Session CO3: Laser-Plasma Coupling

Design and Analysis of Laser Plasma Interaction Experiments Relevant to the National Ignition Facility

Achieving appropriate energetics and symmetry for ignition in indirect drive inertial confinement fusion requires control of stimulated Brillouin and Raman backscatter of the laser beams that heat the hohlraum. This talk describes the computational design and analysis of experiments at the Omega laser facility at LLE to study LPI in NIF-like plasmas. The focus of this study is the interaction of a smoothed beam as it propagates from low-Z gas inside a hohlraum into high-Z material than has been ablated from the hohlraum's wall. We describe the plasma environment of the laser-heated hohlraum as modeled by the HYDRA code and the corresponding linear LPI gains expected for the wall interaction beam, as well as for an on-axis beam that interacts only with the low-Z gas that initially fills the hohlraum. Simulated data of the plasma conditions (temperature and density) and the beam backscatter will be compared to experimental data.   

Simulating Extremely Large-Scale Laser-Plasma Interactions of NIF Experiments

National Ignition Facility (NIF) laser beams will propagate through several mm of plasma between where they enter the hohlraum and where they deposit their energy near the hohlraum wall. Patch simulations, which propagate a small cross-section of the laser beam through the plasma to the wall, demonstrate beam propagation with acceptable levels of laser scatter and spray for 2010 ignition targets. However, these simulations must be normalized to whole beam simulations (C.H. Still et al., this meeting). Such simulations are extremely large calculations, requiring thousands of CPUs on the world's most advanced parallel computers, such as ASC Purple and Blue Gene/L. Simulations of this scale introduce a number of difficulties, including diagnosis. We will show results from these simulations, demonstrating the ability to run effectively on thousands of processors. We will briefly discuss the technologies that enable these calculations and the diagnostics we use to understand them.   

Laser-plasma interactions in 2010 ignition targets

Ignition targets designed for the 2010 ignition campaign at the National Ignition Facility (NIF) have undergone re-design to not only provide good symmetry, but to also reduce the levels of stimulated Brillouin and Raman backscatter, where laser light resonantly scatters off ion acoustic and electron plasma waves, respectively. These high-Z cylinders, which contain a DT capsule coated with a beryllium ablator, are filled with He at 1.3 mg/cc. Both the beryllium ablator and the He gas fill have the potential to promote laser backscatter above previous design levels. To compensate for such effects, the laser focal spot has been increased in area, thereby reducing the laser intensity and hence the propensity to backscatter. Gain exponents for Brillouin and Raman backscatter have been significantly reduced in these new designs. Simulations of beam scatter and propagation have been performed, and show low levels of scatter and spray with fully conditioned beams. Gain exponents, beam spray, and reflectivity as a function of beam conditioning will be presented for these new designs.   

Hohlraum Energetics with Elliptical Phase Plates on OMEGA

A set of 40 elliptical phase plates (E-IDI-300) was produced for hohlraum experiments on OMEGA to increase the coupling of laser energy to x-ray drive by reducing laser scattering losses from stimulated Brillouin and Raman scattering. A series of experiments were conducted with vacuum, gas-filled, and glass-lined Au hohlraums having a single laser entrance hole (``halfraums'') to quantify the effects of E-IDI-300 drive on the radiation temperature $T_{r}$ and the scattering losses. A $\sim $17-eV increase in $T_{r}$ was observed for the gas-filled targets driven with a shaped laser pulse (PS26). Gated ($\sim $80-ps) hard x{\-}ray ($>$8-keV) images of the thin-walled (5-\textit{$\mu $}m) hohlraums are used to illustrate the radiation hydrodynamics. The experimental results will be presented and compared to \textit{LASNEX} simulations. Measured characteristics of the single-beam, E-IDI-300 far-field intensity distribution will also be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Thomson scattering in both the collective and non-collective regime in a laser produced plasma

We present Thomson-scattering measurements of light scattered from ion-acoustic and electron plasma fluctuations in an N$_2$ gas jet plasma. The transition from the collective regime to the non-collective regime for the electron feature is observed by varying the plasma density from 2.5$\times 10^{18}$ cm$^{-3}$ to 1.7$\times 10^{19}$ cm$^{-3}$ and the temperature from 100 eV to 300 eV. These measurements allow an accurate local measurement of both temperature and density. A discussion on the sensitivity to non-maxwellian electron distribution functions will be presented. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore Nation Laboratory under Contract No. W-7405-ENG-48. UCRL-ABS-223005.   

