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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Toroidal modeling of interaction between internal kink mode and plasma flow

Non-linear interaction between the internal kink mode and toroidal plasma rotation is numerically studied using the MARS-Q code. Simulation results show core plasma flow damping due to various toroidal torques, generated by a weakly stable internal kink mode. Here, the 3-D field perturbation induced torques, including the neoclassical toroidal viscous (NTV) torque, as well as that produced by the Maxwell and Reynolds stresses, act as sink terms in the toroidal momentum balance model. The NTV torque is found to play a dominant role in the flow damping in all cases considered in this study. The modification to the internal kink mode structure is observed during the flow damping. Whilst a steady state can be achieved in the coupled mode-flow evolution with a uniform initial flow, a sheared initial flow affects the linear stability of the mode and consequently changes the non-linear evolution. For cases where the steady state solution is achieved, the saturated plasma flow speed critically depends on the initial flow condition as well as the initial amplitude of the internal kink mode but is less sensitive to the on-axis safety factor q0, as long as the latter stays above 1.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Do Impulsive Solar-Energetic-Electron (SEE) Events Drive High-Voltage Charging Events on the Nightside of the Moon?

When the Earth’s moon is in the supersonic solar wind, the darkside of the Moon and the lunar plasma wake can be very dangerous charging environments. In the absence of photoelectron emission (dark) and in the absence of cool plasma (wake), the emission or collection of charge to reduce electrical potentials is difficult. Unique extreme charging events may occur during impulsive solar-energetic-electron (SEE) events when the lunar wake is dominated by relativistic electrons, with the potential to charge and differentially charge objects on and above the lunar surface to very-high negative electrical potentials. In this report the geometry of the magnetic connections from the Sun to the lunar nightside are explored; these magnetic connections are the pathways for SEEs from the Sun. Rudimentary charging calculations for objects in the relativistic-electron environment of the lunar wake are performed. To enable these charging calculations, secondary-electron yields for impacts by relativistic electrons are derived. Needed lunar electrical-grounding precautions for SEE events are discussed. Calls are made 1) for future dynamic simulations of the plasma wake in the presence of time-varying SEE-event relativistic electrons and time-varying solar-wind magnetic-field orientations and 2) for future charging calculations in the relativistic-electron wake environment and on the darkside lunar surface.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2007

Simulation Study of Magnetic Fields generated by the Electromagnetic Filamentation Instability driven by Pair Loading

Using a 3-D relativistic particle-in-cell (RPIC) code, we have investigated particle acceleration associated with a relativistic electron-positron (cold) jet propagating into ambient electron-positron and electron-ion plasmas without initial magnetic fields in order to investigate the nonlinear stage of the Weibel instability. We have also performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which are assumed to be created by the photon annihilation. The growth time and nonlinear saturation levels depend on the initial jet parallel velocity distributions and ambient plasma. Simulations show that the Weibel instability created in the collisionless shocks accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude of densities, currents, electric, and magnetic fields in the electron-ion ambient plasma are larger than those in the electron-positron ambient plasma. We have shown that plasma instabilities driven by these streaming electron-positron pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. These fields maintain a strong saturated level on timescales much longer than the electron skin depth at least for the duration of the simulations. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between electron-positron pairs and ions, and may help explain the origin of large upstream fields in GRB shock.

Nishikawa, K.-I.↗
DOE OSTI2024

Global micro-tearing modes in the wide pedestal of an NSTX plasma

Global linear electromagnetic gyrokinetic simulations of micro-tearing modes (MTMs) growing in the wide pedestal of the ELM-free NSTX discharge #132 588 are reported. Two micro-tearing branches growing near the surfaces of safety factor q = 4 (pedestal top) and q=5–6 (pedestal) are simulated. The frequencies of these MTMs with low toroidal mode number (n ranging from 3 to 6) are in agreement with the ones of quasi-coherent fluctuations present on the magnetic spectrogram. No kinetic ballooning mode (KBM) is observed at experimental values of β; instead, the experimental plasma lies 20% below the critical global KBM threshold. This confirms local simulation results stating that experimental NSTX profiles lie in the KBM stability region. Carbon impurities, which are abundant in this plasma, were found to play a significant role. Neglecting them leads to a ∼50% error in the growth rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Effects of Coulomb coupling on stopping power and a link to macroscopic transport

