Magnetic reconnection releases energy explosively as field lines break and reconnect in plasmas ranging from the Earth's magnetosphere to solar eruptions and astrophysical applications. Collisionless kinetic simulations have shown that this process involves both ion and electron kinetic-scale features, with electron current layers forming nonlinearly during the onset phase and playing an important role in enabling field lines to break 1-4 . In larger two-dimensional studies, these electron current layers become highly extended, which can trigger the formation of secondary magnetic islands 5-10 , but the influence of realistic three-dimensional dynamics remains poorly understood. Here we show that, for the most common type of reconnection layer with a finite guide field, the three-dimensional evolution is dominated by the formation and interaction of helical magnetic structures known as flux ropes. In contrast to previous theories 11 , the majority of flux ropes are produced by secondary instabilities within the electron layers. New flux ropes spontaneously appear within these layers, leading to a turbulent evolution where electron physics plays a central role.Thin current layers are the preferred locations for magnetic reconnection to develop. The most common configuration in nature is guide-field geometry, where the rotation of magnetic field across the layer is less than 180 • . Present theoretical ideas of how reconnection proceeds in these configurations are deeply rooted in early analytical work 11 that, if correct, would imply a direct transition to three-dimensional (3D) turbulence due to a broad spectrum of interacting tearing instabilities. At the core of this idea is the notion that a spectrum of tearing instabilities develops across the initial current sheet for perturbations satisfying the local resonance condition. As these modes grow, the resulting magnetic islands would overlap, leading to stochastic magnetic-field lines and a turbulent evolution. Recently, this type of scenario was proposed as a mechanism for accelerating energetic particles during reconnection 12 . Similar ideas for generating turbulence have been studied in fusion plasmas 13 using resistive magnetohydrodynamics (MHD) and two-fluid 14 models. Alternatively, other researchers have imposed turbulent fluctuations within MHD models in an attempt to understand the consequences 15 . In either case, these results are not applicable to the highly collisionless environment of the magnetosphere, where reconnection is initiated within kinetic ion-scale current layers. The ability to study the self-consistent generation of turbulence during magnetic reconnection with first-principles 3D simulations has only become feasible in the past year.1 Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, 2 University of California San Diego, La Jolla, California 92093, USA. *e-mail: daughton@lanl.gov. tearing instability gives rise to flux ropes as illustrated by an isosurface of the particle density coloured by the magnitude of the curr...

The algorithms, implementation details, and applications of VPIC, a state-of-the-art first principles 3D electromagnetic relativistic kinetic particle-in-cell code, are discussed. Unlike most codes, VPIC is designed to minimize data motion, as, due to physical limitations (including the speed of light!), moving data between and even within modern microprocessors is more time consuming than performing computations. As a result, VPIC has achieved unprecedented levels of performance. For example, VPIC can perform ∼0.17 billion cold particles pushed and charge conserving accumulated per second per processor on IBM’s Cell microprocessor—equivalent to sustaining Los Alamos’s planned Roadrunner supercomputer at ∼0.56 petaflop (quadrillion floating point operations per second). VPIC has enabled previously intractable simulations in numerous areas of plasma physics, including magnetic reconnection and laser plasma interactions; next generation supercomputers like Roadrunner will enable further advances.

