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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Striation pattern of target particle and heat fluxes in three dimensional simulations for DIII-D

The application of resonant magnetic perturbations results in a non-axisymmetric striation pattern of magnetic field lines from the plasma interior which intersect the divertor targets. The impact on related particle and heat fluxes is investigated by three dimensional computer simulations for two different recycling conditions (controlled via neutral gas pumping). It is demonstrated that a mismatch between the particle and heat flux striation pattern (splitting vs. no splitting), as is repeatedly observed in ITER similar shape H-mode plasmas at DIII-D, can be reproduced by the simulations for high recycling conditions at the onset of partial detachment. These results indicate that a detailed knowledge of the particle and energy balance is at least as important for realistic simulations as the consideration of a change in the magnetic field structure by plasma response effects.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Fantastic Striations and Where to Find Them: The Origin of Magnetically Aligned Striations in Interstellar Clouds

Thin, magnetically aligned striations of relatively moderate contrast with the background are commonly observed in both atomic and molecular clouds. They are also prominent in MHD simulations with turbulent converging shocks. The simulated striations develop within a dense, stagnated sheet in the midplane of the post-shock region where magnetically induced converging flows collide. We show analytically that the secondary flows are an inevitable consequence of the jump conditions of oblique MHD shocks. They produce the stagnated, sheet-like sub-layer through a secondary shock when, roughly speaking, the Alfvénic speed in the primary converging flows is supersonic, a condition that is relatively easy to satisfy in interstellar clouds. The dense sub-layer is naturally threaded by a strong magnetic field that lies close to the plane of the sub-layer. The substantial magnetic field makes the sheet highly anisotropic, which is the key to the striation formation. Specifically, perturbations of the primary inflow that vary spatially perpendicular to the magnetic field can easily roll up the sheet around the field lines without bending them, creating corrugations that appear as magnetically aligned striations in column density maps. On the other hand, perturbations that vary spatially along the field lines curve the sub-layer and alter its orientation relative to the magnetic field locally, seeding special locations that become slanted overdense filaments and prestellar cores through enhanced mass accumulation along field lines. In our scenario, the dense sub-layer, which is unique to magnetized oblique shocks, is the birthplace for both magnetically aligned diffuse striations and massive star-forming structures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

A self-consistent hybrid model of kinetic striations in low-current argon discharges

A self-consistent hybrid model of standing and moving striations was developed for low-current DC discharges in noble gases. We introduced the concept of surface diffusion in phase space ( r , u ) (where u denotes the electron kinetic energy) described by a tensor diffusion in the nonlocal Fokker–Planck kinetic equation for electrons in the collisional plasma. Electrons diffuse along surfaces of constant total energy ε = u - eφ ( r ) between energy jumps in inelastic collisions with atoms. Numerical solutions of the 1d1u kinetic equation for electrons were obtained by two methods and coupled to ion transport and Poisson solver. We studied the dynamics of striation formation in Townsend and glow discharges in argon gas at low discharge currents using a two-level excitation-ionization model and a ‘full-chemistry’ model, which includes stepwise and Penning ionization. Standing striations appeared in Townsend and glow discharges at low currents, and moving striations were obtained for the discharge currents exceeding a critical value. These waves originate at the anode and propagate towards the cathode. We have seen two types of moving striations with the two-level and full-chemistry models, which resemble the s and p striations previously observed in the experiments. Simulations indicate that processes in the anode region could control moving striations in the positive column plasma. The developed model helps clarify the nature of standing and moving striations in DC discharges of noble gases at low discharge currents and low gas pressures.

