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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
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

A numerical method for determining highly precise electron energy distribution functions from Langmuir probe characteristics

Electron energy distribution functions (EEDFs) were determined from probe characteristics using a numerical ac superimposed method with a distortion correction of high derivative terms by varying amplitude of a sinusoidal perturbation voltage superimposed onto the dc sweep voltage, depending on the related electron energy. Low amplitude perturbation applied around the plasma potential represented the low energy peak of the EEDF exactly, and high amplitude perturbation applied around the floating potential was effective to suppress noise or distortion of the probe characteristic, which is fatal to the tail electron distribution. When a small random noise was imposed over the stabilized prove characteristic, the numerical differentiation method was not suitable to determine the EEDF, while the numerical ac superimposed method was able to obtain a highly precise EEDF.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

On the velocity variation in atmospheric pressure plasma plumes driven by positive and negative pulses

To better understand the variation in the ''plasma bullet'' velocity, the dynamics of an atmospheric pressure plasma plume driven by positive and negative pulses are investigated in detail. It is found that, before the plasma exits the nozzle, the plasma propagates at a speed of about 30 km/s for both positive and negative pulses. As soon as the plasma exits the nozzle, the plasma propagation speed increases dramatically for both cases. The peak velocity for the case of the positive pulse is much higher than that of the negative pulse, it is approximately 150 km/s and 70 km/s, respectively. According to the optical emission spectra, the acceleration behavior of the plasma bullet when it exits the nozzle is due to the increase in the N{sub 2}{sup +} concentration.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Three-dimensional simulation of efficiency enhancement in free-electron laser with prebunched electron beam

The effect of prebunching of the electron beam and tapering of the wiggler amplitude on the harmonic upconversion in free-electron laser amplifier is studied in three dimensions. A set of coupled nonlinear first-order differential equations that describe the three-dimensional simulation of the system is solved numerically. This set of equation describes self-consistently the longitudinal spatial dependence of radiation waists, curvatures, and amplitudes together with the evaluation of the electron beam. The analysis is related to extreme ultraviolet and x-ray emission. In addition to uniform beam, prebunched electron beam has also been studied. The effect of sinusoidal distribution of entry times for the electron beam on the evolution of radiation is compared with uniform distribution. It is shown that prebunching reduces the saturation length substantially. For efficiency enhancement, the wiggler is set to decrease linearly when the radiation of the third harmonic saturates. The optimum starting point and the slope of tapering of the amplitude of the wiggler are found by a successive run of the code. It was found that tapering can increase the saturated power of the third harmonic considerably.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Ionic wind generation by a wire-cylinder-plate corona discharge in air at atmospheric pressure

A wire-cylinder-plate electrode configuration is presented to generate ionic wind with a dc corona discharge in air at atmospheric pressure. The objective of the work is to maximize the power supplied to the flow in order to increase acceleration while avoiding breakdown. Thus, the proposed experimental setup addresses the problem of decoupling the mechanism of ion generation from that of ion acceleration. Using a wire-plate configuration as a reference, we have focused on improving the topography of the electric field to (1) separate the ionization and acceleration zones in space, and (2) guide the trajectory of charged particles as parallel to the median axis as possible. In the proposed wire-cylinder-plate setup, a dc corona discharge is generated in the space between a wire and two cylinders. The ions produced by the corona then drift past the cylinders and into a channel between two plates, where they undergo acceleration. To maximize the ionic wind it is found that the geometric configuration must be as compact as possible and that the voltage applied must be right below breakdown. Experimentally, the optimized wire-plate reference setup provides a maximum flow velocity of 8 m s{sup -1}, a flow rate per unit electrode length of 0.034 m{sup 2} s{sup -1}, and a thrust per unit electrode length of 0.24 N m{sup -1}. The wire-cylinder-plate configuration provides a maximum flow velocity of 10 m s{sup -1}, a flow rate per unit electrode length of 0.041 m{sup 2} s{sup -1}, and a thrust per unit electrode length of 0.35 N m{sup -1}. This 46% increase in thrust is obtained by increasing the electric power per unit electrode length by only 16% (from 175 to 210 W m{sup -1}), which confirms the gain in efficiency obtained with the decoupled system. In comparison with a simple wire-wire corona configuration, the wire-cylinder-plate configuration increases the ionic wind velocity by up to a factor of 3, and the thrust by an order of magnitude.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Propagation of broadband terahertz pulses through a dense-magnetized-collisional-bounded plasma layer

The terahertz characteristics of a dense-magnetized-collisional-bounded plasma under normal incident are analyzed in this study, which is of practical significance in plasma diagnostics with electromagnetic waves. We theoretically calculate the reflection, absorption, and transmission coefficients for right- and left-handed polarized terahertz waves through a uniform, magnetized, and collisional plasma slab bounded by lossless transparent walls. The power absorption spectra in the frequency range of 0.1-2 THz are given with strong external magnetic fields and different plasma parameters such as plasma density and collisional frequency. Our numerical result is consistent with Jamison's experimental result. It is found that plasma absorption is mainly caused by the collisional absorption and electron cyclotron resonance. Furthermore, the absorption heavily depends on the polarization mode of the terahertz waves when the external magnetic field B is high enough that the election gyrofrequency is near the incident wave frequency. The relationships between the corresponding parameters of the problem are studied numerically.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Mixed time slicing in path integral simulations

