Prof Dmitri Uzdensky

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.

Abstract:

We present results from x-ray imaging of high-aspect-ratio magnetic reconnection experiments driven at the National Ignition Facility. Two parallel, self-magnetized, elongated laser-driven plumes are produced by tiling 40 laser beams. A magnetic reconnection layer is formed by the collision of the plumes. A gated x-ray framing pinhole camera with micro-channel plate detector produces multiple images through various filters of the formation and evolution of both the plumes and current sheet. As the diagnostic integrates plasma self-emission along the line of sight, two-dimensional electron temperature maps ⟨ T e ⟩ Y are constructed by taking the ratio of intensity of these images obtained with different filters. The plumes have a characteristic temperature ⟨ T e ⟩ Y = 240 ± 20 eV at 2 ns after the initial laser irradiation and exhibit a slow cooling up to 4 ns. The reconnection layer forms at 3 ns with a temperature ⟨ T e ⟩ Y = 280 ± 50 eV as the result of the collision of the plumes. The error bars of the plumes and current sheet temperatures separate at 4 ns, showing the heating of the current sheet from colder inflows. Using a semi-analytical model, we survey various heating mechanisms in the current sheet. We find that reconnection energy conversion would dominate at low density ( n e ≲ 7 × 10 18 cm⁻³) and electron-ion collisional drag at high-density ( ≳ 10 19 cm⁻³).

Abstract:

We present 3D fully kinetic shearing-box simulations of pair-plasma magnetorotational turbulence with unprecedented macro-to-microscopic scale separation. While retrieving the expected fluid behavior of the plasma at large scales, we observe a steepening of turbulent spectra at kinetic scales and substantial angular-momentum transport linked with kinetic processes. For the first time, we provide a definitive demonstration of nonthermal particle acceleration in kinetic magnetorotational turbulence agnostically of shearing-box initial conditions by means of a novel strategy exploiting synchrotron cooling.