Vlasov simulation of laser-driven shock acceleration and ion turbulence

We present a Vlasov, i.e. a kinetic Eulerian simulation study of nonlinear collisionless ion-acoustic shocks and solitons excited by an intense laser interacting with an overdense plasma. The use of the Vlasov code avoids problems with low particle statistics and allows a validation of particle-in-cell results. A simple original correction to the splitting method for the numerical integration of the Vlasov equation has been implemented in order to ensure the charge conservation in the relativistic regime. We show that the ion distribution is affected by the development of a turbulence driven by the relativistic "fast" electron bunches generated at the laser-plasma interaction surface. This leads to the onset of ion reflection at the shock front in an initially cold plasma where only soliton solutions without ion reflection are expected to propagate. We give a simple analytic model to describe the onset of the turbulence as a nonlinear coupling of the ion density with the fast electron currents, taking the pulsed nature of the relativistic electron bunches into account

Ionien heijastuminen Maan kvasi-poikittaisella keulasokilla Vlasiator-simulaatiossa

A stream of charged particles known as the solar wind constantly flows with supersonic speed in our solar system. As the supersonic solar wind encounters Earth's magnetic field, a bow shock forms where the solar wind is compressed, heated and slowed down. Not all ions of the solar wind pass through the shock but rather a portion are reflected back upstream. What happens to the reflected ions depends on the magnetic field geometry of the shock. In the case where the angle between the upstream magnetic field and the shock normal vector is small, the reflected ions follow the magnetic field lines upstream and form a foreshock region. In this case the shock is called quasi-parallel. In the case of a quasi-perpendicular shock, where the angle is large, the reflected ions gyrate back to the shock, accelerated by the convection electric field. Upon returning to the shock, the ions have more energy and either pass through the shock or are reflected again, repeating the process. Ion reflection is important for accelerating ions in shocks. In this work we study the properties and ion reflection of the quasi-perpendicular bow shock in Vlasiator simulations. Vlasiator is a plasma simulation which models the interaction between solar wind and the Earth's magnetic field. The code simulates the dynamics of plasma using a hybrid-Vlasov model, where ions are represented as velocity distribution functions (VDF) and electrons as magnetohydrodynamic fluid. Two Vlasiator runs are used in this work. The ion reflection is studied by analysing VDFs at various points in the quasi-perpendicular shock. The analysis is performed with reflections in multiple different frames. A virtual spacecraft is placed in the simulation to study shock properties and ion dynamics, such as the shock potential and ion reflection efficiency. These are compared to spacecraft observations and other simulations to test how well Vlasiator models the quasi-perpendicular bow shock. We find that the ion reflection follows a model for specular reflection well in all tested frames, especially in the plane perpendicular to the magnetic field. In addition, the study was extended to model second specular reflections which were also observed. We conclude that the ions in Vlasiator simulations are nearly specularly reflected. The properties of the quasi-perpendicular bow shock are found to be in quantitative agreement with spacecraft observations. Ion reflection efficiency is found to match observations well. Shock potential investigations revealed that spacecraft observations may have large uncertainties compared to the real shock potential

More than mass proportional heating of heavy ions by supercritical collisionless shocks in the solar corona

We propose a new model for explaining the observations of more than mass proportional heating of heavy ions in the polar solar corona. We point out that a large number of small scale intermittent shock waves can be present in the solar corona. The energization mechanism is, essentially, the ion reflection off supercritical quasi-perpendicular collisionless shocks in the corona and the subsequent acceleration by the motional electric field ${\bf E} = - (1/c) {\bf V} \times {\bf B}$. The acceleration due to ${\bf E}$ is perpendicular to the magnetic field, in agreement with observations, and is more than mass proportional with respect to protons, because the heavy ion orbit is mostly upstream of the quasi-perpendicular shock foot. The observed temperature ratios between O$^{5+}$ ions and protons in the polar corona, and between $\alpha$ particles and protons in the solar wind are easily recovered.Comment: 11 pages, 2 figure

Ion acceleration from laser-driven electrostatic shocks

Multi-dimensional particle-in-cell simulations are used to study the generation of electrostatic shocks in plasma and the reflection of background ions to produce high-quality and high-energy ion beams. Electrostatic shocks are driven by the interaction of two plasmas with different density and/or relative drift velocity. The energy and number of ions reflected by the shock increase with increasing density ratio and relative drift velocity between the two interacting plasmas. It is shown that the interaction of intense lasers with tailored near-critical density plasmas allows for the efficient heating of the plasma electrons and steepening of the plasma profile at the critical density interface, leading to the generation of high-velocity shock structures and high-energy ion beams. Our results indicate that high-quality 200 MeV shock-accelerated ion beams required for medical applications may be obtained with current laser systems.Comment: 33 pages, 12 figures, accepted for publication in Physics of Plasma

Laser-driven shock acceleration of monoenergetic ion beams

We show that monoenergetic ion beams can be accelerated by moderate Mach number collisionless, electrostatic shocks propagating in a long scale-length exponentially decaying plasma profile. Strong plasma heating and density steepening produced by an intense laser pulse near the critical density can launch such shocks that propagate in the extended plasma at high velocities. The generation of a monoenergetic ion beam is possible due to the small and constant sheath electric field associated with the slowly decreasing density profile. The conditions for the acceleration of high-quality, energetic ion beams are identified through theory and multidimensional particle-in-cell simulations. The scaling of the ion energy with laser intensity shows that it is possible to generate $\sim 200$ MeV proton beams with state-of-the-art 100 TW class laser systems.Comment: 13 pages, 4 figures, accepted for publication in Physical Review Letter

Solitary versus Shock Wave Acceleration in Laser-Plasma Interactions

The excitation of nonlinear electrostatic waves, such as shock and solitons, by ultraintense laser interaction with overdense plasmas and related ion acceleration are investigated by numerical simulations. Stability of solitons and formation of shock waves is strongly dependent on the velocity distribution of ions. Monoenergetic components in ion spectra are produced by "pulsed" reflection from solitary waves. Possible relevance to recent experiments on "shock acceleration" is discussed.Comment: 4 pages, 4 figure

Ion-acoustic shocks with reflected ions: modeling and PIC simulations

Non-relativistic collisionless shock waves are widespread in space and astrophysical plasmas and are known as efficient particle accelerators. However, our understanding of collisionless shocks, including their structure and the mechanisms whereby they accelerate particles remains incomplete. We present here the results of numerical modeling of an ion-acoustic collisionless shock based on one-dimensional (1D) kinetic approximation both for electrons and ions with a real mass ratio. Special emphasis is made on the shock-reflected ions as the main driver of shock dissipation. The reflection efficiency, velocity distribution of reflected particles and the shock electrostatic structure are studied in terms of the shock parameters. Applications to particle acceleration in geophysical and astrophysical shocks are discussed.Comment: 6 pages, 7 figures, International Workshop "Complex Plasma Phenomena in the Laboratory and in the Universe", January 19-20, 2015, Rome, Ital

Quasiperpendicular high Mach number Shocks

Shock waves exist throughout the universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasi-perpendicular shocks across two orders of magnitude in Alfven Mach number (MA) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted timescale of ~0.3 {\tau}c, where {\tau}c is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and MA and focus on the magnetic structure of such shocks to further show that for the same MA, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.Comment: Accepted and Published in Physical Review Letters (2015