A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the K shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the L shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.

Each successive generation of X-ray machines has opened up new frontiers in science, such as the first radiographs and the determination of the structure of DNA. State-of-the-art X-ray sources can now produce coherent high-brightness Xrays of greater than kiloelectronvolt energy and promise a new revolution in imaging complex systems on nanometre and femtosecond scales. Despite the demand, only a few dedicated synchrotron facilities exist worldwide, in part because of the size and cost of conventional (accelerator) technology 1 . Here we demonstrate the use of a new generation of laserdriven plasma accelerators 2 , which accelerate high-charge electron beams to high energy in short distances 3-5 , to produce directional, spatially coherent, intrinsically ultrafast beams of hard X-rays. This reduces the size of the synchrotron source from the tens of metres to the centimetre scale, simultaneously accelerating and wiggling the electron beam. The resulting X-ray source is 1,000 times brighter than previously reported plasma wigglers 6,7 and thus has the potential to facilitate a myriad of uses across the whole spectrum of light-source applications.There are a number of proposals to use extreme nonlinear interactions of the latest generation of high-power ultrashort-pulse laser systems to produce beams of high-energy photons with high brightness and short pulse duration. For example, high-order harmonic generation promises trains of coherent pulselets 8 and Compton scattering could extend energies into the γ -regime 9,10 . An alternative proposal has been the use of compact laser-plasma accelerators to drive sources of undulating/wiggling radiation 11 .These accelerators use the plasma wakefield generated by the passage of an intense laser pulse through an underdense plasma 12 . Such wakefields can have intrinsic fields of more than 1,000 times greater than the best achievable by conventional accelerator technology, and thus can accelerate particles to high energies in a fraction of the distance. Recently, it has been demonstrated that at high laser power, the wakefield can be driven to sufficient amplitude to be able to trap large numbers of particles (>100 pC) from the background plasma and accelerate them in a narrow energy spread beam 3-5 , now producing beams of electrons of gigaelectronvoltscale energy of the order of 1 cm (refs 13,14).Such electron sources are of interest to replace the accelerators that drive current synchrotron sources, and typically use multiple periods of alternately poled magnets (undulators or wigglers) to reinforce the synchrotron emission over a length of a few metres. The first demonstrations of wakefield-driven radiation using external wigglers have also been reported, though still being limited to optical or near-optical wavelengths and modest peak brightness 15,16 .However, the particles being accelerated in the plasma accelerator also undergo transverse (betatron) oscillations when subject to the focusing fields of the plasma wave. These oscillations occur at the betatron frequen...

There are many astrophysical and laboratory scenarios where kinetic effects play an important role. These range from astrophysical shocks and plasma shell collisions, to high intensity laser-plasma interactions, with applications to fast ignition and particle acceleration. Further understanding of these scenarios requires detailed numerical modelling, but fully relativistic kinetic codes are computationally intensive, and the goal of one-to-one direct modelling of such scenarios and direct comparison with experimental results is still difficult to achieve. In this paper we discuss the issues involved in performing kinetic plasma simulations of experiments and astrophysical scenarios, focusing on what needs to be achieved for one-to-one direct modeling, and the computational requirements involved. We focus on code efficiency and new algorithms, specifically on parallel scalability issues, namely on dynamic load balancing, and on high-order interpolation and boosted frame simulations to optimize simulation performance. We also discuss the new visualization and data mining tools required for these numerical experiments and recent simulation work illustrating these techniques is also presented.

Ab-initio numerical study of collisionless shocks in electron-ion unmagnetized plasmas is performed with fully relativistic particle in cell simulations. The main properties of the shock are shown, focusing on the implications for particle acceleration. Results from previous works with a distinct numerical framework are recovered, including the shock structure and the overall acceleration features. Particle tracking is then used to analyze in detail the particle dynamics and the acceleration process. We observe an energy growth in time that can be reproduced by a Fermi-like mechanism with a reduced number of scatterings, in which the time between collisions increases as the particle gains energy, and the average acceleration efficiency is not ideal. The in depth analysis of the underlying physics is relevant to understand the generation of high energy cosmic rays, the impact on the astrophysical shock dynamics, and the consequent emission of radiation.

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The acceleration of electrons to approximately 0.8 GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5x10(18) cm(-3) by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10zR), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a0>6, thus driving a highly nonlinear plasma wave.

Collisionless plasma instabilities are fundamental in magnetic field generation in astrophysical scenarios, but their role has been addressed in scenarios where velocity shear is absent. In this work we show that velocity shears must be considered when studying realistic astrophysical scenarios, since these trigger the collisionless Kelvin-Helmholtz instability (KHI). We present the first selfconsistent three-dimensional (3D) particle-in-cell (PIC) simulations of the KHI in conditions relevant for unmagnetized relativistic outflows with velocity shear, such as active galactic nuclei (AGN) and gamma-ray bursts (GRBs). We show the generation of a strong large-scale DC magnetic field, which extends over the entire shear-surface, reaching thicknesses of a few tens of electron skin depths, and persisting on time-scales much longer than the electron time scale. This DC magnetic field is not captured by MHD models since it arises from intrinsically kinetic effects. Our results indicate that the KHI can generate intense magnetic fields yielding equipartition values up to ǫ B /ǫ p ≃ 10 −3 − 10 −2 in the electron time-scale. The KHI-induced magnetic fields have a characteristic structure that will lead to a distinct radiation signature, and can seed the turbulent dynamo amplification process. The dynamics of the KHI are relevant for non-thermal radiation modeling and can also have a strong impact on the formation of relativistic shocks in presence of velocity shears.

The numerical instability observed in the Electromagnetic-Particle-in-cell (EM-PIC) simulations with a plasma drifting with relativistic velocities is studied using both theory and computer simulations. We derive the numerical dispersion relation for a cold plasma drifting with a relativistic velocity and find an instability attributed to the coupling between the beam modes of the drifting plasma and the electromagnetic modes in the system. The characteristic pattern of the instability in Fourier space for various simulation setups and Maxwell Equation solvers are explored by solving the corresponding numerical dispersion relations. Furthermore, based upon these characteristic patterns we derive an asymptotic expression for the instability growth rate. The asymptotic expression greatly speeds up the calculation of instability growth rate and makes the parameter scan for minimal growth rate feasible even for full three dimensions. The results are compared against simulation results and good agreement is found. These results can be used as a guide to develop possible approaches to mitigate the instability. We examine the use of a spectral solver and show that such a solver when combined with a low pass filter with a cutoff value of | k| essentially eliminates the instability while not modifying modes of physical interest. The use of spectral solver also provides minimal errors to electromagnetic modes in the lowest Brillouin zones.