Paul Gibbon

Short-pulse laser - plasma interactions

Recent theoretical and experimental research with short-pulse, high-intensity lasers is surveyed with particular emphasis on new physical processes that occur in interactions with low- and high-density plasmas. Basic models of femtosecond laser - solid interaction are described including collisional absorption, transport, hydrodynamics, fast electron and hard x-ray generation, together with recently predicted phenomena at extreme intensities, such as gigagauss magnetic fields and induced transparency. New developments in the complementary field of nonlinear propagation in ionized gases are reviewed, including field ionization, relativistic self-focusing, wakefield generation and scattering instabilities. Applications in the areas of x-ray generation for medical and biological imaging, new coherent light sources, nonlinear wave guiding and particle acceleration are also examined.

A massively parallel, multi-disciplinary Barnes-Hut tree code for extreme-scale N-body simulations

The efficient parallelization of fast multipole-based algorithms for the N-body problem is one of the most challenging topics in high performance scientific computing. The emergence of non-local, irregular communication patterns generated by these algorithms can easily create an insurmountable bottleneck on supercomputers with hundreds of thousands of cores. To overcome this obstacle we have developed an innovative parallelization strategy for Barnes–Hut tree codes on present and upcoming HPC multicore architectures. This scheme, based on a combined MPI–Pthreads approach, permits an efficient overlap of computation and data exchange. We highlight the capabilities of this method on the full IBM Blue Gene/P system JUGENE at Julich Supercomputing Centre and demonstrate scaling across 294,912 cores with up to 2,048,000,000 particles. Applying our implementation pepc to laser–plasma interaction and vortex particle methods close to the continuum limit, we demonstrate its potential for ground-breaking advances in large-scale particle simulations.

A massively space-time parallel N-body solver

We present a novel space-time parallel version of the Barnes-Hut tree code PEPC using PFASST, the Parallel Full Approximation Scheme in Space and Time. The naive use of increasingly more processors for a fixed-size N-body problem is prone to saturate as soon as the number of unknowns per core becomes too small. To overcome this intrinsic strong-scaling limit, we introduce temporal parallelism on top of PEPCs existing hybrid MPI/PThreads spatial decomposition. Here, we use PFASST which is based on a combination of the iterations of the parallel-in-time algorithm parareal with the sweeps of spectral deferred correction (SDC) schemes. By combining these sweeps with multiple space-time discretization levels, PFASST relaxes the theoretical bound on parallel efficiency in parareal. We present results from runs on up to 262,144 cores on the IBM Blue Gene/P installation JUGENE, demonstrating that the spacetime parallel code provides speedup beyond the saturation of the purely space-parallel approach.

Dominance of Radiation Pressure in Ion Acceleration with Linearly Polarized Pulses at Intensities of 10(21) W cm(-2)

A novel regime is proposed where, by employing linearly polarized laser pulses at intensities 10(21) W cm(-2) (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys. Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch, undergoing a hybrid light-sail-target normal sheath acceleration. Under an appropriate coupling condition between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic C(6+) beams are obtained by linearly polarized laser pulses at intensities of 10(21) W cm(-2).

Spectral shaping of laser generated proton beams

The rapid progress in the field of laser particle acceleration has stimulated a debate about the promising perspectives of laser based ion beam sources. For a long time, the beams produced exhibited quasi-thermal spectra. Recent proof-of-principle experiments demonstrated that ion beams with narrow energy distribution can be generated from special target geometries. However, the achieved spectra were strongly limited in terms of monochromacity and reproducibility. We show that microstructured targets can be used to reliably produce protons with monoenergetic spectra above 2 MeV with less than 10% energy spread. Detailed investigations of the effects of laser ablation on the target resulted in a significant improvement of the reproducibility. Based on statistical analysis, we derive a scaling law between proton peak position and laser energy, underlining the suitability of this method for future applications. Both the quality of the spectra and the scaling law are well reproduced by numerical simulations.

Trends in Supercomputing: The European Path to Exascale

Abstract Recent developments in European supercomputing are reviewed covering both the latest hardware trends and the increasing difficulties faced by scientists in utilising these machines to perform large-scale numerical simulations. These challenges are reflected in the large number of international initiatives which have come into being over the last few years, founded in anticipation of exascale hardware which is foreseen within the next decade. The role of a key institution in supercomputing within these programmes is described using the example of the Julich Supercomputing Centre (JSC), and progress in setting up its own community-oriented support units for scientific computing – Simulation Laboratories – is reported on. Finally, an assessment is made of some common grand challenges and their suitability for scaling to exaflop-scale computation.

Steering UNICORE applications with VISIT

The UNICORE (UNiform Interface to COmputing REsources) software provides a Grid infrastructure together with a computing portal for engineers and scientists to access supercomputer centres from anywhere on the Internet. While UNICORE is primarily designed for the submission and control of batch jobs, it is also feasible to establish an on-line connection between an application and the UNICORE user-client. This opens up the possibility of performing on-line visualization and computational steering of applications under UNICORE control while maintaining the security provided by this system. This contribution describes the design of a steering extension to UNICORE based on the steering toolkit VISIT (VISualization Interface Toolkit). VISIT is a lightweight library that supports bidirectional data exchange between visualizations and parallel applications. As an example application, a parallel simulation of a laser-plasma interaction that can be steered by an AVS/Express application is presented.

Tree-code simulations of proton acceleration from laser-irradiated wire targets

Recent experiments using Terawatt lasers to accelerate protons deposited on thin wire targets are modeled with a new type of gridless plasma simulation code. In contrast to conventional mesh-based methods, this technique offers a unique capability in emulating the complex geometry and open-ended boundary conditions characteristic of contemporary experimental conditions. Comparisons of ion acceleration are made between the tree code and standard particle-in-cell simulations for a typical collisionless “hole boring” scenario in slab geometry. The utility of the gridless approach is emphasized by a series of simulations in “wire” geometry, in which electrons are permitted to circulate around the target at arbitrary distances from the focal region. The simulations reveal a number of features in common with recent experimental observations, including a disclike emission pattern of the MeV protons accelerated away from the wire.

Enhanced hard x-ray emission from microdroplet preplasma

We perform a comparative study of hard x-ray emission from femtosecond laser plasmas in 15μm methanol microdroplets and Perspex target. The hard x-ray yield from droplet plasmas is ≃68 times more than that obtained from solid plasmas at 2×1015Wcm−2. A 10ns prepulse at about 5% of the main pulse appears to be essential for hard x-ray generation from droplets. Hot electron temperature of 36keV is measured from the droplets at 8×1014Wcm−2, whereas a three times higher intensity is needed to obtain similar hot electron temperatures from Perspex plasmas. Particle-in-cell simulations with very long scale-length density profiles support experimental observations.

Tunable Circularly Polarized Terahertz Radiation from Magnetized Gas Plasma

It is shown, by simulation and theory, that circularly or elliptically polarized terahertz radiation can be generated when a static magnetic (B) field is imposed on a gas target along the propagation direction of a two-color laser driver. The radiation frequency is determined by √[ω(p)(2)+ω(c)(2)/4]+ω(c)/2, where ω(p) is the plasma frequency and ω(c) is the electron cyclotron frequency. With the increase of the B field, the radiation changes from a single-cycle broadband waveform to a continuous narrow-band emission. In high-B-field cases, the radiation strength is proportional to ω(p)(2)/ω(c). The B field provides a tunability in the radiation frequency, spectrum width, and field strength.