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.
A bstract. VP1C [L 2]. a first-principles 3d electromngnetic ch arge-conserving rpla ti vis l ic kinetic p mticle-ill-cell (PIC) code, was recently adapted to run Oll Los Alamos's ROHd f'l lll lte r [3), th e first supercomputer to break a petaAop (10 1 ::; Aoating point op erations per seconcl) in t lt e TOP500 supercomputer performance rankiugs. [4 1 \Ve give a brief overview of t hc .uoddi ng capcl bilities and optimization techniques used in VPTC and the comp ut a t ional cll< trclc l ['is ies of pel as u tle supercomputers like Roadrunner. \Ve then discuss tlJrec a pplicatio ns c mlbk,cl by VPIC's unprecedented performance on Roadrunner: nlodeling laser plElSma in lencctioll ill upcomin g inertial confinement fusion experiments at the National Ignition Facil it.y (i\ IF) [0,6], mod e ling short pulse la.ser GeV ion acceleration l7-1O] and modeling recoll nec t ion in n mgllt~tic confinement fusion experiments [11 ].
SUMMARYFor many scientiÿc and engineering applications, it is often desirable to use unstructured grids to represent complex geometries. Unfortunately, the data structures required to represent discretizations on such grids typically result in extremely ine cient performance on current high-performance architectures. Here, we introduce a grid framework using patch-wise, regular reÿnement that retains the exibility of unstructured grids, while achieving performance comparable to that seen with purely structured grids. This approach leads to a grid hierarchy suitable for use with geometric multigrid methods, thus combining asymptotically optimal algorithms with extremely e cient data structures to obtain a powerful technique for large scale simulations.
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