Fokker-Planck transport simulations relevant to Polar-Direct-Drive

Polar-Direct-Drive (PDD) has recently been suggested as a way to perform direct-drive ICF on NIF [1], with minimal beam reconfiguration. Inherent in this scheme is an azimuthal variation in the laser energy deposition and consequently plasma temperature. This has interesting ramifications for thermal transport. We are exploring the nature of non-local heat flow and magnetic-field generation in PDD using the 2-D Vlasov-Fokker- Planck code IMPACT [2]. In particular there is the issue of whether Spitzer-H\"{a}rm theory applies to the lateral heat-flow q$_{\theta }$ driven by azimuthal temperature gradients. We present IMPACT simulations representative of the critical-surface region of an imploding capsule that indicate lateral heat-flow suppression. Interestingly the thermal conductivities for q$_{\theta }$ and q$_{r}$ are not directly linked and the implied flux limiters vary in space and time. We also present results showing spontaneous growth of B-fields via $\nabla $n$\times\nabla $T. \\ \newline [1] S. Skupsky \textit{et al.}, Phys. Plasmas \textbf{11}, 2763 (2004)\newline [2] R. Kingham and A.R. Bell, J. Comput. Phys. \textbf{194}, 1 (2004)   

Laser--Plasma Coupling Near Critical Density in Direct-Drive ICF Plasmas

Laser--plasma coupling near the critical density has a major effect on absorption and heat transport in direct-drive inertial confinement fusion (ICF) plasmas. The characteristic features of the near-critical-density region are steep gradients of plasma density and temperature. This region also includes turning points for laser beams, and laser--plasma coupling is influenced by interference between multiple incident and reflected beams. Modeling with the laser--plasma interaction code\footnote{ A. V. Maximov \textit{et al}., Phys. Plasmas \textbf{11}, 2994 (2004).} for nonlinear beam propagation near the beam turning points allows us to study the generation of magnetic fields caused by crossing gradients of temperature and laser-driven density perturbations, and also the structure of $p$-polarized laser fields that affect the resonance absorption. The modification of transport by laser-driven magnetic fields together with the effect of the laser field on the electron-distribution function are implemented in the hydrodynamic modeling. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Dynamics of high power and long pulse laser propagation and its control in underdense plasmas

The study of intense laser pulse propagation through long underdense plasmas is of crucial importance for inertial confinement fusion (ICF). We have performed a systematic study of long pulse beams ($\tau _L =400ps,I=10^{10}\sim 10^{12}Wcm^{-2})$ propagating through the underdense plasmas ($n_e =10^{19}\sim 10^{20}cm^{-3})$, by controlling two filaments created from the pulses with variable delay and intensity ratio. These experiments have been performed at the LULI laser facility. The results show that the earlier pulse affects the propagation characteristics of the later pulse. The 2D time-resolved sampling camera provides the ability to examine the possibility of enhanced propagation, collimation, and guiding of a trailing pulse induced by an earlier pulse. These facts are of interest for ICF and other applications. In particular, this study opens perspectives, through shaping the pulses temporally, for the control of propagation of long pulses in the low density plasmas that are present within ICF hohlraums.   

Nonlocal Electron Transport in Laser Ablation

Electron transport plays an important role in laser-produced plasmas, especially in laser fusion plasmas, there are some issues to be solved related to the electron transport. The flux-inhibition is an important process in the 1-D problem. Also, in the 2-D problem, the spatial relaxation of the perturbation of the electron flux should be considered. These processes are closely related to the nonlocal electron transport where electron velocity distribution function is not Maxwellian. In order to simulate the nonlocal electron transport in the laser ablation process, we have developed the 1D and 2D nonlocal electron transport simulation codes based on the Fokker-Planck equations. We will compare results of the electron energy conduction calculated by the flux-limited Spitzer-Harm model and those based on the Fokker-Planck simulation in the 1D and 2D condition for the typical laser ablation condition.   