Physics↗
DOE OSTI2018

Ionization instability induced striations in low frequency and pulsed He/H 2 O atmospheric pressure plasmas

In previous work [Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016)] and [Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], 1D kinetic particle-in-cell (PIC) simulations of narrow gap (1 to 4 mm), high frequency (27 MHz) or dc-driven, He/2%H 2 O atmospheric pressure plasmas (APPs) showed an ionization instability resulting in standing striations (spatial oscillations) in the bulk plasma. We developed a steady-state striation theory which showed that the striations are due to non-local electron kinetics. In both the high frequency and dc-driven cases, the equilibrium electron density n 0 in the plasma bulk was stationary. In this work, we first conduct 1D PIC simulations of a 1 mm gap He/2%H 2 O APP, driven by a sinusoidal current at a low frequency of f = 50 kHz such that ω = 2 πf is well below the ionization frequency ν iz . In this case, n 0 varies with time, and we observe a time-varying instability which quasistatically depends on n 0 ( t ). At each phase of the rf cycle, the discharge resembles a dc discharge at the same n 0 . At higher frequencies (200 kHz–1 MHz), ω approaches ν iz , and quasistatic equilibrium at each phase breaks down. The discharge is also driven with a 200 kHz, 50% duty cycle square wave pulse with a short rise and fall time of 0.1 μs in an attempt to directly measure the striation growth rate s during the on-cycle before it saturated. However, the spike in current during the rise time leads to a spike in electron temperature T e and hence ν iz and s at the beginning of the rise which saturated during the beginning of the on-cycle. To predict the instability growth rate and saturation during and after the current spike, we extend our striation theory to include time-varying n 0 , T e , ν iz , as well as terms for the nonlinear saturation and noise floor of the striation amplitude. Furthermore, the time-varying global model predictions are compared to the PIC simulations, showing reasonable agreement.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Striations in moderate pressure dc driven nitrogen glow discharge

Plasma stratification has been studied for more than a century. Despite the many experimental studies reported on this topic, theoretical analyses and numerical modeling of this phenomenon have been mostly limited to rare gases. In this work, a one-dimensional fluid model with detailed kinetics of electrons and vibrationally excited molecules is employed to simulate moderate-pressure (i.e. a few Torrs) dc discharge in nitrogen in a 15.5 cm long tube of radius 0.55 cm. The model also considers ambipolar diffusion to account for the radial loss of ions and electrons to the wall. The proposed model predicts self-excited standing striations in nitrogen for a range of discharge currents. The impact of electron transport parameters and reaction rates obtained from a solution of local two-term and a multi-term Boltzmann equation on the predictions are assessed. In-depth kinetic analysis indicates that the striations result from the undulations in electron temperature caused due to the interaction between ionization and vibrational reactions. Furthermore, the vibrationally excited molecules associated with the lower energy levels are found to influence nitrogen plasma stratification and the striation pattern strongly. A balance between ionization processes and electron energy transport allows the formation of the observed standing striations. Simulations were conducted for a range of discharge current densities from ~0.018 to 0.080 mA cm –2 , for an operating pressure of 0.7 Torr. Parametric studies show that the striation length decreases with increasing discharge current. Furthermore, the predictions from the model are compared against experimental measurements and are found to agree favorably.

42 ENGINEERING↗
DOE OSTI2019

Multi-mode ionization instability induced striations in RF driven He/H 2 O atmospheric pressure plasma (APP) discharges

Previously [E. Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016) and E. Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], one dimensional (1D) particle-in-cell (PIC) simulations of 1 to 4mm gap, He/2%H 2 O atmospheric pressure plasma discharges showed an ionization instability resulting in bulk striations. Assuming that the ionization rate coefficient K iz is related to the root mean square electric field E by K iz &prop; E q , a striation theory showed that q<0 is a necessary condition for the instability. A local calculation yielded q>0, implying that nonlocal electron kinetics are required for the instability. Wider gaps can fit in a wider range of wavelengths, resulting in multi-mode striations. Previously, we assumed a single mean q value for each discharge, and did not calculate q for each mode separately. Here, we develop a wavelength resolved global striation model and apply it to 1D PIC simulations of 4mm gap discharges with input currents J rf = 0.04 to 0.30A/cm 2 . We first examine a base case at 0.23A/cm 2 and observe a mixture of unstable modes within a window of wavelengths λ. At shorter λ, the modes are suppressed by diffusion. At longer λ, we observe a transition to locality in which q becomes less negative with increasing λ, approaching its local positive value and stabilizing the modes. The window of unstable modes shifts to shorter λ with increasing J rf , causing the modes to be suppressed by diffusion at higher J rf . At lower J rf , the decrease in bulk plasma density with decreasing J rf suppresses the striations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

