We proposed experimental and computational approaches to examine kinetic plasma processes relevant to laboratory astrophysics and inertial confinement fusion. We used high-resolution measurements of strong collisional plasma shocks to demonstrate for the first time the existence of kinetic ion flows within the shock front. Our research highlighted the importance of plasma instability and ionization dynamics in strong plasma shocks. Experiments we conducted on the National Ignition Facility (NIF) produced the first evidence of particle acceleration by Weibel-mediated collisionless shocks. Finally, to enhance our understanding of the stagnated plasma conditions in inertial confinement fusion, we also developed new algorithms to compensate for the effects of bulk hot spot velocity on neutron activation diagnostic results obtained on NIF.
Kinetic plasma dynamics has recently emerged as an important branch of high-energy-density (HED) physics. HED plasma conditions are accessible via high-powered lasers and are relevant to a wide range of astrophysical and inertial confinement fusion (ICF) scenarios. Our understanding of HED systems relies on computational modeling, due to the highly dynamic nature of the systems being studied. These systems evolve on rapid (nanosecond) and small (micron) scales. Computer models are based on hydrodynamic equations that assume that the plasma is locally collisional (i.e., particles collide on a scale much shorter than gradient length scales) and thermalized. These simulations neglect the separate behavior of multiple ion species, assuming instead a single ion fluid with an average mass and charge. However, the behavior of astrophysical and laser-produced HED plasmas often contradicts these hydrodynamic assumptions: The particles are not fully thermalized, they travel the scale of the experiment before colliding, or the multiple ion species react differently to the local forces. The kinetic behavior of the individual particles then becomes important to the dynamic evolution of the system as a whole.
Recent research indicates that kinetic motion of particles significantly affects the evolution of temperature and composition in ICF-relevant plasmas (Rinderknecht et al. 2014a, 2014b, 2015). Two varieties of ion kinetic effects impact the evolution of the plasma in this regime: (1) the transition from a collisional state to a collisionless state and (2) the effects of multiple ion species. The transition from the collisional state to the collisionless state occurs in strongly shocked ICF plasmas (Rinderknecht et al. 2015), occurs in supernova shocks (Stockham et al. 2012) believed to accelerate cosmic rays (Ginzburg 1996, Caprioli et al. 2011), and may cause heating of the solar corona (Uzdensky 2007). ICF scenarios are sensitive to the effects of multiple ion species (Rinderknecht et al. 2014a, Kagan and Tang 2014, Rygg et al. 2006, Casey et al. 2012) that may play a role in forming the structure of supernovae and supernova remnants (van Veelan 2009) and in the solar wind (Kasper et al. 2007). New simulation capabilities in high-fidelity physics, kinetic physics, and molecular dynamics are being developed (Graziani et al. 2012). Shocks are a valuable test case for kinetic phenomena because their structure depends on kinetic physics.
During this project, we intended to use HED experimental tools on laser facilities at Lawrence Livermore National Laboratory and elsewhere to investigate the physics of kinetic plasmas. We set three research goals: (1) to analyze and publish the results of collisional shock experiments on the OMEGA laser facility; (2) to perform collisionless shock experiments on NIF and record evidence of shock formation; and (3) to improve our understanding of the conditions under which kinetic mechanisms operate in ICF implosions by developing new algorithms to interpret stagnation asymmetry data. First, in analyzing the data from collisional shock experiments, we showed that the Thomson scattering images (which depict the elastic scattering of electromagnetic radiation by a free charged particle) had recorded the first measurement of non-thermal ion distributions inside a collisional shock front. These data demonstrated the utility of this platform for basic kinetic plasma physics experiments. Second, we conducted collisionless shock experiments on NIF. These experiments were not entirely successful due to the failure of our primary Thomson scattering diagnostic. However, they did produce strong evidence of electron acceleration by Weibel-mediated shocks. Third, we developed new algorithms to interpret the nuclear activation detector (NAD) data in the presence of large measured hot spot velocities on NIF. In ICF, the single shock (resulting from the multiple-shock coalescence) travels in the gas in the form of a strong shock. Such a shock is reflected off the center of the capsule and subsequently off the incoming inner shell surface, which then decelerates. The reflected shock weakens after reflection off the shell. The material of the inner shell surface develops a uniform pressure profile referred to as the hot spot. Our research revealed a correlation between bulk hot spot motion and symmetry of the assembled fuel, an unexpected and important result. In the course of addressing these objectives, we reviewed and summarized the current state of knowledge surrounding kinetic physics in ICF plasmas.
