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The cosmic-ray-driven non-resonant instability of Bell (2004) assumes that both the cosmic rays and the background plasma are collisionless. It is a special case of non-resonant instabilities that are active across a range of collisionalities. In laser-plasmas they manifest themselves as heat flow instabilities and instabilities driven by currents of energetic electrons (Bell, Kingham, Watkins & Matthews, 2020). If the concept of collisions is broadened to include scattering by small-scale magnetic fluctuations we find that the cosmic-ray-driven-instability is active on scalelengths greater than the cosmic ray Larmor radius. I will also describe a new Vlasov-Fokker-Planck code (fastVFP) and show that the collisional Weibel instability can be important when a plasma is non-uniformly heated by a laser (collaborative work with Mark Sherlock, LLNL).
We recently proposed that cosmic rays between the knee and ankle can be produced in young compact stellar clusters. This relies on the efficient production of turbulent magnetic fields in the cluster core, and a fast shock launched from the explosion of a massive star. Some preliminary numerical simulations to expl0re these ideas will be presented. Laboratory experiments using high-power lasers could be exploited to provide additional insight.
Pulsars are neutron stars that emit coherent radio beams out of their magnetic poles. However, the origin and exact mechanism of their coherent radio emission are still under investigation. We exploited plasma bunches, clouds of electron-positron pairs, created during spark events in gap regions by utilizing particle-in-cell simulations streaming instabilities and hot bunches to study the emissions. We found that the main parameter influencing the bunch evolution is the initial drift velocity between electrons and positrons. For zero drift, the bunches can freely expand, and adjacent bunches may overlap in the phase space, forming relativistic streaming instability that produces soliton-like waves. Otherwise, the bunches are constrained from expansion and form strong oscillating local electrostatic fields where plasma particles can oscillate and emit coherent radio waves. We also developed a new post-processing technique for calculation of the emitted radio power and found that the bunches constrained from expansion have similar observational characteristics as those observed for pulsars. The estimated radio spectrum contains a flat part for low frequencies and power-law profiles for higher frequencies. Also, the emitted radiation is relativistically beamed along the pulsar dipole axis and oscillates at microsecond scales.
Hot luminous massive stars usually have stellar winds. These winds are accelerated by light scattering or absorption on different particles. Not all particles are equally efficient in gaining momentum. For massive hot stars, the dominant accelerating mechanism is scattering in resonance spectral lines of elements heavier than hydrogen and helium. The momentum is distributed throughout the matter via Coulomb collisions. Both processes, namely acceleration of particles by radiation and transfer of momentum to other particles, will be described in more detail.
The theory of line radiatively driven stellar winds is being verified only by comparison of theoretical stellar spectra based on sophisticated model atmospheres and winds with the observed ones. Based on this comparison, mass-loss rates and terminal wind velocities as main stellar wind parameters are being derived and subsequently used in further astronomical research.
However, direct laboratory experimental verification of the process of particle acceleration of a gas flow by light has not been done yet.
TBD
The long-term evolution of the orbit of a satellite black hole captured by a massive galactic nucleus has been examined. Repetitive transits across the accretion disk slab create a turbulent wake that needs to be further examined in order to estimate the effects of dragging and the amount of material pushed out of the disk plane.
Fast Radio Bursts (FRBs) are transient and fast (millisecond) episodes of very strong and coherent radio emission. The origin of FRBs is still unknown. In one case, the counterpart in X rays has been identified as a magnetar. We present a scenario where coherent emission is produced in the magnetosphere of a neutron star, when two different outflows from the magnetar interact: a baryon-loaded expanding plasma and an electron positron beam. We want to test, through a controlled laser-plasma experiment, the emission of coherent radio pulses via the interaction of a leptonic beam with cavitons.
The magnetic and radiative versions of the Penrose process of extraction of the rotation energy of a magnetized black hole are discussed and their astrophysical relevance is demonstrated. It is shown that realistic magnetized supermassive black holes with mass larger than 10^{9} Solar masses could accelerate protons up to energy 10^{22}eV, while the Srg A* black hole is able to accelerate protons up to 10^{16}eV. The radiative Penrose process due to the radiation of negative-energy photons in the black hole ergosphere could increase energy of protons, properly moving in the black hole ergosphere, at about one order, leading to observationally interesting signatures at the argosphere boundary.
Interaction of ultraintense lasers with matter results in $\gamma$-photon emission mainly via the multiphoton Compton scattering process, a phenomenon being of primary interest of the recently developed and upcoming multi-petawatt laser facilities. The $\gamma$-ray flashes have always been of interest for a wide portion of the scientific community, regardless if the research focuses on the microcosms or the macrocosms. During the last decade the literature on laser generated $\gamma$-photons saw drastic increase [1-3], with theoretical predictions going along the same path as computer simulations. Generating intense $\gamma$-ray flashes is of particular interest for astrophysical studies [4-5].