Using first-principles fully kinetic simulations with a Fokker-Planck collision operator, it is demon strated that Sweet-Parker reco=ection layers are unstable to a chain of plasmoids (secondary is lands) for Lundquist numbers beyond S ;C; 1000. The instability is increasingly violent at higher Lundquist number, both in terms of the number of plasmoids produced and the super-Alfvenic growth rate. A dramatic enhancement in the reconnect ion rate is observed when the half-thickness of the current sheet between two plasmoids approaches the ion inertial length. During this transi tion, the reconnection electric field rapidly exceeds the runaway limit, resulting in the formation of electron-scale current layers that are unstable to the continual formation of new plasmoids.PACS numbers: 52.35. Vd, 52.35.Py, The conversion of magnetic energy into kinetic energy through the process of magnetic reconnect ion remains one of the most challenging and far reaching problems in plasma physics. One key issue is the scaling of the recon nection dynamics for applications where the system size is vastly larger than the kinetic scales. The magnetohy drodynamics (MHD) model should provide an accurate description of collisional reconnect ion where the resis tive layers are larger than the ion kinetic scale. tween islands approach the ion kinetic scale. This regime is typically referred to as kinetic or fast reconnection since a variety of two-fluid and kinetic models predict rates that are weakly dependent on the system size and dis sipation mechanism [6,7] (the precise scalings are still a subject of controversy [8,9]). In neutral sheet geom etry, both two-fluid simulations [10,11] and theory [12J predict an abrupt transition from the collisional to the kinetic regime when 8 sp ::; d i where d i is the ion inertial length. In this geometry, d i is comparable to the ion gy roradius, and in the kinetic regime d i is also compa.rable to the ion crossing orbit scale.Recently, this transition between collisional and kinetic reconnection was proposed as the central mechanism in regulating coronal heating [13-15J. However, these es timates were based on the assumption of a stable SP layer within the collisional regime. To properly describe the dynamics at high Lundquist number, it is crucial to consider how plasmoid formation ma.y influence the tran sition. To address this problem, this work employs fully kinetic particle-in-cell (PIC) simulations with a Monte Carlo treatment [16] of the Fokker-Planck collision op erator. For Lundquist numbers where the SP layers are stable 5 ;s 1000, this powerful first-principles approach has demonstrated a clear transition between the colli sional and kinetic regimes near the expected thresholdHere we demonstrate that SP layers are in creasingly unstable to plasmoid formation in large-scale systems. The observed growth rate is super-Alfvenic, al lowing the islands to grow to large amplitude before they are convected downstream. A dramatic enhancement in the reconnection rate is observed when the current...

A new laser-driven ion acceleration mechanism using ultrathin targets has been identified from particle-in-cell simulations. After a brief period of target normal sheath acceleration (TNSA) [S. P. Hatchett et al., Phys. Plasmas 7, 2076 (2000)], two distinct stages follow: first, a period of enhanced TNSA during which the cold electron background converts entirely to hot electrons, and second, the “laser breakout afterburner” (BOA) when the laser penetrates to the rear of the target where a localized longitudinal electric field is generated with the location of the peak field co-moving with the ions. During this process, a relativistic electron beam is produced by the ponderomotive drive of the laser. This beam is unstable to a relativistic Buneman instability, which rapidly converts the electron energy into ion energy. This mechanism accelerates ions to much higher energies using laser intensities comparable to earlier TNSA experiments. At a laser intensity of 1021W∕cm2, the carbon ions accelerate as a quasimonoenergetic bunch to 100s of MeV in the early stages of the BOA with conversion efficiency of order a few percent. Both are an order of magnitude higher than those realized from TNSA in recent experiments [Hegelich et al., Nature 441, 439 (2006)]. The laser-plasma interaction then evolves to produce a quasithermal energy distribution with maximum energy of ∼2GeV.

A new laser-driven ion acceleration mechanism has been identified using particle-in-cell (PIC) simulations. This mechanism allows ion acceleration to GeV energies at vastly reduced laser intensities compared with earlier acceleration schemes. The new mechanism, dubbed “Laser Break-out Afterburner” (BOA), enables the acceleration of carbon ions to greater than 2 GeV energy at a laser intensity of only 1021W/cm2, an intensity that has been realized in existing laser systems. Other techniques for achieving these energies in the literature rely upon intensities of 1024W/cm2or above, i.e., 2–3 orders of magnitude higher than any laser intensity that has been demonstrated to date. Also, the BOA mechanism attains higher energy and efficiency than target normal sheath acceleration (TNSA), where the scaling laws predict carbon energies of 50 MeV/u for identical laser conditions. In the early stages of the BOA, the carbon ions accelerate as a quasi-monoenergetic bunch with median energy higher than that realized recently experimentally.