Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session UI2: Invited: ICF III |
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Chair: Denise Hinkel, LLNL Room: Floridian Ballroom AB |
Thursday, October 24, 2019 2:00PM - 2:30PM |
UI2.00001: Direct measurement of hot electron preheat and its spatial distribution in direct drive implosions Invited Speaker: Alison Christopherson In laser fusion, a spherical shell of a low-Z ablator (CH, HDC, Be or others) layered with cryogenic DT ice is accelerated inward on a low adiabat to achieve high fusion yields and areal densities with minimal driver energy. Hot electrons generated from laser--plasma instabilities can severely degrade the implosion performance by preheating the DT fuel, resulting in early decompression of the imploding shell and lower fuel areal density. It is shown that, in direct-drive experiments, the hot-electron energy deposited in the DT fuel can be inferred by comparing the hard x-ray signals between a layered DT implosion and its mass-equivalent all-CH implosion. Since a significant fraction of the ice layer is ablated during the implosion, it is important to assess the spatial distribution of the preheat energy into the fuel, in particular within the unablated fuel which determines the final areal density. The spatial distribution of preheat energy was inferred in two experimental campaigns on OMEGA using warm CH targets with Cu-doped plastic payloads of varying thicknesses. The hard x-rays from the Cu-doped plastic implosions were used to infer the hot electron energy deposited in each layer. A hot electron transport and deposition model was derived to match the hard x-ray spectrum and emission in both warm and cryogenic implosion experiments. The calibrated model is used to assess the areal density degradation due to hot electron preheat. A similar experimental campaign on the NIF using Ge-doped shells has led to the inference of the spatial distribution of preheat energy and provided critical information on the scaling of hot electron preheat at megajoule driver energies . [Preview Abstract] |
Thursday, October 24, 2019 2:30PM - 3:00PM |
UI2.00002: Inferring the Thermal Ion Temperature and Residual Kinetic Energy from Nuclear Measurements in Three-Dimensional Inertial Confinement Fusion. Invited Speaker: Ka Ming Woo In inertial confinement fusion implosion experiments, the presence of residual anisotropic fluid motion within the stagnating hot spot leads to significant variations in ion-temperature measurements using neutron time-of-flight (nTOF) detectors along different lines of sight (LOS). The minimum ion-temperature measurement is typically used as representative of the thermal temperature. However, in the presence of isotropic flows, even the minimum DT neutron-averaged ion temperature is well above the plasma thermal temperature. In this work, it is first shown that by using six LOS measurements, it is possible to accurately determine the true minimum DT ion temperature over 4$\pi $ solid angle and therefore account for the contribution of anisotropic flows. Furthermore, using both DD and DT neutron-averaged temperature measurements, it is possible to determine the contribution of isotropic flows and infer the thermal temperature. Using multimode simulations, it is shown that large isotropic flows drive the ratio of DD to DT neutron-inferred ion temperatures well below unity and approaching the lower bound of 0.8. The minimum of DD neutron-inferred ion temperature is determined from the velocity variance analysis, accounting for the presence of isotropic flows. Being close to the real thermal temperature, the inferred DD minimum ion temperatures demonstrate a strong correlation with the experimental yields in the OMEGA implosion database. An analytical expression is also derived to explain the effect of mode 1 ion-temperature measurement asymmetry on yield degradations caused by the anisotropic flows. Furthermore, residual fluid motion in the shell leads to unconverted kinetic energy and yield degradation. An analytic theory benchmarked with 3-D simulations is developed to relate the residual kinetic energy to the yield degradation. In collaboration with: R. Betti, R. Epstein, O.M. Mannion, C.J. Forrest, J.P. Knauer, V.N. Goncharov, P.B. Radha, D. Patel, K.S. Anderson, J.A. Delettrez, M. Charissis, A. Shvydky, I.V. Igumenshchev, V. Gopalaswamy, A.R. Christopherson, Z.L. Mohamed, D. Cao, H. Aluie, and E.M. Campbell, Laboratory for Laser Energetics, U. of Rochester; R. Yan, U. of Science and Technology of China; P.-Y. Chang, National Cheng Kung U.; A. Bose, MIT; D. Shvarts, Ben Gurion U. of the Negev; J. Sanz, U. Politecnica de Madrid. [Preview Abstract] |