Langmuir wave damping decreases slowly

The onset of stimulated Raman scatter in a single laser speckle occurs (D. S. Montgomery \textit{et al.}, Phys. Plasmas, \textbf{9}, 2311 (2002)) at lower laser intensity, $I$, than predicted by linear theory based on classical Landau damping, $\nu_{\mbox{L}}$, of the SRS daughter Langmuir wave. Does this imply that SRS onset in a speckled laser beam, propagating through long scale length plasma, is also at odds with linear theory? It has been shown (Harvey A. Rose and D. F. DuBois, Phys. Rev. Lett. \textbf{72}, 2883 (1994)) that linear convective gain in speckles with large fluctuations of $I$ about the average, $\left\langle I \right\rangle$, leads to onset at a value of $\left\langle I \right\rangle$, $I_{\mbox{c}}$, small compared to that for onset in a uniform beam. While nonlinear electron trapping effects may occur in very intense speckles, whether or not these effects are sufficient to lower the onset value of $\left\langle I \right\rangle$ below $I_{\mbox{c}}$ depends on how strongly electrons must be trapped before there is significant reduction in $\nu_{\mbox{L}}$. As the amplitude of an SRS daughter Langmuir wave increases, its $\nu_{\mbox{L}}$ decreases by the factor $\nu$/$\omega_{\mbox{b}}$, due to the competition between electron trapping, with electron bounce frequency, $\omega_{\mbox{b}}$, and escape of these trapped electrons by advection out of a speckle's side, at rate $\nu $. This result (Harvey A. Rose and David A. Russell, Phys. Plasmas, \textbf{8}, 4784 (2001)) is valid for $\nu$/$\omega_{\mbox{b}}$ $\ll$1. In this talk I present a nonlinear, transit time damping, calculation of $\nu_{\mbox{L}}$ and find that reduction by a factor of two does not occur until $\omega_{\mbox{b}}$/$\nu \approx$ 5. This slow turn on of trapping effects suggests that the linear calculation of $I_{\mbox{c}}$ is NIF relevant.   

The nonlinear evolution of driven nonlinear ion acoustic waves with kinetic electrons

The stimulated Brillouin scattering (SBS) of laser light from hot plasma drives ion acoustic waves to large amplitudes particularly if the phase velocity is much greater than the ion thermal velocity for all ion species, that is, ZTe/Ti $>>$1 where Z is the charge state of the ion, and T$_{e}$ and T$_{i}$ are the electron and ion temperatures. In fluid simulations of the SBS from CO$_{2}$ and Krypton plasmas, \textit{ad hoc} limits on the amplitude of the driven ion waves were required to match the measured reflectivity. Because ZT$_{e}$/T$_{i }>>$1, ion kinetics are unlikely to play a role in the saturation of ion waves. Here, we study the effect of electron trapping which produces a positive frequency shift in quantitative agreement with theory (see abstract by S. Brunner \textit{et al}., this meeting) and the role of electron kinetics on the decay instability of the driven ion wave. Further, we apply these results to modeling of experiments where ZT$_{e}$/T$_{i }>>$1 [$e.g$., Glenzer \textit{et al.,} PRL \textbf{86}, 2565 (2001), L. Divol, et al., Physics of Plasmas \textbf{10}, 1822 (2003)].   

Magnetic reconnection and plasma dynamics in two beam laser-solid interactions

Measurements using the Vulcan Laser Facility (Rutherford Appleton Laboratory, UK) are presented of a self-generated magnetic reconnection geometry that is created at the surface of a planar solid target by two heater beams (1 ns pulse duration, 1 $\times$ 10$^{15}$ Wcm$^{-2}$). Shadowgraphy and interferometry (using a 10 ps, 263 nm probe beam) and proton deflectometry reveal the formation of high velocity, collimated pairs of jets that originate from the reconnection layer and 0.7 - 1.3 MG magnetic fields at the laser spot edges. Greater jet collimation has been observed using gold target interactions compared to aluminium targets due to radiative cooling effects. Thomson scattering measurements from the reconnection layer are consistent with high electron temperatures (T$_{e}$ $>$ 1 keV) in this region. Similarities and differences to the conditions found in the vicinity of inner hohlraum wall surfaces are discussed.