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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 1,081 records · Page 60
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DOE OSTI2012

Beam-driven three-dimensional electromagnetic strong turbulence

Large scale beam-driven electromagnetic strong turbulence is investigated by numerically solving the three-dimensional electromagnetic Zakharov equations, where turbulence is driven at nonzero wavenumbers k. For electron thermal speeds v{sub e}/c Greater-Than-Or-Equivalent-To 0.1, a significant fraction of driven Langmuir waves undergo electromagnetic decay into electromagnetic waves and ion-acoustic waves so that transverse waves contribute significantly to the total energy density. It is shown that as v{sub e}/c increases, the wavenumber and energy density of transverse waves produced increase. For v{sub e}/c Less-Than-Or-Equivalent-To 0.1, beam-driven turbulence is approximately electrostatic. An approximately periodic cycle is observed, similar to previous two-dimensional electrostatic simulations, in which Langmuir waves are driven to larger mean energy densities until a series of backscatters occurs, shifting the Langmuir waves out of resonance with the driver and decreasing the wavenumber of the Langmuir waves. A low-k condensate results from which wave packets form and collapse, decreasing the mean energy density. Averaging over many of these periods, the statistical properties are calculated and the scaling behavior of the mean energy density is shown to agree well with the electrostatic two-component model prediction. When driven at nonzero k the scaling behavior is shown to depend weakly on v{sub e}/c, in contrast to when strong turbulence is driven at k = 0, where the scalings depend more strongly on v{sub e}/c.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Internal disruptions and sawtooth like activity in Large Helical Device

Large Helical Device (LHD) inward-shifted configurations are unstable to resistive magnetohydrodynamic (MHD) pressure-gradient-driven modes. These modes drive sawtooth like events during LHD operation. In this work, we simulate sawtooth like activity and internal disruptions in order to improve the understanding of these relaxation events and their effect over the device efficiency to confine the plasma, with the aim to improve the LHD present and future operation scenarios minimizing or avoiding the disadvantageous MHD soft and hard limits. By solving a set of reduced non-linear resistive MHD equations, we have studied the evolution of perturbations to equilibria obtained before and after a sawtooth like event in LHD. The equilibrium {beta} value is gradually increased during the simulation until it reaches the experimental value. Sawtooth like events and internal disruption events take place in the simulation for {beta}{sub 0} values between 1% and 1.48%. The main driver of the sawtooth like events is the resonant and non-resonant effect of the (n = 1, m = 3) mode. The instability is stronger for resonant events, and they only appear when {beta}{sub 0} = 1.48%. Internal disruptions are mainly driven by the (n = 1, m = 2) mode, and they extend throughout the whole plasma core. Internal disruption events do not show up when resonant sawtooth like events are triggered.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

A novel local equilibrium model for shaped tokamak plasmas

A model is proposed for a local up-down symmetric equilibrium in the vicinity of a specified magnetic surface with given elongation and triangularity. Different from the Miller's model [R. L. Miller et al., Phys. Plasmas 5, 973 (1998)], the derivative of the Shafranov shift in the present model is self-consistently determined. The equilibrium accounts for all the essential features, like the elongation, the triangularity, and the Shafranov shift etc., of a shaped cross section. Hence, it can be used for investigation of radially localized plasma modes, like reversed shear Alfvenic eigenmodes and ballooning mode, etc., and it is also suitable for local equilibrium construction used for flux tube plasma simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

{delta}f Monte Carlo calculation of neoclassical transport in perturbed tokamaks

Non-axisymmetric magnetic perturbations can fundamentally change neoclassical transport in tokamaks by distorting particle orbits on deformed or broken flux surfaces. This so-called non-ambipolar transport is highly complex, and eventually a numerical simulation is required to achieve its precise description and understanding. A new {delta}f particle orbit code (POCA) has been developed for this purpose using a modified pitch-angle collision operator preserving momentum conservation. POCA was successfully benchmarked for neoclassical transport and momentum conservation in the axisymmetric configuration. Non-ambipolar particle flux is calculated in the non-axisymmetric case, and the results show a clear resonant nature of non-ambipolar transport and magnetic braking. Neoclassical toroidal viscosity (NTV) torque is calculated using anisotropic pressures and magnetic field spectrum, and compared with the combined and 1/{nu} NTV theory. Calculations indicate a clear {delta}B{sup 2} scaling of NTV, and good agreement with the theory on NTV torque profiles and amplitudes depending on collisionality.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Backward mode of the ion-cyclotron wave in a semi-bounded magnetized Lorentzian plasma