We initially intended to interpret spectra of protons from the D–3He fusion reaction to assess the evolution of areal density along the detector line of sight between capsule convergences of 2 to 8 (Taitano et al. 2018). However, the NAD data analysis method proved unexpectedly valuable for understanding hot spot stagnation asymmetry, so we chose to focus on that aspect of the project. The proton spectral comparison is still a candidate for future study.
Our research supports the NNSA goal to advance the science, technology, and engineering competencies that are the foundation of the NNSA mission. It also addresses the Laboratory's research and development challenge in directed energy, as well as its core competency in HED science.
The results of our investigation of collisional plasma shocks are noteworthy for their uniquely high-quality data constraining kinetic plasma physics in an HED regime. Our experiments were conducted under conditions similar to those produced within the low-density DT vapor in ICF implosions, where the mean free path approaches the radius of the vapor. The results of our experiments indicate that neither the duration nor the spatial extent of the shock propagation are sufficient for a shock to fully form in this scenario. Our unique dataset is currently being used to validate particle-in-cell and Fokker-Planck kinetic simulations; these simulation codes are being used to evaluate the residual impact of strong converging shocks on ICF hot spot formation (Taitano et al. 2018).
Our research has contributed to the understanding of recent ICF implosions on NIF. The algorithms developed for interpretation of the NAD data on areal density asymmetries in NIF implosions has revealed an unexpected, significant correlation between residual hot spot velocity and areal density asymmetry, both in magnitude and direction. This signature provides important evidence of a systematic drive asymmetry in NIF implosions, the source of which is currently being investigated.
Our research supports the improvement of predictive capabilities for HED plasma phenomena at the Laboratory and elsewhere. Building on the success of the platforms produced for this project, new collisional shock experiments have been proposed to be conducted at the OMEGA laser facility and NIF to further investigate developing and steady-state shock propagation in collisional plasmas. Velocity-correction algorithms for the NADs have been transferred to the Laboratory's ICF program.
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Casey, D. T., et al. 2012. "Evidence for Stratification of Deuterium-Tritium Fuel in Inertial Confinement Fusion Implosions." Physical Review Letters 108(7): 075002. doi: 10.1103/PhysRevLett.108.075002.
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Rinderknecht, H. G., et al. 2014a. "First Observations of Nonhydrodynamic Mix at the Fuel-Shell Interface in Shock-Driven Inertial Confinement Implosions." Physical Review Letters 112(13): 135001. doi: 10.1103/PhysRevLett.112.135001.
——— . 2014b. "Measurements of Ion Velocity Separation and Ionization in Multi-Species Plasma Shocks." Physics of Plasmas 25(5): 056312. doi: 10.1063/1.5023383.
——— . 2015. "Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions." Physical Review Letters 114(2): 025001. doi: 10.1103/PhysRevLett.114.025001.
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Taitano, W. T., et al. 2018. "Yield Degradation in Inertial-Confinement-Fusion Implosions Due to Shock-Driven Kinetic Fuel-Species Stratification and Viscous Heating."Physics of Plasmas 25(5): 056310. doi: 10.1063/1.5024402.
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Rinderknecht, H. G. 2018. "Velocity Correction for Nuclear Activation Detectors at the NIF." 22nd Topical Conference on High Temperature Plasma Diagnostics, San Diego, CA, April 2018. LLNL-PRES-749411.
——— . 2018. "Progress Toward Astrophysically Relevant, Fully-Formed Collisionless Shock Experiments on OMEGA and the NIF." 12th International Conference on High Energy Density Laboratory Astrophysics (HEDLA), Kurashiki, Japan, May 2018. LLNL-PRES-751809.
——— . 2018. "Shock Front Structure Measurements to Explore Kinetic and Multi-Species Physics." 2018 Kinetic Effects in ICF Workshop, Santa Fe, NM, May 2018. LLNL-PRES-751514.
Rinderknecht, H. G., et al. 2018. "Kinetic Physics in ICF: Present Understanding and Future Directions." Plasma Physics and Controlled Fusion 60(6): 064001. doi: 10.1088/1361-6587/aab79f. LLNL-JRNL-755501.
——— . 2018. "Measurements of Ion Velocity Separation and Ionization in Multi-Species Plasma Shocks."Physics of Plasmas 25(5): 056312. doi: 10.1063/1.5023383. LLNL-JRNL-744851.