Dynamics of relativistic leptonic jets in low-density plasmas

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

The proposed project combines fields of research of high interest and impact, for the scientific community as well as for the general audience. The small-scale reproduction in the laboratory of some of the most energetic objects ever observed in the known Universe - by means of the most intense pulses ever artificially generated - represents indeed an intriguing and attractive scenario, expected to capture the imagination of the layman as well as the academic researcher. We thus envisage that a successful output from this research project would result in the following impact and outreach activities:

1. Publication of high-profile research articles: the proposed research programme is expected to generate data of great interest for a wide range of physical subjects. Firstly, it will help towards the understanding of fundamental plasma physics in the ultra-relativistic regime. The interaction of electron-positron beams with electron-ion plasma, though extensively studied theoretically, is scarcely investigated experimentally, due to obvious challenges that it presents. Secondly, it will represent one of the first examples of effective laboratory-scaled study of astrophysical phenomena. This is a field of extreme interest for the astrophysical community since it is seen as the most probable pathway towards a deeper understanding of astrophysical observations. Finally, the production and study of laser-generated ultra-relativistic positron jets will be of great interest for nuclear and particle physicists since it represent a fundamental step towards the first-ever construction of all-optical anti-matter accelerators and matter-antimatter colliders. These machines would be remarkably cheaper and more compact than conventional accelerators and will finally allow for University-based experimental research on nuclear and particle physics. For these reasons, we believe that our results need to be made accessible to the scientific community via publication in world-wide renowned scientific journals.

2. Presentation of the research results at conferences and seminars: for the above-mentioned reasons, we believe that a more capillary distribution of our results to the scientific community can be reached by presenting our work in conferences and seminars. This will give the possibility of directly engaging with the community, sharing ideas, and building further collaborations to extend our results in the future.

3. Outreach: as mentioned previously, we believe that our research will be of great interest also for the general public. Even though some detailed aspects of our research may be of difficult comprehension for people outside academia, the public can easily appreciate the main scientific ideas behind our research. In this respect, we plan to organise outreach events, such as seminars open to the public, in order to ensure that these ideas can be diffused as widely as possible. We believe that this would also contribute to improve the general conception of science in the general audience.