Molecular dynamics simulations are used to assess the influence of Coulomb coupling on the energy evolution of charged projectiles in the classical one-component plasma. The average projectile kinetic energy is found to decrease linearly with time when ν α /ω p ≲ 10 –2 , where να is the Coulomb collision frequency between the projectile and the medium, and ω p is the plasma frequency. Stopping power is obtained from the slope of this curve. In comparison to the weakly coupled limit, strong Coulomb coupling causes the magnitude of the dimensionless stopping power, ( a/k B T ) dE / dx , to increase, the Bragg peak to shift to several times the plasma thermal speed, and for the stopping power curve to broaden substantially. The rate of change of the total projectile kinetic energy averaged over many independent simulations is shown to consist of two measurable components: a component associated with a one-dimensional friction force and a thermal energy exchange rate. In the limit of a slow and massive projectile, these can be related to the macroscopic transport rates of self-diffusion and temperature relaxation in the plasma. Simulation results are compared with available theoretical models for stopping power, self-diffusion, and temperature relaxation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Impact of hydrofluorocarbon molecular structure parameters on plasma etching of ultra-low-K dielectric

The authors report a systematic study aimed at evaluating the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) films and etching performance of a representative ultra-low-k material, along with amorphous carbon. The precursor gases studied included fluorocarbon and hydrofluorocarbon gases whose molecular weights and chemical structures were systematically varied. Gases with three different degrees of unsaturation (DU) were examined. Trifluoromethane (CHF{sub 3}) is the only fully saturated gas that was tested. The gases with a DU value of one are 3,3,3-trifluoropropene (C{sub 3}H{sub 3}F{sub 3}), hexafluoropropene (C{sub 3}F{sub 6}), 1,1,3,3,3-pentafluoro-1-propene (C{sub 3}HF{sub 5}), (E)-1,2,3,3,3-pentafluoropropene (C{sub 3}HF{sub 5} isomer), heptafluoropropyl trifluorovinyl ether (C{sub 5}F{sub 10}O), octafluorocyclobutane (C{sub 4}F{sub 8}), and octafluoro-2-butene (C{sub 4}F{sub 8} isomer). The gases with a DU value of two includes hexafluoro-1,3-butadiene (C{sub 4}F{sub 6}), hexafluoro-2-butyne (C{sub 4}F{sub 6} isomer), octafluorocyclopentene (C{sub 5}F{sub 8}), and decafluorocyclohexene (C{sub 6}F{sub 10}). The work was performed in a dual frequency capacitively coupled plasma reactor. Real-time characterization of deposition and etching was performed using in situ ellipsometry, and optical emission spectroscopy was used for characterization of CF{sub 2} radicals in the gas phase. The chemical composition of the deposited FC films was examined by x-ray photoelectron spectroscopy. The authors found that the CF{sub 2} fraction, defined as the number of CF{sub 2} groups in a precursor molecule divided by the total number of carbon atoms in the molecule, determines the CF{sub 2} optical emission intensity of the plasma. CF{sub 2} optical emission, however, is not the dominant factor that determines HFC film deposition rates. Rather, HFC film deposition rates are determined by the number of weak bonds in the precursor molecule, which include a ring structure, C=C, C≡C, and C–H bonds. These bonds are broken preferentially in the plasma, and/or at the surface and fragments arriving at the substrate surface presumably provide dangling bonds that efficiently bond to the substrate or other fragments. Upon application of a radio-frequency bias to the substrate, substrate etching is induced. Highly polymerizing gases show decreased substrate etching rates as compared to HFC gases characterized by a lower HFC film deposition rate. This can be explained by a competition between deposition and etching reactions, and an increased energy and etchant dissipation in relatively thicker steady state FC films that form on the substrate surface. Deposited HFC films exhibit typically a high CF{sub 2} density at the film surface, which correlates with both the CF{sub 2} fractions in the precursor molecular structure and the deposition rate. The FC films deposited using hydrogen-containing precursors show higher degrees of crosslinking and lower F/C ratios than precursors without hydrogen, and exhibit a lower etch rate of substrate material. A small gap structure that blocks direct ion bombardment was used to simulate the sidewall plasma environment of a feature and was employed for in situ ellipsometry measurements. It is shown that highly polymerizing precursors with a DU of two enable protection of low-k sidewalls during plasma exposure from oxygen-related damage by protective film deposition. Dielectric film modifications are seen for precursors with a lower DU.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Generation of torsional Alfvén and slow magnetosonic waves by periodic bunches of laser plasma in a magnetised background

