Prof Dmitri Uzdensky

Abstract:

We present two-dimensional particle-in-cell simulations of a magnetised, collisionless, relativistic pair plasma subjected to combined velocity and magnetic field shear, a scenario typical at intermittent structures in plasma turbulence. We create conditions where only the Kelvin–Helmholtz instability (KHI) and drift–kink instability (DKI) can develop, while tearing modes are forbidden. The interaction of DKI and KHI generates qualitatively new structures, marked by a thickened shear layer with very weak electromagnetic field, modulated by KH vortices. Over a range of moderately strong velocity shears explored, the interaction of DKI and KHI results in a significant enhancement of dissipation over cases with only velocity shear or only magnetic shear. Moreover, we observe a new and efficient way of particle acceleration where particles are stochastically accelerated by the motional electric field exterior to the shear layer as they meander in an S-shaped pattern in and out of it. This process takes advantage of the bent geometry of the shear layer caused by the DKI–KHI interaction and is responsible for most of the highest-energy particles produced in our simulations. These results further our understanding of dissipation and particle acceleration at intermittent structures, which are present in plasma turbulence across a wide range of astrophysical contexts such as in active galactic nucleus jet sheaths, potentially relevant to limb-brightened emission, etc., and highlight the sensitivity of dissipation to multiple interacting instabilities, thus providing a strong motivation for further studies of their nonlinear interaction at the kinetic level.

Abstract:

Relativistic magnetic reconnection has been proposed as an important non-thermal particle acceleration (NTPA) mechanism that generates power-law spectra and high-energy emissions. Power-law particle spectra are in general characterised by three parameters: the power-law index, the high-energy cutoff and the low-energy cutoff (i.e. the injection energy). Particle injection into the non-thermal power law, despite also being a critical step in the NTPA chain, has received considerably less attention than the subsequent acceleration to high energies. Open questions on particle injection that are important for both physical understanding and astronomical observations include how the upstream magnetisation influences the injection energy and the contributions of the known injection mechanisms (i.e. direct acceleration by the reconnection electric field, Fermi kicks and pickup acceleration) to the injected particle population. Using fully kinetic particle-in-cell simulations, we uncover these relationships by systematically measuring the injection energy and calculating the contributions of each acceleration mechanism to the total injected particle population. We also present a theoretical model to explain these results. Additionally, we compare two- and three-dimensional simulations to assess the impact of the flux-rope kink and drift-kink instability on particle injection. We conclude with comparisons with previous work and outlook for future work.

Abstract:

ABSTRACT Turbulent, relativistic non-thermal plasmas are ubiquitous in high-energy astrophysical systems, as inferred from broad-band non-thermal emission spectra. The underlying turbulent non-thermal particle acceleration (NTPA) processes have traditionally been modelled with a Fokker–Planck (FP) diffusion–advection equation for the particle energy distribution. We test FP-type NTPA theories by performing and analysing particle-in-cell simulations of turbulence in collisionless relativistic pair plasma. By tracking large numbers of particles in simulations with different initial magnetization and system size, we first test and confirm the applicability of the FP framework. We then measure the FP energy diffusion (D) and advection (A) coefficients as functions of particle energy $\gamma m c^2$, and compare their dependence to theoretical predictions. At high energies, we robustly find $D \sim \gamma ^2$ for all cases. Hence, we fit $D = D_0 \gamma ^2$ and find a scaling consistent with $D_0 \sim \sigma ^{3/2}$ at low instantaneous magnetization $\sigma (t)$, flattening to $D_0 \sim \sigma$ at higher $\sigma \sim 1$. We also find that the power-law index $\alpha (t)$ of the particle energy distribution converges exponentially in time. We build and test an analytic model connecting the FP coefficients and $\alpha (t)$, predicting $A(\gamma) \sim \gamma \log \gamma$. We confirm this functional form in our measurements of $A(\gamma ,t)$, which allows us to predict $\alpha (t)$ through the model relations. Our results suggest that the basic second-order Fermi acceleration model, which predicts $D_0 \sim \sigma$, may not be a complete description of NTPA in turbulent plasmas. These findings encourage further application of tracked particles and FP coefficients as a diagnostic in kinetic simulations of various astrophysically relevant plasma processes like collisionless shocks and magnetic reconnection.

Abstract:

Magnetic field generation in ultraintense laser-solid interactions is studied over a range of laser intensities relevant to next-generation laser facilities (a0=50–500) using two-dimensional (2D) particle-in-cell simulations including strong-field quantum electrodynamic effects. It is found that fields O(0.1) MT (1 GG) may be generated by relativistic electrons traveling along the surface of the target. However, a significant fraction of the energy budget is converted to high-energy photons, approximately 38% at a0=500, greatly reducing the available energy for field generation. A model for the evolution of the target-surface fields is created and the scaling of the field strength with a0 is extracted from a set of 2D simulations. The simulated scaling allows for the estimation of field strengths and the model gives insight into the evolution of the fields on the next generation of laser facilities, a necessary component to the proposal of any future magnetized experiment.

Abstract:

We present the largest 3D particle-in-cell shearing-box simulations of turbulence driven by the magnetorotational instability, for the first time employing the realistic proton-to-electron mass ratio. We investigate the energy partition between relativistically hot electrons and subrelativistic ions in turbulent accreting plasma, a regime relevant to collisionless, radiatively inefficient accretion flows around supermassive black holes such as those targeted by the Event Horizon Telescope. We provide a simple empirical formula to describe the measured heating ratio between ions and electrons, which can be used for more accurate global modeling of accretion flows with standard fluid approaches such as general-relativistic magnetohydrodynamics.