The temporal evolution of 3-m striations in the modified ionosphere

Experiments were performed at Arecibo, Puerto Rico, to investigate the evolution times of 3-m field-aligned striations produced in the ionosphere by powerful high-frequency (HF) radio waves. The results of this investigation are now summarized. First, the striations' rise times are dependent on the HF electric field. The E region data suggest that this dependence is nonlinear. Second, the threshold value of the HF electric field required to produce detectable striations was experimentally determined. At threshold the component of the HF electric field perpendicular to the geomagnetic field is calculated to be 0.09 V/m in the F region and 0.37 V/m in the E region. Third, both the E and the F region data verify theoretical predictions that the striations' decay times are directly proportional to the electron diffusion across B. Finally, a one-to-one correspondence between the growth of the 3-m striations and the decline of the HF-enhanced plasma line during overshoot is sometimes observed.

Coster, A. J.↗
NASA NTRS2000

Statistical Study of Auroral Kilometric Radiation Fine Structure Striations Observed by Polar

We have conducted a statistical survey of a semirandom sample of the auroral kilometric radiation (AKR) data observed by the plasma wave instrument wideband receiver on board the Polar spacecraft. We have determined that AKR fine structure patterns with very narrowband, negative drifting striations occur in approximately 6% of the high-resolution wideband spectrograms when AKR is present. Positive sloping striations are also observed, but at a much lower rate. More than 8200 AKR stripes have been scaled. The stripes are predominantly found in the 40 to 215-kHz frequency range and have a frequency extent of about 4 kHz and a duration of usually less than 2 s. The majority of the stripes have drift rates between -8 and -2 kHz/s, with a peak in the distribution between -6 and -4 kHz/s. There is also a much smaller group of striations with positive drift rates of up to about 5 or 6 kHz/s. We have further investigated the change of drift rate with frequency. Almost all striations are observed in the lowest two frequency bands of the wideband receiver (f < 215 kHz). There is an increase in the statistical drift rate with increasing frequency. The statistical slope of the striations increases with frequency from about -4.4 kHz/s at 75 kHz to about -5.7 kHz/s at 170 kHz. This frequency dependence of the drift rate is consistent, under certain conditions, with a production mechanism stimulated by an upward propagating electromagnetic ion cyclotron wave, as had been suggested earlier. However, such a changing drift rate is also compatible with a stimulated source region that propagates upward along the magnetic field line at the velocity of an ion beam accelerated by a local, upward directed electric field, as is typically observed in the auroral region. An explanation for this association is not apparent at this time.

Menietti, J. D.↗
DOE OSTI2013

EXCITATION OF KINETIC ALFVEN WAVES BY DENSITY STRIATION IN MAGNETO-PLASMAS

Field-aligned density striation is one of the most common inhomogeneity phenomena in magneto-plasmas, such as in the solar coronal plasma and terrestrial auroral plasma. Kinetic Alfven waves (KAWs) can play an important role in the inhomogeneous heating of coronal magneto-plasmas as well as in the local acceleration of auroral energetic electrons. In this paper, we study the dispersion and instability of KAWs in a magneto-plasma with density striation structures. Results show that KAWs become unstable in the presence of the density striation and the corresponding instability has a maximal growth rate at the perpendicular wavelength close to the spatial scale of the density gradient. Related experimental phenomena in both laboratory and space plasmas are discussed. It is suggested that the excitation of KAWs by the density striation of magneto-plasmas can be of potential importance in understanding the physics of the formation of magneto-plasma filaments and their heating mechanisms, which are often present in the terrestrial auroral plasma, the solar coronal plasma, and other astrophysical plasmas.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1985

Some observations on the occurrence of striation heating

Various wind tunnel flow visualization techniques have revealed the presence of striations on the surface of both simple geometries and complex winged entry configurations over a wide range of test conditions. The striations are attributed to streamwise vortices embedded in the boundary layer which produce locally enhanced heating levels. Mechanisms for such vortex formation are suggested based on observation of their occurrence on the simple configurations. These mechanisms are then related to striations observed in the wind tunnel on windward surfaces, swept wing leading edges and on the side fuselage of winged entry bodies. Flight data obtained on the side fuselage of ASSET and Shuttle support the conclusions derived from wind tunnel results. Quantitative criteria for the appearance of striations resulting from some of these mechanisms are presented. It is shown that the presence of embedded vorticity may be a consideration in the design of thermal protection systems for advanced winged entry vehicles.

