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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Suppressing Two-Plasmon Decay with Laser Frequency Detuning

Three-dimensional laser-plasma interaction simulations show that laser frequency detuning by an amount achievable with current laser technology can be used to suppress the two-plasmon decay (TPD) instability and the corresponding hot-electron generation. For the plasma conditions and laser configuration in a direct-drive inertial confinement fusion implosion on the OMEGA laser, the simulations show that ~0.7% laser frequency detuning is sufficient to eliminate TPD-driven hot-electron generation in current experiments. In conclusion, this allows for higher ablation pressures in future implosion designs by using higher laser intensities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1987

Ground testing of array modules for the photovoltaic array space power (PASP) experiment

One of the objectives of the PASP experiment is the verification of cost-effective ground simulations of high-voltage solar array/space-environment interactions by comparing the results with flight data. These ground tests consist of electrical characterization, thermal cycling, and plasma chamber simulations. The results of the latter tests are reported. Five array modules which are representative of the flight arrays were tested. The module types are planar silicon, planar gallium arsenide, planar silicon passivated with an integrally deposited cover glass, mini-Cassegrainian concentrator, and SLATS concentrator. The modules were biased to -1000 V in varying plasma densities from 4 x 103 to 2 x 105 e-/cu cm. Each array was tested in both dark and illuminated conditions with a load resistance. In addition to monitoring arcing during the plasma tests, the arrays were visually inspected and electrically characterized before and after exposure in the chamber. The electrical results are tabulated and briefly discussed.

Morris, Robert K.↗
DOE OSTI2026

Enhancing photoionization rate calculations in low-temperature plasmas using spectral methods

Photoionization plays a central role in the development of streamer discharges and other non-equilibrium plasma phenomena. It creates seed electrons, which are essential for positive streamer propagation, allowing the ionization front to move forward. Because of this, accurate modeling of photoionization is very important for predicting streamer behavior and plasma evolution. The photoionization process in air (N 2 – O 2 mixture) is often described by the Zheleznyak model (1982). This model is usually solved through Helmholtz-type equations that approximate the Zheleznyak photoionization model (Zheleznyak et al. 1982) as Partial Differential Equations (PDEs). Conventional numerical methods, such as the Finite Difference Method (FDM) or Finite Volume Method (FVM), are widely used to solve these equations. Although they are prevalent, the computational cost of these methods due to their need for matrix operations and iterative solver is demanding. To address this challenge, this work develops a spectral solver based on the Fast Fourier Transform (FFT) combined with Discrete Cosine Transform (DCT) and Discrete Sine Transform (DST) to calculate the photoionization rate efficiently in an axisymmetric cylindrical domain. This method naturally satisfies the boundary conditions used in the model and converts the PDE into algebraic ones in spectral space. Thus, avoids the need for iterative matrix solvers. When compared with FDM results, it is demonstrated that the new solver not only maintains accuracy, but also reduces the computational cost, showing a performance increase of approximately 100 compared to FDM over a wide range of problem sizes. The method is parallelized using Message Passing Interface (MPI) and has been integrated into a fluid plasma model for streamer simulation. Here, this FFT-based approach provides a fast and reliable alternative for calculating photoionization in fluid models, helping large-scale plasma simulations run faster and efficiently, and allows higher-resolution simulation without extra computational cost.

Axisymmetric system↗
DOE OSTI2014

On the relationship between quadrupolar magnetic field and collisionless reconnection

Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Simulation analysis of the effects of an initial cone position and opening angle on a cone-guided implosion

In inertial confinement fusion, the implosion process is important in forming a high-density plasma core. In the case of a fast ignition scheme using a cone-guided target, the fuel target is imploded with a cone inserted. This scheme is advantageous for efficiently heating the imploded fuel core; however, asymmetric implosion is essentially inevitable. Moreover, the effect of cone position and opening angle on implosion also becomes critical. Focusing on these problems, the effect of the asymmetric implosion, the initial position, and the opening angle on the compression rate of the fuel is investigated using a three-dimensional pure hydrodynamic code.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