We aim on presenting our two recently developed schemes on $\gamma$-ray flashes that demonstrate high laser to $\gamma$-photon energy conversion efficiency. The first scheme [6-7] employs a tightly focused ($\lambda^3$ regime [8]) radially polarised laser pulse to generate a collimated $\gamma$-photon beam. The properties of the particle-in-cell particles are imported into Monte-Carlo simulations [9], to simulate the interaction of laser-generated particles with a high-Z material (second target) The second scheme [10] introduces a tailored target, usually with a preplasma [11], in the relativistically near-critical regime, to enhance the laser intensity and obtain significantly high laser to $\gamma$-photon energy conversion efficiency with a 10 PW class laser. By considering $\gamma$-photons contained in a small solid angle, it is demonstrated that the nonlinear Breit-Wheeler pair production process can be observed experimentally at current and next-generation high-power laser facilities [12].
References:
[1] T. Nakamura et al., Phys. Rev. Lett. 108, 195001 (2012)
[2] C. P. Ridgers et al., Phys. Rev. Lett. 108, 165006 (2012)
[3] K. V. Lezhnin et al., Phys. Plasmas 25, 123105 (2018)
[4] M. J. Rees et al., Mon. Not. R. Astron. Soc. 258, 41p-43p (1992)
[5] S. V. Bulanov et al., Plasma Phys. Rep. 41, 1-51 (2015)
[6] P. Hadjisolomou et al., Phys. Rev. E 104, 015203 (2021)
[7] P. Hadjisolomou et al., J. Plasma Phys. 88, 1 (2022)
[8] G. Mourou et al., Plasma Phys. Rep. 28, 12 (2002)
[9] D. Kolenaty et al., Phys. Rev. Res. 4, 023124 (2012)
[10] P. Hadjisolomou et al., Sci. Rep. 12, 17142 (2022)
[11] I. Tsygvintsev et al., Matem. Mod. 34, 3-12 (2023)
[12] A. Macleod et al., Phys. Rev. A 107, 012215 (2023)
TBD
I will start with a brief introduction summarizing my view of what has already been learnt about particle acceleration in both astrophysical settings and in laser-plasma interactions using the test-particle approach. I will then present some recent results on test-particle acceleration at relativistic shock fronts using both analytical and Monte-Carlo techniques [1,2,3]. Contrary to previous expections, these suggest that, in the relativistic regime, perpendicular shocks are just as efficient as parallel shocks. They also reveal that large-scale structure in the unshocked plasma may lead to a significant enhancement of the acceleration efficiency and a hardening of the accelerated particle spectrum in localised regions.
References
[1] Giacinti, G., Kirk, J.G., Astrophysical Journal 863, 18 (2018)
[2] Kirk, J.G., Reville B., Huang Z.Q., Monthly Notices of the Royal Astronomical Society 509, 1022 (2022)
[3] Huang, Z-Q., Reville B., Kirk, J.G., Giacinti, G., Monthly Notices of the Royal Astronomical Society 522, 4959 (2023)
When applied to compute the density jump of a shock, the standard magnetohydrodynamic (MHD) formalism assumes that all the upstream material passes downstream, together with the momentum and energy it carries, and that pressures are isotropic. In a collisionless shock, shock-accelerated particles going back and forth around the front can invalidate the first assumption. In addition, an external magnetic field can sustain stable pressure anisotropies, invalidating the second assumption. Therefore, it is unclear whether or not the density jump of a collisionless shock fulfills the MHD jump. Posible consequences in ICF will be commented.
Astrophysically relevant super-critical quasi-perpendicular magnetized collisionless shocks can be produced and characterized in experiments using high power lasers irradiating solid targets, a background gas jet and a magnetic pulser. In this configuration the presence of both the external magnetic field and the background gas is crucial to observe the development of the collisionless shock structures and the associated particle energization. Results from recent experimental campaigns in this configuration will be presented as well as numerical simulations shedding light on these processes. The scaling of these results to higher energy laser facilities and to oblique magnetized collisionless shocks will also be discussed.
Higher intensity lasers can be used to accelerate protons to several tens of MeV. These protons can be sent on a secondary target to produce neutrons through nuclear reactions. Results from recent experimental campaigns on Apollon and PETAL will be presented characterizing the neutron sources produced. The perspectives of using these sources for studies on the rapid neutron capture process will be discussed.
