• Propagation of ultra relativistic fireball beam into plasma

    Ab initio PIC simulations of astrophysical plasmas have provided new directions to identify the role of plasma instabilities in the formation of collisionless shock, the particle acceleration processes, and the generation of magnetic field structures compatible with the intense radiation bursts of synchrotron radiation.
    It has been known that the interaction of ultra-relativistic beam with plasma drives several plasma instabilities. There are three possible main scenarios depending on the direction of the wave-vector respect to the flow; when the wave-vector is aligned to the flow, the purely two-stream instability driven by the peaked nature of the velocity distribution dominates; when the wave-vector is normal to the flow, the filamentation instability is excited; finally, if the wave-vector is at an arbitrary angle respect to the flow, then it is the oblique instability to play a role. As a consequence of all these instabilities, the beam breaks up into narrow and…

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  • Disruption effect

    One of the experiments performed in particle physics consist in colliding two counter-streaming beams, one composed of electrons and the other of positrons. One relativistic beam, composed by charged particles of the same kind, is stable and the coulomb repulsive force is counteracted as 1/2 by the confining magnetic field generated by the charges motion. However when two beams collide the oncoming beam field acts on the bunch as a thin lens focusing the particles on the axis of motion. This focusing produces what is called pinching of the beam: the beam density increases in the focusing spot. After pinching the beam defocuses.

    During the interaction, the beam may suffer none, one or multiple pinching according to the strength of the oncoming beam field and to the particles energy. The presence of these pinching spots alters the beam density profile; at the end of the interaction, the density profile results…

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  • Propagation of quasi-neutral beam into plasma

    Ultra-relativistic outflows are of interest not only for astrophysics but also to understand the interaction of relativistic beams with plasmas. The interaction of particle beams (e− and e+) with plasmas has been studied in the context of plasma wakefield accelerators magnetic field generation via current filamentation instability in GRBs scenarios, particle acceleration in collisionless shocks and inertial confinement fusion.
    It is known that the interaction of relativistic particle beams with plasmas drives several instabilities such as the current filamentation, two stream and oblique instability. In particular, the first one, known also as electromagnetic instability, is able to drive strong transverse magnetic fields that break up the beam into narrow and high current density filaments and lead to the emission of synchrotron radiation.
    Experimentally probing such complex phenomenon is still an open challenge and especially the creation of the astrophysical conditions in laboratory is difficult. However, present laboratory facilities, such as the…

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  • Does light fly straight? In an optical tornado, it spins.

    An intense optical vortex beam, where light possess orbital angular momentum, can be understood as an optical analogue of a tornado, because of a spinning motion of photons in these beams. This exotic dynamics gives rise to exciting phenomena, with profound implications in science. The onset of many of their fascinating properties arises with nonlinear behaviours, which can be exploited only at very high laser intensities and in nontrivial optical media with higher order optical nonlinearities. Plasmas are an example of a nonlinear optical media where the striking nonlinear properties of twisted photons could be explored in depth. Plasmas are also an ideal medium to generate and amplify these beams to very high intensities, beyond breakdown thresholds of conventional materials. The image shows the result of a three dimensional particle-in-cell simulation using the massively parallel, fully relativistic particle-in-cell code Osiris, in a configuration that shows an intense beam of…

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  • Filamentation of Solid target due to interaction of a Laser

    Formation of counter-streaming plasma flow inside solid target
    The movie shows the temporal evolution of the magnetic fields when a 140 fs laser pulse with focused intensity (I = 1016 W/cm2) hits on solid target (ne ~ 1022 cm−3). As the laser hits the target it generates fast electrons. In the beginning of the simulation, we initialised a warm background plasma electron isotropic temperature distribution. The background plasma ions form an immobile cold fluid background. The laser-plasma interaction then introduces an anisotropy in the electron temperature distribution. This anisotropy causes the growth of the Weibel instability, which consists in the formation of several electron current filaments that lead to the magnetic field structures shown in the movie. At the end of the movie, we can also observe the saturation of the Weibel instability, as the magnetic fields amplitudes stop growing exponentially. The Weibel instability is thought to play a critical…

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  • An efficient light-matter-light converter

