Residency Abstracts

Implosions at NIF are measured with a wide array of instruments, such as the neutron imaging system and the neutron activation diagnostics. A reconstruction technique to build a three-dimensional dynamic picture of the implosion based on those measurements would help us better understand our experiments. However, because our instruments provide data in such different forms, it's difficult to combine them using conventional computational methods. A machine-learning surrogate trained on a 3D rocket-piston model provides a way to quickly build a consistent 3D dynamic picture of an implosion given data from disparate sources.

The primary project goals comprised measurement of the activity of a Sm-146 sample produced at the Triumph Accelerator via cryogenic Decay Energy Spectroscopy (DES / Q-Spec) as well as research and development into cryogenic radiation sensor improvement. The decay and extinction of Sm-146 and Sm-147 int Nd-142 and Nd-143 respectively are important benchmarks for early solars-system chronology and are used for dating events such as the formation of the first solid bodies, lunar re-solidification, and formation of the Martian crust and mantle. Despite this importance for nuclear dating, the half-life value of Sm-146 remains controversial. Previous half-life measurements yielded 103 and 68 million years. The goals for the first residency were to conduct a total activity counting of the Sm-146 sample as part of a new half-life measurement utilizing the DES technique. During my residency I accomplished the decay counting and measured an activity of 20.3 +/- 0.2 mBq. The sample has been given to the radio chemistry group which will perform mass spectroscopy to determine the sample’s mass. From the two numbers the half-life can be extracted.

Modern energetic lasers interacting can accelerate electrons in gases or solids to high energies. Self-focusing and plasma instabilities within this electron beam generate strong electromagnetic fields that can lead to ultrafast ion acceleration and/or synchrotron x-ray emission. We are conducting laser-target simulations and experiments to model and image the strong fields produced in these experiments. We also plan to harness these fields in future experiments to accelerate ions and produce energetic x-rays which will enable ultrafast radiography many high energy density plasmas.

Intense lasers can create strong electromagnetic fields in plasmas that accelerate particles to high energies. We created a movie of this interaction in which we can measure the strength of these electromagnetic fields and check our theoretical models.

Extreme states of matter at high densities and temperatures can be found in astrophysical systems such as young stellar objects, stellar atmospheres, and interiors, as well as inertial confinement fusion experiments. Similar extreme conditions can also be found in the interaction of astrophysical flows, specifically in areas where the plasma is compressed or shocked when such flows converge and collide. Our objective is to develop a new target platform that accesses extreme plasma regimes in a novel way via the convergence of deflagrating plasma jets. The experiment fields hollow cylindrical and spherical shells with a deuterated plastic (CD) layer lined on the interior of the shell. That interior is uniformly illuminated by laser light, generating a fully-ionized plasma at high temperature (~keV). The resulting plasma expands inwards and converges at the central axis (cylinder) or point (sphere) of the shell. There the opposing plasma streams interpenetrate, stagnate, and thermalize, converting their directed kinetic energy into thermal energy and reaching extreme densities and temperatures. The details of this thermalization process are not well-understood, and are highly dependent on the plasma collisionality. The initial plasma fronts reaching the point of stagnation are nearly collisionless, and the streams are expected to interpenetrate significantly, leading to a wide interaction region. The depth of this interpenetration significantly affects the resulting stagnation conditions. The objective is to run these experiments at the National Ignition Facility at LLNL. In support of this goal, we are developing simulations to better understand the behavior of the plasma in the system, help design the experimental configuration, and predict the resulting stagnation conditions. In order to capture the interpenetration and kinetic behavior of this system, we are running simulations using a hybrid particle-in-cell (PIC) approach. In a hybrid PIC simulation, the ions are treated as a collection of macroparticles, allowing for kinetic behavior. These effects, which we expect to be important to this interaction, cannot be captured in typical hydrodynamic codes. We are using the commercial code Chicago [1] to run these simulations. Experiments have recently been conducted at the OMEGA Laser Facility in Rochester, NY to test these targets. We have demonstrated that single-fluid hydrodynamics codes such as HYDRA, which assume the plasma is highly collisional everywhere, are unable to reproduce the results of these experiments. HYDRA predicted that only the first thin layer of liner material (~1 micron) contributes to the neutron yield, whereas experiments demonstrated that a much larger region matters. Simplified 1D multi-fluid and kinetic ion simulations in Chicago are able to better reproduce the observed behavior, although they don’t yet fully explain experimental results.

High-repetition rate (HRR) laser systems are coming online around the world and will enable a faster rate of learning in high energy density science. HRR lasers will be very important to research in laser-driven ion sources, where high intensity laser-matter interactions are harnessed to create compact particle accelerators. To support this growing technology, we will need high repetition rate diagnostics, diagnostic analysis and new data-driven tools in order to optimize and tune these sources for a wide variety of applications relevant to high energy density science, material science and stockpile stewardship. As part of my second residency, I had the privilege learn from LLNL experts in experiments, simulation and machine learning to push forward this vision of HRR science.