Using numerical simulation, the influence of parameters of a magnetised background plasma on the intensity of quasi-stationary Alfvén and slow magnetosonic waves produced by periodic bunches of a laser plasma in a magnetic field tube is investigated. It is shown that in a weak magnetic field, the waves propagate together, carrying a flow of rotating plasma, and in a strong field, the bunches form simultaneously two types of waves propagating with different velocities. (selected papers presented at the viii symposium 'mplp-2018')

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Physics-assisted Latent Space Dynamics Learning for Stiff Collisional-radiative Models

Collisional-radiative (CR) models describe the atomic processes in a plasma by tracking the population density in the ground and excited states for each charge state of the atom/ion. These models predict important plasma properties such as charge state distributions and radiative emissivity and opacity. Accurate descriptions of the CR balance of the plasma are essential in fusion whole device modeling, especially when significant impurities are introduced into the plasmas. In an integrated fusion plasma and CR simulation, a CR model, which is a high-dimensional stiff ODE, needs to be solved on each grid point in the configuration space, and can overwhelm the overall computational cost. In this work, we propose a machine-learning-based method that discovers a latent space and learns its corresponding latent dynamics, which can capture the essential physics to make accurate predictions at much lower online computational cost. The proposed approach is physics-assisted, due to its combination of a physical latent space and a data-driven latent space. It has been demonstrated that the proposed architecture can predict both the full-order CR dynamics and a physical quantity of interest accurately.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Experimental Simulation of Meteorite Ablation during Earth Entry Using a Plasma Wind Tunnel

Three different types of rocks were tested in a high enthalpy air plasma flow. Two terrestrial rocks, basalt and argillite, and an ordinary chondrite, with a 10 mm diameter cylindrical shape were tested in order to observe decomposition, potential fragmentation, and spectral signature. The goal was to simulate meteoroid ablation to interpret meteor observation and compare these observations with ground based measurements. The test flow with a local mass-specific enthalpy of 70 MJ kg{sup −1} results in a surface heat flux at the meteorite fragment surface of approximately 16 MW m{sup −2}. The stagnation pressure is 24 hPa, which corresponds to a flight condition in the upper atmosphere around 80 km assuming an entry velocity of 10 km s{sup −1}. Five different diagnostic methods were applied simultaneously to characterize the meteorite fragmentation and destruction in the ground test: short exposure photography, regular video, high-speed imaging with 10 kHz frame rate, thermography, and Echelle emission spectroscopy. This is the first time that comprehensive testing of various meteorite fragments under the same flow condition was conducted. The data sets indeed show typical meteorite ablation behavior. The cylindrically shaped fragments melt and evaporate within about 4 s. The spectral data allow the identification of the material from the spectra which is of particular importance for future spectroscopic meteor observations. For the tested ordinary chondrite sample a comparison to an observed meteor spectra shows good agreement. The present data show that this testing methodology reproduces the ablation phenomena of meteoritic material alongside the corresponding spectral signatures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Development and transferability of neural-network models for plasma-surface interactions

