[en] How much speedup can we expect for large scientific parallel programs running on supercomputers. For insight into this problem we extend the parallel processing environment currently existing on the Cray X-MP (a shared memory multiprocessor with at most four processors) to a simulated N-processor environment, where N greater than or equal to 1. Several large scientific parallel programs from Los Alamos National Laboratory were run in this simulated environment, and speedups were predicted. A speedup of 14.4 on 16 processors was measured for one of the three most used codes at the Laboratory

[en] In the National Ignition Facility (NIF) ICF point design, the cylindrical hohlraum target is illuminated by multiple laser beams through two laser entrance holes on the ends. According to simulations by LASNEX and HYDRA plasma created inside the hohlraum will stream out of the LEH, accelerate to supersonic speeds and then fan out radially. Inside the hohlraum, flows are subsonic. Forward Brillouin scattering can transfer energy between pairs of laser beams (0 and 1) if the following frequency matching condition is satisfied: ω₀ - ω₁ = (k₀ - k₁) · V + |k₀ - k₁| c_s (1) where ω_0.1 and k_0.1 are the frequencies and wave-numbers of the two laser beams, V is the plasma flow velocity and c_s is the local ion sound speed. In the nominal case of equal frequency beams, this requires the component of the plasma flow velocity transverse to the bisector of the beam directions to be sonic, with the resulting transfer being to the downstream beam. In the NIF beam geometry, this is from the outer to inner cones of beams. The physics of this transfer is the same as in beam bending; the difference being that in the case of beam bending the effect is to redistribute power to the downstream side of the single beam. Were significant power transfer to occur in the point design, the delicately tuned implosion symmetry would be spoiled. To directly compensate for the transfer, the incident beam powers would have to be adjusted. The greatest vulnerability in the point design thus occurs at 15.2ns, when the inner beams are at their peak power and are at their nominal design power limit. In this situation, some other means of symmetry control would be required, such as re-pointing. At 15.2ns, the envelope focal intensities of the outer and inner beams are approximately 10¹⁵ and 6.7 10¹⁴ W/cm² respectively. There is little absorption or diffractive spreading of the beams in the crossing region, so these intensities are also representative there. The outer beams are at higher intensity, despite their lower power, because of their smaller spot-size required to clear the LEH at their steeper angle of incidence. The approximate locations of the crossing beams are shown in Fig. (1). There are four cones of NIF beams entering each LEH. The eight quads at 50^o and 44.5^o to the hohlraum axis are termed the outer beams; four quads at 30^o and 23^o form the inner beams. Each cone is arranged symmetrically around the hohlraum axis, staggered in azimuthal angle. Condition (1) is thus a constraint on the 3D plasma flow velocity that differs modestly, depending on precisely which pair of overlapping beams is considered. In order for the correct component of the flow to be sonic, the total flow in general will be supersonic. The potential resonant surfaces therefore lie outside the LEH

[en] Two-dimensional simulations with the BZOHAR hybrid code (kinetic PIC ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability. The simulation physics model provides a first-principles description of several nonlinearities that affect SBBS saturation: wave breaking and ion trapping (and the associated nonlinear frequency shift of the ion wave and nonlinear modification of the ion collisionless kinetic dissipation), two-ion-wave decay instability, harmonic generation, and pump depletion. The simulations address the interplay of these nonlinearities in affecting SBBS saturation as a function of the population of resonant ions, which is controlled by ZT_e/T_i in a single ion species plasma. The competition of various saturation mechanisms and the contrast between one-dimensional and two-dimensional physics are of particular interest. We also examine the role of ponderomotive filamentation in these simulations. The peak reflectivities in two dimensions are less than in one dimension. The two-dimensional reflectivities and ion wave amplitudes in simulations with strong backscatter relax to very small values in times corresponding to less than 40 ps in experimentally relevant conditions, while one-dimensional simulations with the same physical parameters sustain high reflectivities and concomitant ion wave amplitudes for much longer times. Two-dimensional physics facilitates higher ion wave dissipation rates that account for the relaxation and suppression of SBBS. After the crash of the SBBS in two dimensions, the longitudinal ion velocity distribution function supports significant ion Landau damping (because of the combination of finite negative slope and a large tail of energetic ions), which is shown in the simulations to be the critical element in the continued suppression of SBBS after the crash

[en] We study stimulated Raman scattering (SRS) in laser-fusion conditions with the Eulerian Vlasov code ELVIS. Back SRS from homogeneous plasmas occurs in subpicosecond bursts and far exceeds linear theory. Forward SRS and re-scatter of back SRS are also observed. The plasma wave frequency downshifts from the linear dispersion curve, and the electron distribution shows flattening. This is consistent with trapping and reduces the Landau damping. There is some acoustic (ω ∝ κ) activity and possibly electron acoustic scatter. Kinetic ions do not affect SRS for early times but suppress it later on. SRS from inhomogeneous plasmas exhibits a kinetic enhancement for long density scale lengths. More scattering results when the pump propagates to higher as opposed to lower density

