，并可能有助于理解宇宙射线和伽马射线的超高亮度爆发。尽管这些结构激发了人们的基本兴趣，但它们的关键特性（如成分，密度和能量）仍然缺乏彻底的了解。这是因为，尽管进行了广泛的理论建模和观测，但显然不可能获得现场相关的物理量。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，尽管它们典型的相对百分比和光谱还没有完全确定。一个间接推断它们特性的方法是通过监测它们与星系际空间的相互作用。尽管星系际空间代表了自然界中观察到的最接近纯真空的空间，（它的平均密度大约是每立方厘米一个粒子），它的密度仍然不完全为零;已经观察到，在如此巨大的距离上，即使存在这样的低密度介质也会影响天体物理学喷流诱导细丝的动力学，不连续扩展和弯曲。通过了解在什么条件下可以触发这些不稳定性，从而可以推断这些喷流的特征。因此，很明显，对低密度气体中电子-正电子喷流传播的深入研究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充满活力的现象是可扩展的：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，有可能在实验室中利用受控的小规模复制来研究天体物理喷流。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-正电子喷流的可能性，其特征与天体物理喷流相似。此外，我们还证明了通过简单的改变装置，将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）调整到中性电子-正电子束（几乎没有电荷，因此没有磁场）的可能性。我们的目标是探测这些激光驱动的电子-正电子喷流通过不同密度的背景等离子体的传播。我们的目的是研究不同的不稳定性触发的气体密度的函数（即密度，可比和更稀薄的电子-正电子射流）和电子和正电子在光束中的相对百分比。这将使我们能够通过实验研究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，而且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