THE EFFECT OF ANISOTROPIC VISCOSITY ON COLD FRONTS IN GALAXY CLUSTERS

Cold fronts—contact discontinuities in the intracluster medium (ICM) of galaxy clusters—should be disrupted by Kelvin-Helmholtz (K-H) instabilities due to the associated shear velocity. However, many observed cold fronts appear stable. This opens the possibility of placing constraints on microphysical mechanisms that stabilize them, such as the ICM viscosity and/or magnetic fields. We performed exploratory high-resolution simulations of cold fronts arising from subsonic gas sloshing in cluster cores using the grid-based Athena MHD code, comparing the effects of isotropic Spitzer and anisotropic Braginskii viscosity (expected in a magnetized plasma). Magnetized simulations with full Braginskii viscosity or isotropic Spitzer viscosity reduced by a factor f ∼ 0.1 are both in qualitative agreement with observations in terms of suppressing K-H instabilities. The rms velocity of turbulence within the sloshing region is only modestly reduced by Braginskii viscosity. We also performed unmagnetized simulations with and without viscosity and find that magnetic fields have a substantial effect on the appearance of the cold fronts, even if the initial field is weak and the viscosity is the same. This suggests that determining the dominant suppression mechanism of a given cold front from X-ray observations (e.g., viscosity or magnetic fields) by comparison with simulations is not straightforward. Finally, we performed simulations including anisotropic thermal conduction, and find that including Braginskii viscosity in these simulations does not significantly affect the evolution of cold fronts; they are rapidly smeared out by thermal conduction, as in the inviscid case.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Electron and ion kinetic effects on non-linearly driven electron plasma and ion acoustic waves

Fully non-linear kinetic simulations of electron plasma and ion acoustic waves (IAWs) have been carried out with a new multi-species, parallelized Vlasov code. The numerical implementation of the Vlasov model and the methods used to compute the wave frequency are described in detail. For the first time, the nonlinear frequency of IAWs, combining the contributions from electron and ion kinetic effects and from harmonic generation, has been calculated and compared to Vlasov results. Excellent agreement of theory with simulation results is shown at all amplitudes, harmonic generation being an essential component at large amplitudes. For IAWs, the positive frequency shift from trapped electrons is confirmed and is dominant for the effective electron-to-ion temperature ratio, Z T{sub e}/T{sub i} Greater-Than-Or-Equivalent-To 10 with Z as the charge state. Furthermore, numerical results demonstrate unambiguously the dependence [R. L. Dewar, Phys. Fluids 15, 712 (1972)] of the kinetic shifts on details of the distribution of the trapped particles, which depends in turn on the conditions under which the waves were generated. The trapped particle fractions and energy distributions are derived and, upon inclusion of harmonic effects, shown to agree with the simulation results, completing a consistent picture. Fluid models of the wave evolution are considered but prove unable to capture essential details of the kinetic simulations. Detrapping by collisions and sideloss is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Particle-in-cell simulations of multi-MeV pulsed X-ray induced air plasmas at low pressures

A full kinetic modelling of the charge particles dynamics generated upon the irradiation of an air-filled cavity by a multi-MeV pulsed x-ray is performed. From the calculated radiative source generated by the ASTERIX generator, we calculated the electromagnetic fields generated by x-ray induced air plasmas in a metallic cavity at different pressures. Simulations are carried out based on a Particle-In-Cell interpolation method which uses 3D Maxwell-Vlasov calculations of the constitutive charged species densities of air plasmas at different pressures at equilibrium. The resulting electromagnetic fields within the cavity are calculated for different electron densities up to 4 × 10{sup 10 }cm{sup −3}. For each air pressure, we show electronic plasma waves formation followed by Landau damping. As electron density increases, the calculations exhibit space-charged neutralization and return current formation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Efficient data acquisition and training of collisional-radiative model artificial neural network surrogates through adaptive parameter space sampling

Abstract Effective plasma transport modeling of magnetically confined fusion devices relies on having an accurate understanding of the ion composition and radiative power losses of the plasma. Generally, these quantities can be obtained from solutions of a collisional-radiative (CR) model at each time step within a plasma transport simulation. However, even compact, approximate CR models can be computationally onerous to evaluate, and in-situ evaluation of these models within a larger plasma transport code can lead to a rigid bottleneck. As a way to bypass this bottleneck, we propose deploying artificial neural network (ANN) surrogates to allow rapid evaluation of the necessary plasma quantities. However, one issue with training an accurate ANN surrogate is the reliance on a sufficiently large and representative training and validation data set, which can be time-consuming to generate. In this work we explore a data-driven active learning and training routine to allow autonomous adaptive sampling of the problem parameter space to ensure a sufficiently large and meaningful set of training data is assembled for the network training. As a result, we can demonstrate approximately order-of-magnitude savings in required training data samples to produce an accurate surrogate.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2013

