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 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.

This paper presents a consistent quantum mechanical model of Child-Langmuir (CL) law, including electron exchange-correlation interaction, electrode's surface curvature, and finite emitter area. The classical value of the CL law is increased by a larger factor due to the electron tunneling through the space-charge potential, and the electron exchange-correlation interaction becomes important when the applied gap voltage Vg and the gap spacing D are, respectively, on the order of Hartree energy level, and nanometer scale. It is found that the classical scaling of Vg(3/2) and D(-2) is no longer valid in the quantum regime, and a new scaling of Vg(1/2) and D(-4) is established. The smooth transition from the classical regime to the quantum regime is also demonstrated.

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 ].

A new laser-driven ion acceleration mechanism has been identified in particle-in-cell simulations of high-contrast-ratio ultraintense lasers with very thin (10s of nm) solid targets [Yin et al., Laser and Particle Beams 24, 291 (2006); Yin et al., Phys. Plasmas 13, 072701 (2007)]. After a brief period of target normal sheath acceleration (TNSA), “enhanced” TNSA follows. In this stage, the laser rapidly heats all the electrons in the target as the target thickness becomes comparable to the skin depth and enhanced acceleration of the ions results. Then, concomitant with the laser penetrating the target, a large accelerating longitudinal electric field is generated that co-moves with the ions. This last phase has been termed the laser “breakout afterburner” (BOA). Earlier work suggested that the BOA was associated with the Buneman instability that efficiently converts energy from the drift of the electrons into the ions. In this Brief Communication, this conjecture is found to be consistent with particle-in-cell simulation data and the analytic dispersion relation for the relativistic Buneman instability.