Fast electron scattering on plasma ions due to stimulated Bremsstrahlung is investigated and modelled. Comparison with Coulomb scattering, at non-relativistic energies, suggests that stimulated Bremsstrahlung scattering can be dominant and observable in radiation-driven, low density, large scale astrophysical plasmas. The conditions are met in the flaring solar corona. The effect of the solar microwave radiation on fast-electron scattering is evaluated from a parameterized flaring corona model. We find that stimulated Bremsstrahlung greatly enhances the fast-electron scattering frequency in the flare magnetic loop, leading the transport of deka-keV electrons to occur in the diffusion regime, characterized by significant precipitation rates. This prediction is consistent with the interpretation of the above-loop-top hard X-ray and microwave emissions from the X3.1 flare of August 24, 2002. Our analysis indicates that stimulated Bremsstrahlung may play an essential role in the dynamics of fast electrons trapped in solar flare loops.
We investigate the effect of azimuthal and axial ambient magnetic fields on the structure and evolution of a magnetized blast wave. The blast wave is driven by a central source of energy and forms a shell that results from the accumulation of interstellar matter behind the shock front. A similarity form of the ambient magnetic field is assumed to obtain self-similar solutions. The model is studied separately for both azimuthal and axial magnetic fields and applied to stellar wind bubbles and supernova remnants respectively using 1D numerical simulations. When the magnetic field is present, the forward shock front goes slower in the azimuthal case and faster in the axial one. For both tangential orientations, the thickness of the shell increases with the magnetic strength. In the azimuthal case, the thermal energy can be converted to magnetic energy near the inner boundary of the shell. Thus the temperature drops and the magnetic field increases at the tangential discontinuity of the stellar wind bubble. In the axial case of a supernova remnant, the numerical solution always follows a special curve in the parameter space given by the self-similar model.
We investigate the evolution of the separation between an adiabatic and a radiative shock. The contact discontinuity between the two gases is subject to the Rayleigh- Taylor and Richtmyer-Meshkov instability with radiative cooling. These dynamics will be studied experimentally for the first time in the optically thin regime using a laser-driven shock configuration. The shock will propagate in a box filled with helium and xenon gases separated by a thin membrane. The shocked helium will be the adiabatic reverse shock and the shocked xenon will be the radiative transmitted shock.
We carried out an experiment to generate photoionised plasma using line radiation. Using the high-power ns laser to irradiate the Ag-coated CH foil to produce an X-ray line source, which then photoionise Ar gas. Produced Ar-photoionised plasma achieved photoionisation parameter > 100 erg-cm s-1, the regime of interest of several astrophysical cases.
We demonstrate that the use of a keV X-ray line source, rather than the usual quasi-blackbody radiation fields normally used in such experiments, has allowed for generating the same ratio of inner-shell to outer-shell photoionization as that expected from a blackbody source with ~keV spectral temperature.
We have compared results from our in-house plasma modelling code with those from Cloudy and found moderately good agreement for the time evolution of electron temperature and average ionisation.
Astrophysical shock waves are among the most powerful particle accelerators in the Universe. Generated by violent interactions of supersonic plasma flows with the interstellar or intergalactic medium, shocks are inferred to heat the plasma, amplify magnetic fields, and accelerate electrons and protons to highly relativistic speeds. However, the exact mechanisms that control magnetic field amplification and energy partition in shocks remain a mystery. This is particularly challenging for high Mach number shocks, such as those associated with supernova remnants or gamma-ray bursts, where the shock structure cannot be directly resolved from observations. I will discuss recent progress in using the combination of fully kinetic simulations and laser-driven laboratory experiments to study high-Mach number collisionless shocks. In particular, I will present results on magnetic field amplification and energy partition, and discuss how experimental measurements are helping benchmark models of the shock microphysics.
TBD
Transport processes are virtually ubiquitous in engineering fluid and plasma problems but their properties are not always well-determined, particularly when complex microphysics is at play. One outstanding example is heat flux, which according to both laser plasma experiments performed at NIF and more recently measurements of astrophysical plasmas becomes strongly suppressed with respect to predictions from Spitzer-Härm when the electron mean-free-path approaches the temperature gradient scale-length. While such information is contained in the results of microscopic-scale numerical simulations close to first principles or experiments it remains in a form that is not suitable for macroscopic modelling. Here we leverage machine learning to produce micro-physics informed transport flux representations applicable to a macroscopic/fluid model description. We address convergence issues arising from noisiness of deep neural networks representations in numerical schemes. Our results apply most specifically to astrophysical plasmas where severe flux reduction may occur while arguably locality of the heat flux function is maintained and represent a promising initial step towards filling the gap between micro and macro description in this important area of modelling.