[en] The inactivating effect of far UV light on the unicellular blue-green alga Gloeocapsa alpicola could be totally reversed by exposure to blue light immediately after irradiation. However, if the irradiated cells were held in the dark before exposure to blue light, reversal became progressively less efficient, and almost disappeared after 60-80 h holding. Caffeine and acriflavine inhibited loss of photoreversibility, suggesting an involvement of excision functions. Chloramphenicol and rifampicin slightly increased the rate of loss of photoreversibility, indicating that inducible functions play only a minor role. Split UV dose experiments indicated that light-dependent repair remained operational during dark liquid holding. These results provide preliminary evidence for dark repair in G. alpicola. (author)

[en] A steady-state model for Brillouin and Raman backscatter along a laser ray path is presented. The daughter plasma waves are treated in the strong damping limit, and have amplitudes given by the (linear) kinetic response to the ponderomotive drive. Pump depletion, inverse-bremsstrahlung damping, bremsstrahlung emission, Thomson scattering off density fluctuations, and whole-beam focusing are included. The numerical code Deplete, which implements this model, is described. The model is compared with traditional linear gain calculations, as well as 'plane-wave' simulations with the paraxial propagation code pF3D. Comparisons with Brillouin-scattering experiments at the Omega Laser Facility show that laser speckles greatly enhance the reflectivity over the Deplete results. An approximate upper bound on this enhancement is given by doubling the Deplete coupling coefficient. Analysis with Deplete of an ignition design for the National Ignition Facility (NIF), with a peak radiation temperature of 285 eV, shows encouragingly low reflectivity. Doubling the coupling to bracket speckle effects suggests a less optimistic picture. Re-absorption of Raman light is seen to be significant in this design

[en] We developed a reduced description of kinetic effects that is included in a fluid model of stimulated Brillouin backscattering (SBS) in low Z plasmas (e.g. He, Be). Following hybrid-PIC simulations, the modified ion distribution function is parametrized by the width (delta) of the plateau created by trapping around the phase velocity of the SBS-driven acoustic wave. An evolution equation is derived for (delta) , which affects SBS through a frequency shift and a reduced Landau damping. This model recovers the linear Landau damping value for small waves and the time-asymptotic nonlinear frequency shift calculated by Morales and O'Neil. Finally we compare our reduced model with Bzohar simulations of a Be plasma representative of experiments that have shown evidence of ion trapping

[en] In modern 2D and 3D PIC simulations relevant to National Ignition Facility (NIF) parameters, the low frequency magnetic fields associated with the localized fast electron currents generated by Stimulated Raman Scatter have been identified. We consider electron plasma densities from 0.1 to 0.2 of critical density (n_c) and electron plasma temperatures (T_e) from a few keV to over 10 keV in simulations with space scales corresponding to a laser speckle in modeling with our massively parallel PIC code 23. These magnetic fields are ∼ 1 MG, Then the electrons accelerated by the Raman process are magnetized with their Lamor radii on the order of a speckle width. The transport of these hot electrons out of the speckle then becomes a more complex process than generally assumed

[en] An extensive study of the stimulated Brillouin scattering (SBS) in helium-hydrogen plasmas has been performed using a gas jet at the Janus Laser Facility. We observe three regions of reflectivity by varying the probe intensity from 10¹⁴ to 10¹⁶: saturated region, linear region, and near SBS threshold region. In the linear regime, adding small amounts of H to a He plasma reduces the SBS reflectivity by a factor of 4

[en] Beam Synchronous Acquisition (BSA) provides a common infrastructure for aligning data to each individual beam pulse, as required by the Linac Coherent Light Source (LCLS). BSA allows 20 independent acquisitions simultaneously for the entire LCLS facility and is used extensively for beam physics, machine diagnostics and operation. BSA is designed as a part of LCLS timing system and is currently an EPICS record based implementation, allowing timing receiver EPICS applications to easily add BSA functionality to their own record processing. However the lack of real-time performance of EPICS record processing and the increasing number of BSA devices has brought real-time performance issues. The major reason for the performance problem is due to the lack of separation between time-critical BSA upstream processing and non-critical downstream processing. We are improving BSA with thread level programming, breaking the global lock in each BSA device, adding a queue between upstream and downstream processing, and moving out the non-critical downstream to a lower priority worker thread. We are also investigating the use of multiple worker threads for parallel processing in Symmetric Multi-Processor (SMP) system. (author)