This project involved developing and applying a methodology for determining the drive pressure in ramp-wave compression experiments. This drive pressure acts as an equivalent pressure that encapsulates the effect of the drive mechanism (e.g., magnetic drive, high-energy laser drive, etc.) at a surface upstream of the experimental setup and is essential for performing forward hydrodynamics simulations of the experiment. This methodology iteratively produces an optimized drive pressure using 1-D hydrodynamics simulation of a given ramp-wave compression experiment. I developed much of this methodology during my previous residency. During this current residency, I was able to take the drive-pressure profiles that I had developed and apply them to modeling the phase transition kinetics of the liquid-solid transition that occurs in the water samples involved in a number of experiments conducted at a variety of compression rates. This allowed for a multi-scale simulation of these experiments where both the phase transition kinetics and the hydrodynamics of the compression process are accounted for. Using these simulations, I gained significant insight into how the initial temperature and the compression rate affect the kinetics of the phase transition in these types of ramp-compression experiments. I gained further insight into the compression-rate dependence of the phase transition kinetics by investigating the transient kinetics in a theoretical study conducted across a wide range of compression rates that correspond to gas-gun driven impactor, magnetic-drive, and laser-drive ramp-compression experiments. As part of this investigation, various scaling parameters were developed to aid in understanding the compression-rate dependence of the phase transition kinetics. The results of this investigation were directly compared to results from ramp-wave compression experiments.

Extreme states of matter at high densities and temperatures can be found in many types of astrophysical systems, as well as inertial confinement fusion experiments. Our objective is to develop a new target platform that accesses extreme plasma regimes in a novel way via the convergence of deflagrating plasma jets. The experiment will field hollow cylindrical and spherical shells with a deuterated plastic (CD) layer lined on the interior of the shell. The interior of the shell is uniformly illuminated by laser light, generating a fully-ionized plasma at high temperature (~keV). The resulting plasma expands inwards and converges at the central axis (cylinder) or point (sphere) of the shell. There the opposing plasma streams interpenetrate, stagnate, and thermalize, converting their directed kinetic energy into thermal energy and reaching extreme densities and temperatures. The details of this thermalization process are not well-understood, and are highly dependent on the plasma collisionality. The initial plasma fronts reaching the point of stagnation are nearly collisionless, and the streams are expected to interpenetrate significantly, leading to a wide interaction region. The depth of this interpenetration significantly affects the resulting stagnation conditions. We are interested both in the fundamental physics of the interaction, as well as the potential of the target platform for use as a novel neutron source.

The Advanced Radiographic Capability (ARC) at the National Ignition Facility (NIF) recently demonstrated that it was able to produce high proton energies via the target normal sheath acceleration (TNSA) mechanism (see Mariscal et al POP 2019). This result was extremely exciting since most laser driven proton acceleration experiments were conducted with short pulse length (sub -picosecond) pulses with small laser focal spots (<10 microns FWHM). In contrast, ARC was able to generate higher than expected proton energies with multi-picosecond pulses and a relatively large focal spot (~ 10s of microns FWHM). My work this summer was working to support this ARC proton acceleration work with related experiments at the TITAN laser at the Jupiter Laser Facility. Most work describing how proton energy scales with laser parameters (pulse length, energy etc.) is done is the sub-picosecond regime. Our task this summer was to investigate how proton energies scale in the regime that ARC inhabits (long pulse lengths, large focal spot). In addition to gathering this data set, I began my work on designing the Thomson Parabola Charged Particle Spectrometer (TP-CPS), which is a proposed diagnostic for NIF-ARC. This diagnostic will be crucial for future ion acceleration measurements at NIF. This summer was used to get feedback on my preliminary design from the NIF/NIF-ARC community.

This project involved modeling the phase transition kinetics of water to the high-pressure ice VII phase for a suite of experiments carried out at the Sandia Thor-64 pulsed power machine by Nissen and Dolan [J. Appl. Phys. 126, 015903 (2019)]. A good portion of the work focused on developing a new method for determining the drive pressure that the drive mechanism (e.g., magnetic drive, laser drive, etc.) exerts on the target material (for our case water). After developing a new optimization methodology to determine this drive pressure, the ramp-wave drive pressure was determined for each of the six Nissen and Dolan experiments. Each of the Nissen and Dolan experiments involves a target side with water (where a liquid--solid phase transition is induced) and a control target side that lacks the water sample. We used the control side in these experiments to develop the drive pressure, as the kinetics of the transition do not need to be accounted for, and subsequently applied this same drive pressure to the water side (in theory the drive pressures should be equal). This allowed us to investigate and simulate the kinetics of the phase transition that occurs on the water side with an accurate drive pressure derived from our new drive-pressure optimization methodology.