The backward modes of the surface ion-cyclotron wave are investigated in a semi-bounded magnetized Lorentzian plasma. The dispersion relation of the backward mode of the surface ion-cyclotron wave is obtained using the specular reflection boundary condition with the plasma dielectric function. The result shows that the nonthermal effect suppresses the wave frequency as well as the group velocity of the surface ion-cyclotron wave. It is also found that the nonthermal effect on the surface ion-cyclotron wave increases with an increase of the wave number. In addition, it is found that the propagation domain of the surface ion-cyclotron wave increases with an increase of the ratio of the electron plasma frequency to the electron gyrofrequency. It is also found that the nonthermal effect increases the propagation domain of the surface ion-cyclotron wave in a semi-bounded magnetized Lorentzian plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Enhancing extreme ultraviolet photons emission in laser produced plasmas for advanced lithography

Current challenges in the development of efficient laser produced plasma sources for the next generation extreme ultraviolet lithography (EUVL) are increasing EUV power and maximizing lifetime and therefore, reducing cost of devices. Mass-limited targets such as small tin droplets are considered among the best choices for cleaner operation of the optical system because of lower mass of atomic debris produced by the laser beam. The small diameter of droplets, however, decreases the conversion efficiency (CE) of EUV photons emission, especially in the case of CO{sub 2} laser, where laser wavelength has high reflectivity from the tin surface. We investigated ways of improving CE in mass-limited targets. We considered in our modeling various possible target phases and lasers configurations: from solid/liquid droplets subjected to laser beam energy with different intensities and laser wavelength to dual-beam lasers, i.e., a pre-pulse followed by a main pulse with adjusted delay time in between. We studied the dependence of vapor expansion rate, which can be produced as a result of droplet heating by pre-pulse laser energy, on target configuration, size, and laser beam parameters. As a consequence, we studied the influence of these conditions and parameters on the CE and debris mass accumulation. For better understanding and more accurate modeling of all physical processes occurred during various phases of laser beam/target interactions, plasma plume formation and evolution, EUV photons emission and collection, we have implemented in our heights package state-of-the art models and methods, verified, and benchmarked against laboratory experiments in our CMUXE center as well as various worldwide experimental results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Energy enhancement of proton acceleration in combinational radiation pressure and bubble by optimizing plasma density

The combinational laser radiation pressure and plasma bubble fields to accelerate protons are researched through theoretical analysis and numerical simulations. The dephasing length of the accelerated protons bunch in the front of the bubble and the density gradient effect of background plasma on the accelerating phase are analyzed in detail theoretically. The radiation damping effect on the accelerated protons energy is also considered. And it is demonstrated by two-dimensional particle-in-cell simulations that the protons bunch energy can be increased by using the background plasma with negative density gradient. However, radiation damping makes the maximal energy of the accelerated protons a little reduction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Electron geodesic acoustic modes in electron temperature gradient mode turbulence

In this work, the first demonstration of an electron branch of the geodesic acoustic mode (el-GAM) driven by electron temperature gradient (ETG) modes is presented. The work is based on a fluid description of the ETG mode retaining non-adiabatic ions and the dispersion relation for el-GAMs driven nonlinearly by ETG modes is derived. A new saturation mechanism for ETG turbulence through the interaction with el-GAMs is found, resulting in a significantly enhanced ETG turbulence saturation level compared to the mixing length estimate.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Electron acoustic waves in a magnetized plasma with kappa distributed ions

Electron acoustic solitary waves in a two component magnetized plasma consisting of fluid cold electrons and hot superthermal ions are considered. The linear dispersion relation for electron acoustic waves is derived. In the nonlinear regime, the energy integral is obtained by a Sagdeev pseudopotential analysis, which predicts negative solitary potential structures. The effects of superthermality, obliquity, temperature, and Mach number on solitary structures are studied in detail. The results show that the superthermal index {kappa} and electron to ion temperature ratio {sigma} alters the regime where solitary waves can exist. It is found that an increase in magnetic field value results in an enhancement of soliton electric field amplitude and a reduction in soliton width and pulse duration.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Dissipative electromagnetic solitary waves in collisional plasmas