Plasma-surface interactions are increasingly critical to modern technologies; yet, accurate molecular dynamics simulations remain limited by the capabilities of interatomic potentials. Deep Potentials (DPs) promise to revolutionize the field by providing a systematic method for producing accurate interatomic potentials. The primary challenge of DP development is selecting a dataset, which efficiently spans the set of atomic environments one expects to encounter in the subsequent molecular dynamics simulations. The computational cost of density functional theory calculations, which are the typical basis for DP development, makes it impossible to directly verify the quality of a given DP. To address this challenge, we explore the development of a deep-learned interatomic potential, “DeepREBO,” trained to reproduce the behavior of the REBO2 empirical potential, enabling direct validation of training methodology and transferability. Using an active learning framework, we begin with a minimal dataset and iteratively expand it to train a Deep Potential-Smooth Edition model that faithfully reproduces REBO2 results for 25 eV hydrogen bombardment of diamond (001), a particularly challenging case. We show that small, carefully curated datasets can outperform large, unguided ones, with effective models requiring fewer than 15 000 snapshots. Subsequent transferability tests demonstrate that while DeepREBO generalizes well to diamond (111) surfaces, performance degrades for amorphous carbon or higher-energy impacts, highlighting the need for use-case-specific training data. We also evaluate methods to improve short-range repulsion. This study outlines best practices for training robust deep potentials and underscores the importance of dataset design for predictive plasma simulations.

Ab-initio molecular dynamics↗
NASA NTRS2020

Distributed Spacecraft Autonomy (DSA): Development of Swarm Autonomy Capability and Scalability for Spacecraft

The Distributed Spacecraft Autonomy project is developing a suite of software tools that enable an operator to command and receive data from a swarm as a single entity, enable a swarm to autonomously coordinate its actions via distributed decision making and reactive closed-loop control, and model swarm behavior in the presence of anomalies or failures. Our use case is the mapping of the electron density of the ionosphere using radio tomography by coordinating the selection of appropriate GPS channels, and by recording Total Electron Count (TEC)measurements. DSA will be demonstrated on board the NASA Ames Starling mission a swarm of four small, LEO spacecraft, scheduled to launch in 2021. We will also perform a ground demonstration with simulated and hardware-in-the-loop elements, to validate the tools for controlling swarms of up to 100 assets.The capability to communicate autonomously between the swarm satellites is demonstrated via a sophisticated simulation architecture. Historical Plasma sphere TEC data obtained via dual-band Novatel GPS Receivers are utilized as a representative input data set for the swarm. The representative TEC data and GPS satellite observability information is fed to the autonomous software package in place of a true real-time ground data collection process. The swarm satellites actively share status updates amongst one another and utilize multi-agent decision making to optimally identify regions of interest in the TEC distribution. The software,aware of the bandwidth limitations of the swarm satellites, prioritizes explorative measurements,which define the range of observability for the satellites, as well as exploitative measurements,which focus on maximizing the observance potential of regions with prolonged, elevated TEC density. The science of this study can ultimately be used to determine the dynamics and coupling of Earth's magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs. The findings can be applied to the imaging of critical, transient phenomena in the magnetosphere in later missions. Meanwhile, the swarm autonomy capabilities have far reaching potential in future satellite missions.As an experimental demonstration of the autonomous capabilities of the network, a message is first printed within a core Flight Executive (cFE) application. Two cFE applications that communicate with one another within the same core Flight System (cFS) are shown.Communication between mission applications on the internal cFE bus is extended to utilize Data Distribution Service (DDS) for vehicle-to-vehicle networking. The DDS middle ware provides reliable delivery, routing, and topic subscription features over User Data gram Protocol (UDP).Leveraging Linux containerization, a networked set of satellite instances are generated by script to simulate swarm behavior. Swarm commanding and synchronization through the network is demonstrated under various topologies and data-loss conditions. Finally, autonomous swarms calability from 2 satellites to 100 satellites is shown.