PARTICLE ACCELERATION DURING MAGNETOROTATIONAL INSTABILITY IN A COLLISIONLESS ACCRETION DISK

Particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk was investigated by using a particle-in-cell simulation. We discuss the important role that magnetic reconnection plays not only on the saturation of MRI but also on the relativistic particle generation. The plasma pressure anisotropy of p > p{sub ||} induced by the action of MRI dynamo leads to rapid growth in magnetic reconnection, resulting in the fast generation of nonthermal particles with a hard power-law spectrum. This efficient particle acceleration mechanism involved in a collisionless accretion disk may be a possible model to explain the origin of high-energy particles observed around massive black holes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1983

Simulations of beam excited minor species gyroharmonics in the Porcupine experiment

An active experiment for the study of wave particle interaction in the presence of a perpendicular beam type distribution is considered. The experiment involves the injection of a beam perpendicular to the magnetic field. Beam parameters can then be varied while local plasma parameters and associated wave phenomena are measured. Such an experiment was conducted as part of the Porcupine project. The present investigation has the objective to explain theoretically and by means of plasma particle simulations, some of the wave observations obtained. In the experiment, a main rocket payload and four subpayloads were employed. The subpayloads were ejected radially in different directions at an altitude of 240 km. One of the spinning subpayloads contained a plasma gun which emitted a beam of 200 eV xenon ions toward the main payload. The data recorded on the main payload revealed the existence of electrostatic waves with peaks at hydrogen gyroharmonics. A series of computer simulations approximating the xenon experiment was performed. The interpretation of the data is discussed.

Roth, I.↗
DOE OSTI2013

Influence of metastable atoms in the simulation of hollow cathode discharge

The characteristics of hollow cathode discharge are investigated by using two-dimensional fluid model combined with a transport model for metastable atoms (F-M model) in argon. It shows that the stepwise ionization is one of main important mechanism for electrons production. The distribution of electric potential, density of electrons, ions, and metastable atoms are calculated with a pressure of 10 Torr and a voltage of 250 V. The peak density of electron and ion is 1.2×10{sup 13} cm{sup −3}, and the peak density of metastable atoms is 3.5×10{sup 13} cm{sup −3}. The results obtained in F-M model are compared with that in fluid model (without metastable atoms involved). Metastable atoms are found to play an important role in the discharge. In addition, with the increase of pressure and voltage, the percentage of stepwise ionization in the total ionization increase, and the difference of discharge characteristics simulated by these two kinds of models rises.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Effects of non-equilibrium ionization and excitation on radiation absorption in plasma plumes induced by ablation of metal targets with nanosecond laser pulses

Ionization and radiation absorption in nanosecond laser-induced plasma plumes are routinely modeled using the Saha–Boltzmann equilibrium ionization model (EQM). However, the equilibrium assumption can be inaccurate during the laser pulse when non-equilibrium effects significantly impact radiation absorption. In the present work, the EQM and non-equilibrium collisional-radiative model (CRM) are compared to reveal the effect of plasma non-equilibrium on radiation absorption in non-homogeneous plumes and degree of plasma shielding. Simulations of plume expansion induced by irradiation of a copper target in 1 atm argon background gas with a 10 ns Gaussian pulse at a fluence from 8 Jcm −2 to 14 Jcm −2 are performed with a hybrid computational model that couples a lumped particle direct simulation Monte Carlo method with either CRM or EQM. The simulations show that the EQM strongly underestimates the effects of ionization and radiation absorption compared to CRM and, contrary to the CRM, predict strong ionization of the background gas. The differences between the models are explained by the qualitatively different coupling between plume expansion and dynamics of ionization and exitation processes in the CRM and EQM under conditions when the characteristic times for most radiation- and electron-induced processes are longer than the pulse duration. The CRM-based predictions are also found to agree much better with available experimental data. In conclusion, these results indicate that the model of Saha–Boltzmann equilibrium cannot be used for reliable prediction of the degree of plasma shielding in plumes induced by nanosecond laser pulses or for processing results of spectroscopic measurements at early stages of expansion of such plumes.

