We study the energetics of wake excitation during the laser–plasma interaction in application to laser wakefield accelerators. We find that both the wake amplitude and the accelerating efficiency ...(transformer ratio) can be maximized by properly shaping the longitudinal profile of the driving laser pulse. The corresponding family of laser pulse shapes is derived in the nonlinear regime of laser–plasma interaction. Such shapes provide theoretical upper limit on the magnitude of the wakefield and efficiency by allowing for uniform photon deceleration inside the laser pulse. We also construct realistic optimal pulse shapes that can be produced in finite-bandwidth laser systems.
One scenario for the generation of fast radio bursts (FRBs) is magnetic reconnection in a current sheet of the magnetar wind. Compressed by a strong magnetic pulse induced by a magnetar flare, the ...current sheet fragments into a self-similar chain of magnetic islands. Time-dependent plasma currents at their interfaces produce coherent radiation during their hierarchical coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations to compute the efficiency of the coherent emission and to obtain frequency scalings. Consistent with expectations, a fraction of the reconnected magnetic field energy, \(f\sim 0.002\), is converted to packets of high-frequency fast magnetosonic waves which can escape from the magnetar wind as radio emission. In agreement with analytical estimates, we find that magnetic pulses of \(10^{47}\text{erg}\;\text{s}^{-1}\) can trigger relatively narrowband GHz emission with luminosities of approximately \(10^{42}\text{erg}\;\text{s}^{-1}\), sufficient to explain bright extragalactic FRBs. The mechanism provides a natural explanation for a downward frequency drift of burst signals, as well as the \(\sim 100\;\text{ns}\) substructure recently detected in FRB 20200120E.
Experiments investigating the physics of interpenetrating, collisionless, ablated plasma flows have become an important area of research in the high-energy-density field. In order to evaluate the ...feasibility of designing experiments that will generate a collisionless shock mediated by the Weibel instability on the National Ignition Facility (NIF) laser, computer simulations using the Center for Radiative Shock Hydrodynamics (CRASH) radiation-hydrodynamics model have been carried out. This paper reports assessment of whether the experiment can reach the required scale size while maintaining the low interflow collisionality necessary for the collisionless shock to form. Comparison of simulation results with data from Omega experiments shows the ability of the CRASH code to model these ablated systems. The combined results indicate that experiments on the NIF are capable of reaching the regimes necessary for the formation of a collisionless shock in a laboratory experiment.
The role of the non-resonant firehose instability in conditions relevant to the precursors of supernova remnant shocks is considered. Using a second order tensor expansion of the Vlasov-Fokker-Planck ...equation we illustrate the necessary conditions for the firehose to operate. It is found that for very fast shocks, the diffusion approximation predicts that the linear firehose growth rate is marginally faster than its resonant counterpart. Preliminary hybrid MHD-Vlasov-Fokker-Planck simulation results using young supernova relevant parameters are presented.
A heat flux in a high-\(\beta\) plasma with low collisionality triggers the whistler instability. Quasilinear theory predicts saturation of the instability in a marginal state characterized by a heat ...flux that is fully controlled by electron scattering off magnetic perturbations. This marginal heat flux does not depend on the temperature gradient and scales as \(1/\beta\). We confirm this theoretical prediction by performing numerical particle-in-cell simulations of the instability. We further calculate the saturation level of magnetic perturbations and the electron scattering rate as functions of \(\beta\) and the temperature gradient to identify the saturation mechanism as quasilinear. Suppression of the heat flux is caused by oblique whistlers with magnetic-energy density distributed over a wide range of propagation angles. This result can be applied to high-\(\beta\) astrophysical plasmas, such as the intracluster medium, where thermal conduction at sharp temperature gradients along magnetic-field lines can be significantly suppressed. We provide a convenient expression for the amount of suppression of the heat flux relative to the classical Spitzer value as a function of the temperature gradient and \(\beta\). For a turbulent plasma, the additional independent suppression by the mirror instability is capable of producing large total suppression factors (several tens in galaxy clusters) in regions with strong temperature gradients.
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