The propagation of linearly polarized electromagnetic (EM) waves in a collisional plasma is studied using multiple scale perturbation technique in a weakly nonlinear regime. A complex linear dispersion relation and a complex group velocity are obtained for EM waves propagating in a plasma and their dependence on system parameters is investigated. It is shown that the amplitude of EM pulse is governed by an envelope equation similar to a cubic complex Ginzburg-Landau equation. A traveling bright solitary wave solution for envelope equation is found, its existence condition in parameter space is explored and variation of its profile with system parameters is manipulated. Monitoring temporal evolution of traveling solitary wave solution provides more insight into the nature of this solution and ensures that depending on the parameters of the system, solitary wave solution may behave like a stationary soliton or may exhibit the behavior of a breathing soliton.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Propagation characteristics of a Gaussian laser beam in plasma with modulated collision frequency

The propagation characteristics of a Gaussian laser beam in cold plasma with the electron collision frequency modulated by laser intensity are presented. The nonlinear dynamics of the ponderomotive force, which induce nonlinear self-focusing as opposed to spatial diffraction, are considered. The effective dielectric function of the Drude model and complex eikonal function are adopted in deriving coupled differential equations of the varying laser beam parameters. In the framework of ponderomotive nonlinearity, the frequency of electron collision in plasmas, which is proportional to the spatial electron density, is strongly interrelated with the laser beam propagation characteristics. Hence, the propagation properties of the laser beam and the modulated electron collision frequency distribution in plasma were studied and explained in depth. Employing this self-consistent method, the obtained simulation results approach practical conditions, which is of significance to the study of laser-plasma interactions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Non-local gyrokinetic model of linear ion-temperature-gradient modes

The non-local properties of anomalous transport in fusion plasmas are still an elusive topic. In this work, a theory of non-local linear ion-temperature-gradient (ITG) drift modes while retaining non-adiabatic electrons and finite temperature gradients is presented, extending the previous work [S. Moradi et al., Phys. Plasmas 18, 062106 (2011)]. A dispersion relation is derived to quantify the effects on the eigenvalues of the unstable ion temperature gradient modes and non-adiabatic electrons on the order of the fractional velocity operator in the Fokker-Planck equation. By solving this relation for a given eigenvalue, it is shown that as the linear eigenvalues of the modes increase, the order of the fractional velocity derivative deviates from two and the resulting equilibrium probability density distribution of the plasma, i.e., the solution of the Fokker-Planck equation, deviates from a Maxwellian and becomes Levy distributed. The relative effect of the real frequency of the ITG mode on the deviation of the plasma from Maxwellian is larger than from the growth rate. As was shown previously the resulting Levy distribution of the plasma may in turn significantly alter the transport as well.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

The interaction of quasi-monoenergetic protons with pre-compressed inertial fusion fuels

The interaction of a quasi-monoenergetic proton beam with a pre-compressed plasma is studied in the context of inertial fusion fast ignition (FI). Based on fundamental principles, a kinetic model is developed by considering hard collisions, nuclear scattering, and the contribution due to collective processes. The penetration depth, longitudinal straggling, and the transverse blooming are evaluated by solving the Boltzmann transport equation using the multiple scattering theory. The stopping power, transport scattering cross sections, and convenient expressions for the angular moments of the proton distribution function have been used in modeling the collisional proton transport in a three-dimensional (3D) Monte Carlo code. The transport of a proton beam with a quasi-monoenergetic energy =10 MeV is studied for pre-compressed deuterium-tritium plasma with an average density of {rho}=400 g cm{sup -3} and temperatures T=1 keV, 5 keV, and 10 keV. The net effects of multiple scattering are to reduce the penetration from 1.028 to 0.828 g cm{sup -2} with range straggling {rho}{Sigma}{sub R}=0.044 g cm{sup -2} and beam blooming {rho}{Sigma}{sub B}=0.272 g cm{sup -2}, for 10 MeV protons in a {rho}=400 g cm{sup -3} plasma at T = 5 keV. This model can be used for quantitatively assessing ignition requirements for proton fast ignition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Potential of a plasma bound between two biased walls