Physics - Plasma physics↗
DOE OSTI2019

Structural transformations of graphene exposed to nitrogen plasma: quantum chemical molecular dynamics simulations

Nitrogen-doped graphene (N-graphene) has been intensively studied for tailoring the electronic property of the graphene, because different nitrogen configurations influence the electronic properties of N-graphene in different ways. However, atomically precise control of the nitrogen configurations during the doping process remains a challenge in the synthesis of N-graphene. Moreover, additional structural transformations of the graphene carbon network structure as a side-effect of plasma doping are little understood and are as of yet uncontrollable. Therefore, we theoretically investigated the nitrogen doping process of graphene for a range of nitrogen atom incident kinetic energies in nonequilibrium quantum chemical molecular dynamics (QM/MD) simulations. We observed and characterized prominent configurations of N-containing graphene. In analogy to similar, earlier studies of graphene plasma hydrogenation, we observed an Eley–Rideal associative desorption mechanism during the graphene plasma nitrogenation, producing molecular nitrogen. Especially for graphitic-N (Gr-N) and Stone–Wales-defect-N (SW-N) configurations, which are frequently observed in experimental studies, we discovered two typical chemical reaction mechanisms which were well categorized by two key processes: adsorption of primary nitrogen dopant and collision with a secondary nitrogen dopant. Here, we discussed effects of the incident nitrogen energy on the formation mechanism, and propose a method to generate of Gr-N and SW-N configurations selectively by tuning the conditions with respect to the two key formation processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1981

Physics of auroral arc formation; Proceedings of the Chapman Conference on Formation of Auroral Arcs, Fairbanks, AK, July 21-25, 1980

The results of satellite, balloon-borne, and ground-based observations of auroral phenomena are discussed in terms of the morphology of auroral arcs and the behavior and characteristics of auroral electrons and ions. Further attention is given to auroral electric fields and field aligned currents, and to the development of models of auroral potential structures and energization of auroral particles. A simulation of space plasma phenomena is presented, along with numerical simulations of auroral potential structures and related phenomena, including the V-potential double layers and auroral arc deformations, and a simulation of auroral arcs. Plasma waves observed on auroral field lines and in laboratory conditions are reported, and theoretical studies of waves and turbulence in auroral plasmas are described.

Akasofu, S.-I.↗
NASA NTRS1991

The plasma physics of shock acceleration

The history and theory of shock acceleration is reviewed, paying particular attention to theories of parallel shocks which include the backreaction of accelerated particles on the shock structure. The work that computer simulations, both plasma and Monte Carlo, are playing in revealing how thermal ions interact with shocks and how particle acceleration appears to be an inevitable and necessary part of the basic plasma physics that governs collisionless shocks is discussed. Some of the outstanding problems that still confront theorists and observers in this field are described.

Jones, Frank C.↗
DOE OSTI2013

Characterisation of a MeV Bremsstrahlung x-ray source produced from a high intensity laser for high areal density object radiography

Results of an experiment to characterise a MeV Bremsstrahlung x-ray emission created by a short (<10 ps) pulse, high intensity (1.4 × 10{sup 19} W/cm{sup 2}) laser are presented. X-ray emission is characterized using several diagnostics; nuclear activation measurements, a calibrated hard x-ray spectrometer, and dosimeters. Results from the reconstructed x-ray energy spectra are consistent with numerical simulations using the PIC and Monte Carlo codes between 0.3 and 30 MeV. The intense Bremsstrahlung x-ray source is used to radiograph an image quality indicator (IQI) heavily filtered with thick tungsten absorbers. Observations suggest that internal features of the IQI can be resolved up to an external areal density of 85 g/cm{sup 2}. The x-ray source size, inferred by the radiography of a thick resolution grid, is estimated to be approximately 400 μm (full width half maximum of the x-ray source Point Spread Function)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1973

Simulator test to study hot-flow problems related to a gas cooled reactor

An advance study of materials, fuel injection, and hot flow problems related to the gas core nuclear rocket is reported. The first task was to test a previously constructed induction heated plasma GCNR simulator above 300 kW. A number of tests are reported operating in the range of 300 kW at 10,000 cps. A second simulator was designed but not constructed for cold-hot visualization studies using louvered walls. A third task was a paper investigation of practical uranium feed systems, including a detailed discussion of related problems. The last assignment resulted in two designs for plasma nozzle test devices that could be operated at 200 atm on hydrogen.

Poole, J. W.↗