Thursday, October 24, 2019 3:00PM - 3:30PM |
UI2.00003: Hydro Scaling of Direct-Drive Cylindrical Implosions at the OMEGA and the National Ignition Facility Invited Speaker: Sasi Palaniyappan Deceleration-phase Rayleigh-Taylor instability (RTI) growth during the inertial confinement fusion (ICF) implosions affects the implosion performance significantly as it mixes the cold ablator material into the fuel. Precise measurements of such instability growth are essential for both validating the existing simulation codes and improving our predictive capability. RTI measurements on the inner surface of a spherical shell are limited and are often inferred indirectly at limited convergence. In contrast, cylindrical implosions allow for direct and precise measurements of the inner surface while retaining the effects of convergence, which are known to modify RTI growth rates through Bell-Plesset effects. We have performed direct-drive cylindrical implosions experiments at both the Omega and the NIF laser facilities using scaled targets. RTI growth is demonstrated to be scale-invariant between the cylindrical targets at OMEGA and similar targets at the NIF that are scaled up by a factor of 3 in the radial dimension. Single-mode (m$=$20) instability growth factors of \textasciitilde 17 are measured at a convergence ratio CR\textasciitilde 2.5 with nearly identical mode growth at both scales. The measurements are in agreement with RAGE rad-hydro simulations. In addition, we have also developed Bayesian-Inference-Engine (BIE) methods to correct the parallax effects in the measurements allowing a more precise comparison between the experimental data and the simulations. The simulations use an \textit{ad hoc} laser drive multiplier to account for cross-beam energy transfer and laser-plasma interaction physics that are not currently modeled. The same 0.8 multiplier matches both the OMEGA and NIF-scale targets, despite the disparate plasma length scales, with strong implications for scaling of direct-drive ICF implosions. Designs for higher convergence cylindrical implosions, CR\textasciitilde 10-15, are currently underway. [Preview Abstract] |
Thursday, October 24, 2019 3:30PM - 4:00PM |
UI2.00004: Nonlinear electron and ion dynamics in the saturation of cross-beam energy transfer Invited Speaker: Lin Yin Cross-beam energy transfer (CBET) allows crossing laser beams to exchange energy. Understanding the nonlinear saturation of CBET, including effects of wave-particle interaction with ions and electrons, excitation of forward stimulated Raman scattering (FSRS), and speckle geometry, is important for controlling low-mode asymmetry in ICF implosions. The nonlinear dynamics of CBET for multi-speckled laser beams is examined using VPIC simulations under NIF-like conditions. The simulations show CBET saturates on a fast (\textasciitilde 10s of ps) time scale through ion trapping and excitation of oblique FSRS in the seed beam. Ion trapping reduces wave damping and speckle interaction increases wave coherence length to scales much larger than the speckle width, together enhancing energy transfer, whereas ion acoustic wave (IAW) breakup increases wave damping and contributes to CBET saturation. The seed beam can also become unstable to oblique FSRS, which leads to beam deflection and a frequency downshift. FSRS saturates on fast (\textasciitilde ps) time scales by electron plasma wave self-focusing, leading to enhanced side-loss hot electrons with energy exceeding 300 keV. Such electrons may contribute to preheat but can be mitigated by introducing density gradients. Scaling simulations show that CBET, as well as FSRS and hot electrons, increase with beam average intensity, beam diameter, and crossing area, but that CBET is limited by excitation of FSRS, IAW breakup, and pump depletion. FSRS deflects the seed beam energy by \textgreater 40{\%} of incident beam energy and puts a few-{\%} of incident beam energy into hot electrons. FSRS therefore limits the efficacy of CBET for symmetry tuning at late stages in the implosion and may account for some of the ``missing energy'' inferred in implosions with gas-filled hohlraums. Collaborators: B. J. Albright, D. J. Stark, D. Nystrom, R. F. Bird, K. J. Bowers [Preview Abstract] |