Fugate, Jason↗
NASA NTRS2011

Magnetohydrodynamic Turbulence and Reconnection in the Magnetotail

We have used a global MHD simulation with high spatial resolution to investigate the origin and properties of turbulence in the plasma sheet. In this simulation we imposed a steady southward IMF with a magnitude of 5 nT at the upstream simulation boundary for more than three hours followed by ninety minutes of northward IMF of the same magnitude. The solar wind number density was 20 cm-3, the thermal pressure was 20 pPa, and the velocity was 500 km/s in the x direction. The moderately high dynamic pressure confined the magnetotail to the high-grid resolution region. Even for these nominal solar wind parameters and steady driving the plasma sheet became turbulent. The power spectral densities and probability distribution functions computed from the simulations were comparable to those obtained from spacecraft observations. The largest scale vortices were associated with reconnection outflows and, in the southward IMF case, with the diversion of high speed flows in the near-Earth region. Both time and space domain analyses revealed that there were three scales present, the large scale of the driving processes, the intermediate inertial scale and the dissipative scale.

El-Alaoui, Mostafa↗
DOE OSTI2016

Whistler turbulence heating of electrons and ions: Three-dimensional particle-in-cell simuations

In this study, the decay of whistler turbulence in a collisionless, homogeneous, magnetized plasma is studied using three-dimensional particle-in-cell simulations. The simulations are initialized with a narrowband, relatively isotropic distribution of long wavelength whistler modes. A first ensemble of simulations at electron beta ${\beta }_{{\rm{e}}}$ = 0.25 and ion-to-electron mass ratio ${m}_{{\rm{i}}}$/${m}_{{\rm{e}}}$ = 400 is carried out on a domain cube of dimension $L{\omega }_{\mathrm{pi}}$/c = 5.12 where ${\omega }_{\mathrm{pi}}$ is the ion plasma frequency. The simulations begin with a range of dimensionless fluctuating field energy densities, ${\epsilon }_{{\rm{o}}}$, and follow the fluctuations as they cascade to broadband, anisotropic turbulence which dissipates at shorter wavelengths, heating both electrons and ions. The electron heating is stronger and preferentially parallel/antiparallel to the background magnetic field ${{\boldsymbol{B}}}_{{\rm{o}}};$ the ion energy gain is weaker and is preferentially in directions perpendicular to ${{\boldsymbol{B}}}_{{\rm{o}}}$. The important new results here are that, over 0.01 < ${\epsilon }_{{\rm{o}}}$ < 0.25, the maximum rate of electron heating scales approximately as ${\epsilon }_{{\rm{o}}}$, and the maximum rate of ion heating scales approximately as ${\epsilon }_{{\rm{o}}}^{1.5}$. A second ensemble of simulations at ${\epsilon }_{{\rm{o}}}$ = 0.10 and ${\beta }_{{\rm{e}}}$ = 0.25 shows that, over 25 < ${m}_{{\rm{i}}}$/${m}_{{\rm{e}}}\;$< 1836, the ratio of the maximum ion heating rate to the maximum electron heating rate scales approximately as ${m}_{{\rm{e}}}$/${m}_{{\rm{i}}}$.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Performance Analysis of PIConGPU: Particle-in-Cell on GPUs using NVIDIA’s NSight Systems and NSight Compute