A simple and efficient scheme is presented for using different time slices for different degrees of freedom in path integral calculations. This method bridges the gap between full quantization and the standard mixed quantum-classical (MQC) scheme and, therefore, still provides quantum mechanical effects in the less-quantized variables. Underlying the algorithm is the notion that time slices (beads) may be 'collapsed' in a manner that preserves quantization in the less quantum mechanical degrees of freedom. The method is shown to be analogous to multiple-time step integration techniques in classical molecular dynamics. The algorithm and its associated error are demonstrated on model systems containing coupled high- and low-frequency modes; results indicate that convergence of quantum mechanical observables can be achieved with disparate bead numbers in the different modes. Cost estimates indicate that this procedure, much like the MQC method, is most efficient for only a relatively few quantum mechanical degrees of freedom, such as proton transfer. In this regime, however, the cost of a fully quantum mechanical simulation is determined by the quantization of the least quantum mechanical degrees of freedom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Nonlinear oscillation behavior of a driven gyrotron backward-wave oscillator

Controlling the phase and frequency of a gyrotron backward-wave oscillator (gyro-BWO) by means of injection-locking techniques is of practical importance. This study employed a nonlinear self-consistent time-independent code to analyze the nonlinear oscillation behavior of a driven gyro-BWO. There are three regimes in the driven gyro-BWO, including amplification, injection-locked oscillation, and mode competition regimes. Based on the theory of nonlinear oscillation, the amplification and injection-locked oscillation modes are the stable modes and compete with each other in the mode competition regime. An oscillator plane of the driven gyro-BWO is elucidated in the paper. This work demonstrates for the first time that the amplification mode transits to the injection-locked oscillation mode in the driven gyro-BWO. Moreover, the signification efficiency enhancement of the driven gyro-BWO over the free-running efficiency is found.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Theoretical investigation of the spectrum and conversion efficiency of short wavelength extreme-ultraviolet light sources based on terbium plasmas

The emission spectrum and conversion efficiency of laser-produced terbium plasmas are theoretically investigated on the basis of computational atomic data. The theoretically calculated spectrum reproduces the main peak of observed spectrum at {lambda}=6.5 nm, which originates from 4d-4f transitions of near palladiumlike ions (Tb{sup 19+}). A simple model of the isothermal expansion of terbium plasma suggests that efficient emission can be achieved by pumping the plasma with a laser pulse at an intensity of approximately one order of magnitude greater than that used with tin sources at {lambda}=13.5 nm.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Kinetics of illuminated complex plasmas considering Mie scattering by spherical dust particles with a size distribution

In this paper Mie scattering of light by dust particles having Mathis, Rumpl, and Nordsieek power law distribution of size has been incorporated in the formulation of the kinetics of an illuminated complex plasma which takes into account the ionization of neutral atoms by an external agency, ion-electron recombination, photoemission of electrons by the dust particles, and accretion of electrons and ions on the surface of the particles; the number and energy balance of the constituent species has also been taken into account. An interesting conclusion is that unlike the usual case (when Mie scattering is neglected) the charge on a particle is not proportional to the radius and that for certain sets of parameters the smaller particles are negatively charged while the larger particles, carry positive charge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Parametric Decay of Pump Waves into two Linear Modes in SINP MaPLE Device

Parametric decay of incident waves of ion cyclotron frequency range into linear modes is observed in experiment performed in the SINP MaPLE device where nitrogen plasma produced by ECR discharge. Along with a mode in drift wave frequency range, sideband of the incident waves are observed when amplitude of the exciter signal goes above a threshold value. Sideband of the second harmonic is also seen. Preliminary studies point towards excitation of ion Bernstein wave. Details of the experimental results are presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Advances and Current Challenges in the Theory of Zonal-Flow Generation

Some remarks are made about the use of modern statistical formalism in the calculation of the zonal-flow growth rate and the backreaction of zonal flows on drift waves. The intimate connection between the zonal growth-rate calculation and the derivation of a statistical formula for eddy viscosity in two-dimensional neutral fluids is emphasized, as is the the role of Casimir invariants in disparate-scale expansion. Also stressed is the importance of random Galilean invariance in the calculation of the triad interaction time between short-wavelength drift waves and long-wavelength flows.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Synergy of Edge Shear Layer and Micro Turbulence in the Dynamics of the Density Limit of H-mode Discharges