Kipp, H. W.↗
DOE OSTI2023

Spatiotemporal measurements of striations in a glow discharge’s positive column using laser-collisional induced fluorescence

Here we have observed the behavior of striations caused by ionization waves propagating in low-pressure helium DC discharges using the non-invasive laser-collision induced fluorescence (LCIF) diagnostic. To achieve this, we developed an analytic fit of collisional radiative model (CRM) predictions to interpret the LCIF data and recover quantitative two-dimensional spatial maps of the electron density, n e , and the ratios of LCIF emission states that can be correlated with T e with the use of accurate distribution functions at localized positions within striated helium discharges at 500 mTorr, 750 mTorr, and 1 Torr. To our knowledge, these are the first spatiotemporal, laser-based, experimental measurements of n e in DC striations. The n e and 447:588 ratio distributions align closely with striation theory. Constriction of the positive column appears to occur with decreased gas pressure, as shown by the radial n e distribution. We identify a transition from a slow ionization wave to a fast ionization wave between 750 mTorr and 1 Torr. These experiments validate our analytic fit of n e , allowing the implementation of an LCIF diagnostic in helium without the need to develop a CRM.

42 ENGINEERING↗
NASA NTRS1974

Barium cloud evolution and striation formation in the magnetospheric release on September 21, 1971

The joint NASA-Max Planck Institute Barium Ion Cloud (BIC) Experiment on September 21, 1971 involved the release of 1.7 kg of neutral barium at an altitude of 31,500 km at a latitude of 6.93 deg N. and a longitude of 74.40 deg W. A theoretical model describing the barium neutral cloud expansion and the ion cloud formation is developed. The mechanism of formation of the striational features observed in the release is also discussed. Two candidate instabilities, which may contribute to striation formation, are examined. The drift instability stemming from the outwardly directed drag force exerted on the ions by the outstreaming neutrals is rejected on the grounds that the ion density is too low during the collision-dominated phase of the cloud expansion to support this kind of instability. The joint action of Rayleigh-Taylor and flute instabilities plausibly accounts for the observed striational structure. This same mechanism may well be operative at times of sudden injection of plasma into the inner magnetosphere during geomagnetic storms and may thus contribute to the formation of field-alined inhomogeneities which serve as whistler ducts.

Adamson, D.↗
NASA NTRS1985

MCZ: Striations in CZ silicon crystals grown under various axial magnetic field strengths

Suppression of fluid flow instabilities in the melt by the axial magnetic field in Czochralski silicon crystal growth (AMCZ) is investigated precisely by a high-sensitivity striation etching in conjunction with temperature measurements. The magnetic strength (B) was varied up to 4.0 kG, incremented in 0.5 kG/5 cm crystal length. The convection flow was substantially suppressed at B 1.0 kG. A low oxygen level of 2-3 ppma and a high resistivity of 400 ohm-cm is achieved in the AMCZ silicon crystals at B 1.0 kG. Details of the striation formation as a function of B are presented. Computer simulation of the magnetohydrodynamics of the AMCZ silicon crystal growth are discussed briefly with regard to the solute, especially oxygen segregation at B=0, 1.0, and 2.0 kG. Earlier studies in the inverted Bridgman growth of InSb and Ge, which have established the cause and effect relationship between the convection in the melt and the striation formation as well as the suppression of the convections in the melt by transverse magnetic field are reviewed.

