Relazione sulle attività svolte durante il Dottorato Anna Grassi
This Phd thesis concerns the study of collisionless relativistic shocks in magnetised plasma. This research area is of great importance both for the astrophysical community and for the laser-plasma interaction one. In the astrophysical context, relativistic collisionless shocks and the resulting high-energy radiation emission have been identified as the main source of supra-thermal high-energy particles. The purpose of this thesis project is to investigate, by means of analytical models and numerical simulations, the range of laser and plasma parameters and the possible configurations, allowing for laser-driven collisionless shocks generation. Emphasis is placed on the role of magnetic fields, either externally imposed or self-generated by the plasma instabilities. The Weibel or filamentation instability is believed to play a central role in most of the astrophysical events in the formation of collisionless shocks, providing the magnetic turbulences required to build-up the shock front.
I initially focused on the kinetic and hydrodynamic model of magnetized plasmas, computing the analytical dispersion relation and the growth rate for the Weibel instability in the presence of an external magnetic field. The theoretical results obtained have proved to be a useful benchmark for the numerical code I am currently using and developing. Subsequently, an analytical investigation of the late non-linear stage of the instability has been carried out and validated via numerical simulations. This work has been presented both as seminar and poster in several conferences and workshops. An article has been published on Physical Review E.
The validation of the analytical results of the thesis relies on extensive Particle-In-Cell (PIC) simulations and requires the use of dedicated, massively parallel, numerical tools. PIC codes are well established and widely used for modelling the kinetics of plasma. The theoretical group of LULI, of which I am a member, is heavily involved in the development of the code SMILEI (An article has been published on Computer Physics Communications).
In these years I actively participated in the implementation of new numerical schemes in the code such as advanced solvers for the Maxwell’s equations to correctly describe the dynamics of ultra-relativistic particles, and new physical modules, as the radiation reaction force. An article on this topic is in preparation, in collaboration with the Institut de Recherche en Astrophysique et Planétologie, in Toulouse.
Numerical simulations are an essential tool not only to understand the physical mechanisms, but also to design and interpret experimental campaigns. Since the physical mechanisms involved are largely multi-scale, these simulations are numerical extremely expensive and requires the use of thousands of processors in parallel. Using massively parallel supercomputers, on major national and European scientific computing facilities, is therefore essential. A major effort is needed to support the experiences and this is one of the main purpose of the theoretical group at LULI. Two main experimental configurations are currently under investigation for the production of Weibel-mediated collisionless shock. The first one relies upon the interaction of two
counterstreaming plasma flows generated from the ablation of foil targets by nanosecond, high-energy (kJ-class) lasers. A recent experiment on Omega laser facility showed the characteristic filamentary structures of the Weibel instability, but up to now there is no evidence of the shock formation. During a collaboration with the experimental team at LULI, I performed simulations in support of the experimental campaign at TITAN laser facility (Livermore, USA). This study seems to suggest that an external magnetic field might be required to accelerate the development of the instability and observe a shock formation in this configuration with the current laser facilities. The second experimental configuration that has been identified, relies on the interaction of an ultra-intense laser with an overdense plasma. The experimental requirements still need to be properly investigated. In the last year I have performed a series of kinetic simulations varying the laser and
plasma parameters, in order to highlight the role of different acceleration mechanisms in the generation of filamentation instability and the subsequent shock front formation.
Finally, I have proposed a scheme to produce, in the laboratory, the ion-Weibel-instability with the use of an ultra-high-intensity laser. The produced flows are faster and denser than in current experiments, leading to a larger growth rate and stronger magnetic fields. These results are important for the LPI at very high intensity and have been published on Physical Review E. Articoli Pubblicati o in preparazione
[1] A. Grassi , L. Fedeli , A. Sgattoni and A. Macchi. Vlasov simulation of laser-driven shock acceleration and ion turbulence. Plasma Phys. Control. Fusion 58 (2016) 034021
[2] A. Grassi , M. Grech, F. Amiranoff, A. Macchi and C. Riconda, Electron Weibel instability in relativistic counterstreaming plasmas with flow-aligned external magnetic fields. PHYSICAL REVIEW E 95, 023203
(2017).
[3] A. Grassi , M. Grech, F. Amiranoff, A. Macchi and C. Riconda, Radiation-Pressure-Driven Ion-Weibel Instability and Collisionless Shocks. PHYSICAL REVIEW E 96, 033204(2017).
[4] J. Dérouillat, A. Beck, T. Vinci, M. Chiaramello, A. Grassi, F. Pérez, I. Plotnikov, N. Aunais, J. Dargent, C. Riconda and M. Grech , SMILEI: a collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation, in print on Computer Physics Communications.
[5] I. Plotnikov, A. Grassi, M. Grech, Perpendicular relativistic shocks in magnetized pair plasma, To be
submitted.
Conferenze/Workshop (oral presentations)
[1] May 2017– PLAS@PAR Young Researcher’s day, Paris, France. (Oral Presentation) [2] Mar 2017– 8ème Forum Laser et Plasma,Aussois, France. (Poster Presentation)
[3] Dec 2016– Seminar at Institut Jean Lamour, Université de Lorraine, Nancy, France. (Oral Presentation) [4] Nov 2016– Seminar at SLAC National Accelerator Laboratory, Menlo Park CA, USA. (Oral Presentation) [5] Nov 2016– 58th Annual Meeting of the APS Division of Plasma Physics, San Jose CA, USA. (Poster Presentation)
[6] Oct 2016– 1st Chalmers Workshop on Laser Plasma Theory, Gothen- burg, Sweden. (Invited Presentation)
[7] Sept 2016– PRACE Autumn School "Modern HPC Development for Scientists and Engineers", Hagenberg, Austria. (Poster Presentation)
[8] May 2016– 11th International Conference on High Energy Density Laboratory Astrophysics, Stanford CA, USA. (Poster Presentation)
[9] Jan 2016– Scientific Day of PLAS@PAR, Paris, France. (Oral Presentation)
[10] Dec 2015— Atelier "Accélération de particules en astrophysique des hautes énergies", Grenoble, France.
[11] Jun 2015– 42th European Physical Society Conference on Plasma Physics, Lisbon, Portugal. (Poster Presentation)
[12] Jun 2015– 7ème Forum Laser et Plasma, Île de Porquerolles, France. (Poster Presentation) [13] Aprl 2015– Seminar at the Centre Lasers Intenses et Applications, Bordeaux, France. (Oral Presentation)
[14] Oct 2015– Collisionless shock workshop, Paris, France. (Oral Presentation)
School and other educational experiences
[1] Sept 2016— PRACE Autumn School "Modern HPC Development for Scientists and Engineers", Research Institute for Symbolic Computation, Hagenberg, Austria.
[2] Jun 2016— PATC course "High-Performance computing with Python", Julich Supercomputing Centre, Julich, Germany.
[3] Nov 2015— Fête de la Science, stand of the LABEX Plas@Par "Place aux Plasmas!", UPMC, Paris, France. Three days were dedicated to the explanation of basic concepts of plasma physics and simple experiments, to the students of the primary school and general public.
[4] Oct 2015— Journée présentation d'entreprises - Miltenyi Biotec, Cycle "Connaissance des organisations", Département Formation et Carrières, UPMC, Paris France.
[5] Jan/Apr 2015— 10 Seminars on “Histoire des sciences, histoire de l’innovation” Institut de mathématiques de Jussieu, UPMC, Paris, France.
[6] Sept 2014— 3rd LA3NET "Advanced School on Laser Applications at Accelerators", Spanish Pulsed Lasers Centre (CLPU), Salamanca, Spain.