PIConGPU, Particle In Cell on GPUs, is an open source simulations framework for plasma and laser-plasma physics used to develop advanced particle accelerators for radiation therapy of cancer, high energy physics and photon science. While PIConGPU has been optimized for at least 5 years to run well on NVIDIA GPU-based clusters, there has been limited exploration by the development team of potential scalability bottlenecks using recently updated and new tools including NVIDIA’s NVProf tool and the brand-new NVIDIA NSight Suite (Systems and Compute) tools. PIConGPU is a highly optimized application that runs production jobs at scale on a system Oak Ridge Leadership Facility’s (OLCF) Summit supercomputer (using the full machine at 4600 nodes; at 98% of GPU utilization on all ~28000 NVIDIA Volta GPUs). PIConGPU has been selected as one of the the eight applications for OLCF’s coveted Center for Accelerated Application Readiness (CAAR) program aimed at the facility’s Frontier supercomputer (OLCF’s first exascale system to launch in 2021), to partner with our vendors (primary vendors: AMD and Cray/HPE) ensuring that Frontier will be able to perform large-scale science when it opens to users in 2022. To this effect, performance engineers on the PIConGPU team wanted to dive deep into the application to understand at the finest granularity, which portions of the code could be further optimized to exploit the hardware on Summit at it’s maximum potential and also to elucidate which key kernels should be tracked and optimized for the CAAR effort to port this code to Frontier. Any bottlenecks that are observed via performance profiling on Summit are likely to also impact scalability on the Frontier-dev system and the Frontier Early Access (EA) system. Additionally, the engineers wanted to take a closer look at the newest NVIDIA profiling tools which allows us to identify the most useful features on these tools and will provide an opportunity to compare it to new AMD and Cray’s performance analysis tool releases and provide feedback to our vendor partners on what features are most important and mission critical for CAAR efforts. The primary goal of this report is to focus on the evaluation of PIConGPU’s most time-intensive kernels using NVProf and NSight Suite. Three kernels, Current Deposition (also known as Compute Current), Particle Push (Move and Mark), and Shift Particles are known to be some of the most time-consuming kernels in PIConGPU. The Current xi Deposition kernel and Particle Push kernel both set up the particle attributes for running any physics simulation with PIConGPU, so it is crucial to improve the performance of these two kernels. In this report, we measure single GPU metrics for the three kernels, offer high level takeaways from the conducted analysis, and compare the profiling data from NSight Compute to that of NVProf. This analysis was performed using a grid size of 240 x 272 x 224, and 10 time steps with the Mid-November Figure of Merit (FOM) run setup. The Traveling Wave Electron Acceleration (TWEAC) science case used in this run is a representative science case for PIConGPU. This execution can also be used for baseline analysis on AMD MI50/ MI60 systems. As of the time of writing, the PIConGPU application has limited use for features of NSight Systems, so this report will mainly focus on insights garnered from NSight Compute. For this analysis, we run the “full” metric set available in NSight Compute version 2020.1.2 and use NSight Systems version 2020.3.1 to generate the application timeline.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2012

Vlasov Simulation of Electrostatic Solitary Structures in Multi-Component Plasmas

Electrostatic solitary structures have been observed in the Earth's magnetosheath by the Cluster spacecraft. Recent theoretical work has suggested that these solitary structures are modeled by electron acoustic solitary waves existing in a four-component plasma system consisting of core electrons, two counter-streaming electron beams, and one species of background ions. In this paper, the excitation of electron acoustic waves and the formation of solitary structures are studied by means of a one-dimensional electrostatic Vlasov simulation. The present result first shows that either electron acoustic solitary waves with negative potential or electron phase-space holes with positive potential are excited in four-component plasma systems. However, these electrostatic solitary structures have longer duration times and higher wave amplitudes than the solitary structures observed in the magnetosheath. The result indicates that a high-speed and small free energy source may be needed as a fifth component. An additional simulation of a five-component plasma consisting of a stable four-component plasma and a weak electron beam shows the generation of small and fast electron phase-space holes by the bump-on-tail instability. The physical properties of the small and fast electron phase-space holes are very similar to those obtained by the previous theoretical analysis. The amplitude and duration time of solitary structures in the simulation are also in agreement with the Cluster observation.