    One of the most striking nonlinear quantum effect is the production of an electron-positron pair by an electric field in vacuum. The probability of this process was determined by Schwinger in 1951 and shows that the production of electron-positron pairs is exponentially small for electric fields small compared to the Schwinger field (which is the field necessary for an electron or positron to gain its rest mass energy in a Compton length). Unfortunately, despite the tremendous progress observed in the development of ultra-intense lasers in the last decades, the electric fields generated by current lasers are many orders weaker than the Schwinger field, so that many effects have exponentially small probabilities and are therefore unobservable. One can nevertheless observe certain nonlinear quantum effects in fields small compared to the Schwinger field by using ultra relativistic particles such that the field amplitude in the rest frame of the particles will…

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  • Magnetic field generation: the Biermann battery in action

    Magnetic fields are ubiquitous throughout the universe, but their origin is still an open question. One of the most common explanations to this problem is that weak seed magnetic fields are amplified via turbulent mixing.  Misaligned temperature and density gradients generated in shocks can provide this seed magnetic field via the Biermann battery mechanism.
    This mechanism of magnetic field generation also plays an important role in laser-solid experiments. Lasers can ionize a solid film of metal or plastic and generate a temperature gradient with a maximum temperature in the center of the beam. The plasma has a density gradient with a decreasing density away from the target. As the newly ionized plasma expands, these perpendicular gradients drive the Biermann battery and can generate magnetic fields as high as megagauss. These fields have been detecte, for example, in experiments using the Omega EP lasers in Rochester, as well as in experiments…

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  • Generation of miniature magnetised shockwaves

    Solar wind is the name commonly associated with the stream of hot ionized gas, composed of free electrons and protons, released from the sun’s atmosphere. After its emission, the solar wind propagates in interplanetary space and interacts with several objects in our solar system, namely multi-scale magnetic fields. The interaction between the solar wind and earth’s magnetic field is responsible for the formation of our magnetosphere, which deflects the plasma around our planet. Earth’s magnetosphere prevents the solar wind from reaching earth’s atmosphere, protecting us from hazardous space radiation. Therefore, the detailed study of this interaction is relevant to understand space weather phenomena, which impact, for example, satellites operation.
    In general, the typical scales of planetary magnetic fields are much bigger than the kinetic scales associated with the solar wind plasma, and this interaction is considerably well described using macroscopic frameworks like magnetohydrodynamics (MHD). However, when the magnetized object is…

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  • Brillouin amplification: towards next generation laser energy densities

    High-intensity lasers beams allow to create high-energy-density (HED) conditions, that mimic extreme astrophysical scenarios, in controlled laboratory settings. These exciting experiments give us a closer look at complex astrophysical processes that would otherwise have to be inferred indirectly by their radiation collected by our telescopes. Advances in the generation of higher laser powers and intensities is highly desirable, since these will give access to novel astrophysical conditions and exotic  physical regimes of HED science, like “boiling the vacuum”.
    Further increasing current state-of-the are laser energy densities is highly expensive using solid state optics. Unfeasibly massive gratings, mirrors and lenses are required to manage impinging laser intensities below their damage threshold. Plasma-based laser amplifiers, leveraging on parametric processes like stimulated Brillouin backscattering, overcome the optical damage limitations of solid state optics by several orders of magnitude, providing a promising alternative to the production of next generation laser energy densities.
    Stimulated Brillouin backscattering…

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  • Competition between self-modulation and hosing instability in self-modulated wakefield acceleration

    As conventional particle acceleration techniques are hitting technological limits, plasma based acceleration is emerging as a leading technology in future generations of higher energy, compact particle accelerators. Although initially proposed more than 30 years ago by Prof. T. Tajima and Prof. J. Dawson at the University of California in Los Angeles (UCLA), the first ground breaking plasma acceleration experimental results appeared in 2005. Plasma acceleration is presently an active field of research, being pursuit by several leading laboratories (e.g. SLAC, DESY, RAL, LOA, LBNL).
    Electron or positron acceleration in plasma waves is similar to sea wave surfing. Plasma accelerators use an intense laser pulse or particle bunch (boats on water) as driver to excite relativistic plasma waves (sea waves for surfing). Accelerating structures are sustained by plasma electrons and are not affected by physical boundary effects as in conventional accelerators. The plasma accelerator only lasts for the driver transit time…

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