Session YI2: Laser-plasma Interactions, Kinetic Effects and Invited Post-deadline

Saturation of multi-beams laser plasma interactions by turbulent ion heating

Overlapping multiple laser beams in plasmas, as is often done in ICF or HEDP experiments, can lead to a rich variety of laser-plasma interactions. In particular, plasma-induced power transfer between overlapping laser beams has been successfully used to tune the implosion symmetry of ignition capsules since the beginning of the National Ignition Campaign in 2009. However, recent experiments have shown that power transfer is saturated, even though the associated plasma waves are typically too small (dn/n$\sim$ 10$^{-4}$) to trigger non-linear saturation mechanisms. In this presentation, we will show that overlapping multiple laser beams in plasmas can lead to strong turbulent ion heating and saturation of laser-plasma interactions. The heating rate is found to be of several keVs/ns, which can have a significant effect on the local hydrodynamics conditions at the entrance holes of ignition targets where the laser beams overlap. The ion heating has also been identified as the main saturation mechanism for cross-beam power transfer observed in NIF experiments, reducing the linear gains by as much as 4-5x. It also prevents reamplification of the stimulated Brillouin scattering generated inside the targets by the incoming laser beams at the entrance holes, which had been a concern for NIF experiments but could never be demonstrated experimentally. This mechanism was investigated using a new 3D ``gridless'' particle code with binary collisions. A reduced model based on the weak turbulence theory was also developed, and is found to be in good agreement with the particle code's results. The implementation of this reduced model in radiative-hydrodynamics codes will be presented.   

Experimental validation of the two-plasmon-decay common-wave process

Direct-drive inertial confinement fusion requires multiple overlapping laser beams that can drive the two-plasmon-decay (TPD) instability. When multiple overlapping laser beams with polarization smoothing are used, the total energy in TPD-generated hot electrons was shown to scale with the overlapped intensity.\footnote{C. Stoeckl\textit{ et al.}, Phys. Rev. Lett. \textbf{90}, 235002 (2003).} This scaling would not be expected if the beams drive the TPD independently according to the single-plane wave growth rates. Experiments were conducted on OMEGA EP, in large-scale-length plasmas, to validate the common-wave process, where the total energy in hot electrons is measured to be similar when one or two polarized beams are used at the same overlapped intensity and significantly reduced when four beams with the same overlapped intensity are used.\footnote{D. T. Michel \textit{et al}., ``Experimental Validation of the Two-Plasmon-Decay Common-Wave Process,'' submitted to Physical Review Letters.} A theoretical model of the common-wave process shows that multiple laser beams can share an electron-plasma wave in the region bisecting the electromagnetic wave vectors. For two beams, this region defines a plane; for four beams, it defines a line. In this region, the convective gain of the dominant mode is proportional to the overlapped intensity and a geometric factor. When the TPD instability is saturated, the hot-electron temperature increases rapidly (25 keV to 90 keV), consistent with Zakharov simulations.\footnote{D. H. Froula\textit{ et al.}, Phys. Rev. Lett. \textbf{108}, 165003 (2012).} This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.\\[4pt] In collaboration with A. V. Maximov, R. W. Short, J. F. Myatt, W. Seka, J. A. Delettrez, R. Follett, S. X. Hu, A. A. Solodov, C. Stoeckl, B. Yaakobi, and D. H. Froula (Laboratory for Laser Energetics, Univ. of Rochester).   

A Novel Multi-Dimensional Vlasov-Fokker-Planck Code for Modeling Electron Transport in High Energy Density Plasmas

The unexpected macroscopic behaviour of laser-irradiated plasmas is often attributable to complex kinetic phenomena that depend on the detailed structure of the electron distribution function. For example, the non-local transport of electrons can have a dramatic effect on the temperature and pressure profile of inertial confinement fusion targets. To explore the kinetic physics of high energy density plasmas we have developed the parallel relativistic 2D3P Vlasov-Fokker-Planck code OSHUN [1] that incorporates a spherical harmonic expansion of the electron distribution function. The expansion is truncated such that the necessary resolution in momentum space is retained for a given problem. Finite collisionality results in rapid decay of the high-order harmonics, thereby providing a natural truncation mechanism for the expansion. The code has both implicit and fully explicit electromagnetic field-solvers and employs a rigorous ``linearized'' Fokker-Planck collision operator. OSHUN has been benchmarked against well-known problems, in the highly kinetic limit to model collisionless relativistic instabilities, and in the hydrodynamic limit to recover transport coefficients. We will demonstrate the applicability and limitations of the code by discussing a number of studies we have recently undertaken with relevance to shock ignition and the national ignition facility. \\[4pt] [1] M. Tzoufras, A. R. Bell, P. A. Norreys, F. S. Tsung, ``\textit{A Vlasov-Fokker-Planck code for high energy density physics},'' J. Comp. Phys. 230, 17, 6475-6494 (2011)   