magnetohydrodynamics (MHD)↗
DOE OSTI2022

Voltage waveform tailoring for high aspect ratio plasma etching of SiO 2 using Ar/CF 4 /O 2 mixtures: Consequences of ion and electron distributions on etch profiles

The quality of high aspect ratio (HAR) features etched into dielectrics for microelectronics fabrication using halogen containing low temperature plasmas strongly depends on the energy and angular distribution of the incident ions (IEAD) onto the wafer, as well as potentially that of the electrons (EEAD). Positive ions, accelerated to high energies by the sheath electric field, have narrow angular spreads and can penetrate deeply into HAR features. Electrons typically arrive at the wafer with nearly thermal energy and isotropic angular distributions and so do not directly penetrate deeply into features. These differences can lead to positive charging of the insides of the features that can slow etching rates and produce geometric defects such as twisting. In this work, we computationally investigated the plasma etching of HAR features into SiO 2 using tailored voltage waveforms in a geometrically asymmetric capacitively coupled plasma sustained in an Ar/CF 4 /O 2 mixture at 40 mTorr. The tailored waveform consisted of a sinusoidal wave and its higher harmonics with a fundamental frequency of 1 MHz. We found that some degree of control of the IEADs and EEADs is possible by adjusting the phase of higher harmonics φ through the resulting generation of electrical asymmetry and electric field reversal. However, the IEADs and EEADs cannot easily be separately controlled. The control of IEADs and EEADs is inherently linked. The highest quality feature was obtained with a phase angle φ = 0° as this value generated the largest (most negative) DC self-bias and largest electric field reversal for accelerating electrons into the feature. That said, the consequences of voltage waveform tailoring (VWT) on etched features are dominated by the change in the IEADs. Although VWT does produce EEADs with higher energy and narrower angular spread, the effect of these electrons on the feature compared to thermal electrons is not large. Finally, this smaller impact of VWT produced EEADs is attributed to thermal electrons being accelerated into the feature by electric fields produced by the positive in-feature charging.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Scrape-off layer plasma and neutral characteristics and their interactions with walls for FNSF

Simulations of the heat flux on plasma facing components from exhausting core plasma are reported for two possible Fusion Nuclear Science Facility (FNSF) divertor configurations. One configuration utilizes divertor plates strongly inclined with respect to the poloidal magnetic flux surfaces like that planned for ITER and results in a partially detached divertor-plasma. The second configuration has divertor plates orthogonal to the flux surfaces, which leads to a fully detached divertor-plasma if the width of the divertor region is sufficient. Both configurations use scrape-off layer impurity seeding to yield an acceptable peak heat flux of ~10 MW/m 2 or smaller on the divertor plates and chamber walls. The roles of recycled hydrogenic atoms and molecules are investigated and distribution of sputtering tungsten throughout the edge region modeled. Furthermore, the simulations are performed with the UEDGE 2D transport code to model both plasma and neutral components with supplementary neutral modeling performed with the DEGAS 2 Monte Carlo code.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Variational symplectic particle-in-cell simulation of nonlinear mode conversion from extraordinary waves to Bernstein waves

In this paper, the nonlinear mode conversion of extraordinary waves in nonuniform magnetized plasmas is studied using the variational symplectic particle-in-cell simulation. The accuracy of the nonlinear simulation is guaranteed by the long-term accuracy and conservativeness of the symplectic algorithm. The spectra of the electromagnetic wave, the evolution of the wave reflectivity, the energy deposition profile, and the parameter-dependent properties of radio-frequency waves during the nonlinear mode conversion are investigated. It is illustrated that nonlinear effects significantly modify the physics of the radio-frequency injection in magnetized plasmas. The evolutions of the radio-frequency wave reflectivity and the energy deposition are observed, as well as the self-interaction of the Bernstein waves and mode excitations. Even for waves with small magnitude, nonlinear effects can also become important after continuous wave injections, which are common in the realistic radio-frequency wave heating and current drive experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