Theoretical model for the physical mechanism of Greenwald density limit is presented. The importance of the role of shear layer associated with Zonal Flows (ZFs) and edge turbulence driven by high-m drift resistive ballooning mode (high-m DRBM) for Greenwald scaling is established. We have studied the multiscale interaction of zonal flows (ZFs) in the background of high-m DRBM turbulence. When the collision damping weak and model predicts unrealistically low value of saturated primary turbulence, we have studied the tertiary instability of ZFs, which take the place of collisional damping of ZFs. A self-consistent, simplified low-dimensional model of these interactions is constructed and derived the density scaling closed to Greenwald limit, which is based on experimental observations i.e., n{sub G{alpha}} I {sub P} /a{sup 2}(where n{sub G}, the plasma density and I{sub P}, the current).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Systematic investigation of self-absorption and conversion efficiency of 6.7 nm extreme ultraviolet sources

We have investigated the dependence of the spectral behavior and conversion efficiencies of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6x10{sup 12} W/cm{sup 2} at an operating wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target. Moreover, the lower-density results in a narrower spectrum and therefore improved spectral purity. It is shown to be important to use a low initial density target and/or to produce low electron density plasmas for efficient extreme ultraviolet sources when using high-Z targets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Plasma dispersion function for a Fermi-Dirac distribution

A plasma dispersion function (PDF) is defined for a nonrelativistic Fermi-Dirac distribution and its properties are explored. The degree of degeneracy is described by a parameter {xi}=e{sup {mu}}{sub e}{sup /T}{sub e}, for electrons, with {mu}{sub e}/T{sub e} large and negative in the nondegenerate limit, and large and positive in the completely degenerate limit. The PDF is denoted Z(y,{xi}), where the variable y={omega}/{radical}(2)kV{sub e}, is the argument of the conventional PDF, Z(y)=Z(y,0), for a Maxwellian distribution. In the completely degenerate limit, Z(y,{xi}) approaches a logarithmic function that depends on the Fermi temperature and is independent of T{sub e}. Analytic approximations to Z(y,{xi}) are derived in terms of polylogarithmic functions for y{sup 2}>>1 and for y{sup 2}<<1.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

rf mode switching in a relativistic magnetron with diffraction output

The relativistic magnetron with diffraction output (RMDO) has demonstrated nearly 70% efficiency in recent simulations. This letter reports a rapid mode switching technique in the RMDO using a low power, short-pulse, external single frequency signal. The MAGIC electromagnetic finite-difference-time-domain particle-in-cell code used in simulations demonstrated that an input signal of 300 kW is sufficient to switch neighboring modes in a gigawatt output power A6 RMDO with a transparent cathode, whereas for the original A6 magnetron configuration with radial extraction driven by a transparent cathode 30 MW is required. This frequency agility adds additional versatility to this high power microwave source.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Numerical analysis of electronegative plasma in the extraction region of negative hydrogen ion sources

This numerical study focuses on the physical mechanisms involved in the extraction of volume-produced H{sup -} ions from a steady state laboratory negative hydrogen ion source with one opening in the plasma electrode (PE) on which a dc-bias voltage is applied. A weak magnetic field is applied in the source plasma transversely to the extracted beam. The goal is to highlight the combined effects of the weak magnetic field and the PE bias voltage (upon the extraction process of H{sup -} ions and electrons). To do so, we focus on the behavior of electrons and volume-produced negative ions within a two-dimensional model using the particle-in-cell method. No collision processes are taken into account, except for electron diffusion across the magnetic field using a simple random-walk model at each time step of the simulation. The results show first that applying the magnetic field (without PE bias) enhances H{sup -} ion extraction, while it drastically decreases the extracted electron current. Secondly, the extracted H{sup -} ion current has a maximum when the PE bias is equal to the plasma potential, while the extracted electron current is significantly reduced by applying the PE bias. The underlying mechanism leading to the above results is the gradual opening by the PE bias of the equipotential lines towards the parts of the extraction region facing the PE. The shape of these lines is due originally to the electron trapping by the magnetic field.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Ionization in inductively coupled argon plasmas studied by optical emission spectroscopy

Contribution of stepwise ionization to total ionization was experimentally investigated in low-pressure inductively coupled argon plasmas. In the pressure range 3-50 mTorr, optical emission spectroscopy was employed to determine metastable fractions (metastable density relative to ground state density) by measuring the emission intensity of selected lines. The measured metastable fractions were in good agreement with the calculation, showing a dependence on the discharge pressure. The rate of stepwise ionization was estimated from the excited level densities (measurements and model predictions) and their ionization rate coefficients. It is observed that at relatively low discharge pressures (<10 mTorr) the ionization is mainly provided by the direct ionization, whereas at higher pressure the stepwise ionization is predominant with increasing absorbed power.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Comment on 'On the ionization equilibrium of hot hydrogen plasma and thermodynamic consistency of formulating finite partition functions' [Phys. Plasmas 17, 062701 (2010)]

Zaghloul [Phys. Plasmas 17, 062701 (2010)] reconsiders the occupation probability formalism in plasma thermodynamics and claims inconsistencies in previous models. I show that the origin of this incorrect claim is an omission of the configurational factor from the partition function.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