Thursday, October 24, 2019 4:00PM - 4:30PM |
UI2.00005: Measurements of the Effects of Isolated Surface Defects on Laser Accelerated Targets Invited Speaker: Calvin Zulick Initial imperfections in ICF targets, amplified by hydrodynamic instabilities can lead to asymmetric target compression and mixing of the ablator material into the fuel. Previous efforts to understand perturbation growth have largely focused on studies of uniformly imposed patterns such as sinusoidal modulations or surface roughness of targets. However, implosions on the NIF and OMEGA have raised questions about the effect on target performance of localized large amplitude, or “isolated,” perturbations such as fill tubes, target mount structures, pits, and other localized target defects. A better understanding of the evolution of such non-linear seeds is necessary to mitigate their effects. To this end, hydrodynamic growth of isolated defects, with characteristic widths of $1$ to $30 \mu$m and depths of $100$ nm to $25$ $\mu$m, was studied using planar plastic targets ablatively accelerated by the Nike KrF laser. The target defects were machined in CH foils using femtosecond laser ablation. The high drive uniformity of ISI-smoothed Nike beams allows hydrodynamics to be dominated by the imposed target features with negligible laser imprint. X-ray backlighting using monochromatic curved crystal imaging was used to obtain high resolution, large-field-of-view measurements of areal mass evolution while x-ray sidelighting provided images of axial structures such as jets. The growth rate of the perturbations, which can be followed by late time “hole closure” of the areal density perturbation, was observed to vary with the shock strength and initial feature sizes. Side-on images showed rear-surface jet formation and enhanced self-emission on the front surface, originating at the isolated defects and propagating back toward the laser, indicating that the coronal plasma was being perturbed. The experimental results are compared to simulations using the FASTRAD3D rad-hydro code. [Preview Abstract] |
Thursday, October 24, 2019 4:30PM - 5:00PM |
UI2.00006: Novel criteria for efficient Raman and Brillouin amplification of laser beams in plasma Invited Speaker: Raoul Trines Twenty years have passed since the seminal paper on Raman amplification in plasma by Malkin, Shvets and Fisch [1]. While Raman amplification has been explored very successfully in theory and simulations [2], no significant Raman amplification of a laser pulse beyond 0.1 TW or 6\% efficiency has been achieved [3], and there exists only one report of Brillouin amplification beyond 1 TW [4]. In this paper, we investigate one aspect of Raman and Brillouin amplification that has been consistently overlooked until now: the parameters and quality of the initial seed pulse. We have developed new criteria for the initial seed pulse in Raman and Brillouin amplification, and show through analytic theory and numerical simulations, that the energy gain and efficiency of the amplification will be significant if and only if these criteria are met. We will analyze the plasma-based Raman and Brillouin amplification experiments carried out to date, and show that the input seed pulses in all but one of these experiments fall short of our criteria, which is the likely explanation for the poor efficiency obtained in them. Finally, we apply our findings to the results of the most promising Raman and Brillouin amplification experiments available [3, 4] to test how well those conform to our model. [1] V.M. Malkin, G. Shvets and N.J. Fisch, Phys. Rev. Lett. {\bf 82}, 4448 (1999). [2] R. M. G. M. Trines \emph{et al.}, Nature Physics {\bf 7}, 87 (2011). [3] J. Ren \emph{et al.}, Nature Physics {\bf 3}, 732 (2007). [4] J.-R. Marqu\`es \emph{et al.}, Phys. Rev. X {\bf 9}, 021008 (2019). [Preview Abstract] |
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