An analytical study is presented for an one-dimensional, steady-state plasma bound between two perfectly absorbing walls that are biased with respect to each other. Starting from a description of the plasma sheaths formed at both walls, an expression relating the bulk plasma potential to the wall currents is derived, showing that the plasma potential undergoes an abrupt transition when currents cross a critical value. This result is confirmed by numerical simulations performed with a particle-in-cell code.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Experimental and computational study of complex shockwave dynamics in laser ablation plumes in argon atmosphere

We investigated spatio-temporal evolution of ns laser ablation plumes at atmospheric pressure, a favored condition for laser-induced breakdown spectroscopy and laser-ablation inductively coupled plasma mass-spectrometry. The 1064 nm, 6 ns pulses from a Nd:YAG laser were focused on to an Al target and the generated plasma was allowed to expand in 1 atm Ar. The hydrodynamic expansion features were studied using focused shadowgraphy and gated 2 ns self-emission visible imaging. Shadowgram images showed material ejection and generation of shock fronts. A secondary shock is observed behind the primary shock during the time window of 100-500 ns with instabilities near the laser cone angle. By comparing the self-emission images obtained using fast photography, it is concluded that the secondary shocks observed in the shadowgraphy were generated by fast moving target material. The plume front estimates using fast photography exhibited reasonable agreement with data obtained from shadowgraphy at early times {<=}400 ns. However, at later times, fast photography images showed plume confinement while the shadowgraphic images showed propagation of the plume front even at greater times. The structure and dynamics of the plume obtained from optical diagnostic tools were compared to numerical simulations. We have shown that the main features of plume expansion in ambient Ar observed in the experiments can be reproduced using a continuum hydrodynamics model which provided valuable insight into the expansion dynamics and shock structure of the plasma plume.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Paraxial Wentzel-Kramers-Brillouin method applied to the lower hybrid wave propagation

The paraxial Wentzel-Kramers-Brillouin (pWKB) approximation, also called beam tracing method, has been employed in order to study the propagation of lower hybrid waves in a tokamak plasma. Analogous to the well-know ray tracing method, this approach reduces Maxwell's equations to a set of ordinary differential equations, while, in addition, retains the effects of the finite beam cross-section, and, thus, the effects of diffraction. A new code, LHBEAM (lower hybrid BEAM tracing), is presented, which solves the pWKB equations in tokamak geometry for arbitrary launching conditions and for analytic and experimental plasma equilibria. In addition, LHBEAM includes linear electron Landau damping for the evaluation of the absorbed power density and the reconstruction of the wave electric field in both the physical and Fourier space. Illustrative LHBEAM calculations are presented along with a comparison with the ray tracing code GENRAY and the full wave solver TORIC-LH.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Impact of Xe partial pressure on the production of excimer vacuum ultraviolet emission for plasma display panels

In this work, the effect of the Xe partial pressure on the excimer vacuum ultraviolet (VUV) emission intensity of the plasma display panels is investigated, both by measuring the spectral emission directly and by two-dimensional simulations. Experimentally, we find that at the high Xe partial pressure levels, there is an supra-linear increase of excimer VUV radiation and that determines the strong increase of luminance at the high pressures and high voltage. Due to the increase of the luminance and the almost unchanged discharge current, the luminous efficacy strongly increases with the Xe partial pressure. In addition, we also investigated the dynamics of the VUV generation, by measuring the decay time of the excimer VUV light as a function of the gas pressure. It is found that the decay time decreases with the increase of gas pressure. The spatial characteristics of the excimer VUV emission are also discussed. Different from the Ne and near-infrared emission, the excimer VUV emission is generated near the surface of the electrodes and increases uniformly on both sides of the anode and cathode (i.e., the bulk plasma region). Most importantly, it is found that the VUV production occurs during the afterglow period, while it is almost zero at the moment of the discharge itself. From the simulations, it can be seen that the Xe{sub 2}*({sup 3}{Sigma}{sub u}{sup +}) excimer species, which are generated from Xe*(1s{sub 5}), play a dominant role in the excimer VUV emission output at the high Xe partial pressure. The two-dimensional simulations also show that the strong increase of Xe excimer excitation states in the case of high pressure is mainly the result of the high conversion efficiency of the Xe excimer states, especially in the afterglow period. Due to the high conversion efficiency of Xe excitation species to Xe excimer species by the high collision rate in the case of high pressure, there is a strong increase of excimer VUV production, especially from the cathode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