A Bright Neutron Source Driven by a Short Pulse Laser

Neutrons are a unique tool to alter and diagnose material properties, and to exciting nuclear reactions, for many applications. Accelerator based spallation sources provide high neutron fluxes for research, but there is a growing need for more compact sources with higher peak brightness, whether fast or moderated neutrons. Intense lasers promise such as source, readily linkable to other experimental facilities, or deployable outside a laboratory setting. We present experimental results on the first short-pulse laser-driven neutron source powerful enough for radiography. A novel laser-driven ion acceleration mechanism (Breakout Afterburner), operating in the relativistic transparency regime, is used. Based on the mechanism's advantages, a laser-driven deuteron beam is used to achieve a new record in laser-neutron production, in numbers, energy and directionality. This neutron beam is a highly directional pulse $<$ 1 ns at $\sim$ 1 cm from the target, with a flux $>$ 40/$\mu^2$, and thus suitable for imaging applications with high temporal resolution. The beam contained, for the first time, neutrons with energies of up to 150 MeV. Thus using short pulse lasers, it is now possible to use the resulting hard x-rays and neutrons of different energies to radiograph an unknown object and to determine its material composition. Our data matches the simulated data for our test samples.   

Recent results from the first polar direct drive plastic capsule implosions on NIF

Polar direct drive (PDD) offers a simplified platform for conducting strongly driven implosions on NIF to investigate mix, hydro-burn and ignition-relevant physics. Its successful use necessitates a firm understanding and predictive capability of its implosion characteristics including hydro performance, symmetry and yield. To assess this capability, the first two PDD implosions of deuterium filled CH capsules were recently conducted at NIF. The P2 Legendre mode symmetry seen in these implosions agreed with pre-shot predictions even though the 700kJ drive energy produced intensities that far exceeded thresholds for both Raman and Brillouin stimulated scattering. These shots were also the first to employ image backlighting driven by two laser quads. Preliminary results indicate that the yield from the uncoated 2.25 mm diameter, 42 $\mu$m thick, CH shells was reduced by about a factor of two owing to as-shot laser drive asymmetries. Similarly, a small ($sim$50 $\mu$m) centroid offset between the upper and lower shell hemispheres seen in the first shot appears to be indicative of the laser quad energies. Overall, the implosion trajectories agreed with pre-shot predictions of bangtime. The second shot incorporated an 80 ?m wide,10 ?m deep depression encircling the equator of the capsule. This engineered feature was imposed to test our capability to predict the effect of high-mode features on yield and mix. A predicted yield reduction factor of 3 was not observed.\\[4pt] In collaboration with P. A. Bradley, J. A. Cobble, P. Hakel, S. C. Hsu, N. S. Krasheninnikova, G. A. Kyrala, G. R. Magelssen, T. J. Murphy, K. A. Obrey, R. C. Shah, I. L. Tregillis and F. J. Wysocki of Los Alamos National Laboratory; M. Marinak, R. Wallace, T. Parham, M. Cowan, S. Glenn, R. Benedetti and the NIF Operations Team of Lawrence Livermore National Laboratory; R. S. Craxton and P. W. McKenty of the Univ. Rochester; P. Fitzsimmons and A. Nikroo of General Atomics; H. Rinderknecht, M. Rosenberg, and M. G. Johnson, MIT; Work supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under contract DE-AC52-06NA25396.   

Comparison of the Zero Turbulence Manifold with results from the JET and MAST tokamaks

Calculation of the threshold (the zero turbulence manifold) for subcritical turbulence in the presence of a toroidal sheared flow in the zero magnetic shear regime has revealed a strong dependence on the ratio of the magnetic safety factor q to the inverse aspect ratio epsilon; it is shown that the lower the value of this ratio, the higher the ion temperature gradient that can be reached, for example, in transport bifurcations. The theoretically calculated manifold is compared to results from the JET and MAST tokamaks; it is shown that the qualitative trends are remarkably similar between theory and experiment, and that in addition there is reasonable quantitative agreement in the predicted values of the temperature gradient. Experimental data also shows that at the highest values of the temperature gradient, the density fluctuation levels are lowest; the increase in temperature gradient may therefore be attributed to turbulence suppression. In summary, it is to be expected that a regime of low magnetic shear (i.e., flat safety factor profile), low safety factor and/or high inverse aspect ratio, and high toroidal flow shear will allow the highest temperature gradients to be achieved with a fixed level of heat input.