Kim, G. K. M.↗
NASA NTRS1988

Diffusional decay of striations

This paper investigates the amount of decay of growth striations in bulk crystals grown from melts and solutions, due to solid-state diffusion during the cooling of the crystal to room temperature. A model is developed based on the assumption of a sinusoidal variation in the solid composition at the interface of a semiinfinite ingot growing at constant velocity, and the damping behavior of this variation is followed as it moves away from the solid-liquid interface. Numerical results are presented showing that an increase in the diffusivity causes the striations to penetrate deeper into the solid and, consequently causes the distance for 99.9 percent decay to increase. Analytical results are also presented for the decay of striations in an isothermal ingot induced by a sinusoidal fluctuation in the rate of crystal growth.

Gray, R. T.↗
DOE OSTI2019

Measurement of the electron energy distribution in moving striations at low gas pressures

The time-resolved Electron Energy Distribution Functions (EEDFs) have been measured at different phases of moving striations in a positive column of DC discharge in argon gas. A very low gas pressure of 10 mTorr, a high energy resolution (to resolve the low energy part of the EEDF), and the dynamic range up to 3–4 orders of magnitude (to resolve the EEDF tail) with a temporal resolution of 2.5 μs distinguish our work from previous publications. The measured EEDFs reveal drastic changes in time of their low energy parts with the formation of a low energy peak. The observed EEDF dynamics is explained in the framework of nonlocal electron kinetics as electric field reversals and the trapping of low-energy electrons in potential wells propagating with striation along the discharge tube. Finally, the formation of the low energy peak in the EEDF is similar to that in rf capacitive and inductive discharges at low gas pressures where the low-energy electrons are trapped in the potential well created by the ambipolar electric field and cannot penetrate into the areas of electron heating by strong rf electric fields

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1978

Theory of short-scale field-aligned density striations due to ionospheric heating

The theoretical saturation spectrum of parametrically excited Langmuir waves in a locally uniform ionosphere is shown by the present calculations to produce, by ohmic dissipation, short-scale field-aligned density striations. The spectrum of the calculated striations is not inconsistent with observations of field-aligned scatter of VHF and UHF waves in ionospheric modification experiments if local increases of the pump field due to focusing are invoked.

Lee, M.-C.↗
NASA NTRS1997

Striations, Polish, and Related Features from Clasts in Impact-Ejecta Deposits and the "Tillite Problem"

Proximal ejecta deposits related to three large terrestrial impacts, the 14.8-Ma Ries impact structure in Germany (the Bunte Breccia), the 65-Ma Chicxulub impact structure in the Yucatan (the Albion and Pook's Hill Diamictites in Belize) and the mid-Tertiary Azuara impact structure in Spain (the Pelarda Fm.) occur in the form of widespread debris-flow deposits most likely originating from ballistic processes. These impact-related diamictites typically are poorly sorted, containing grain sizes from clay to large boulders and blocks, and commonly display evidence of mass flow, including preferred orientation of long axes of clasts, class imbrication, flow noses, plugs and pods of coarse debris, and internal shear planes. Clasts of various lithologies show faceting, various degrees of rounding, striations (including nailhead striae), crescentic chattermarks, mirror-like polish, percussion marks, pitting, and penetration features. Considering the impact history of the Earth, it is surprising that so few ballistic ejecta, deposits have been discovered, unless the preservation potential is extremely low, or such materials exist but have been overlooked or misidentified as other types of geologic deposits . Debris-flow diamictites of various kinds have been reported in the geologic record, but these are commonly attributed to glaciation based on the coarse and poorly sorted nature of the deposits and, in many cases, on the presence of clasts showing features considered diagnostic of glacial action, including striations of various kinds, polish, and pitting. These diamictites are the primary evidence for ancient ice ages. We present evidence of the surface features on clasts from known proximal ejecta debris-flow deposits and compare these features with those reported in diamictites. interpreted as ancient glacial deposits (tillites). Our purpose is to document the types of features seen on clasts in diamictites of ejecta origin in order to help in the interpretation of the origin of ancient diamictites. The recognition of characteristic features in clast populations in ancient diamictites may allow identification and discrimination of debris-flow deposits of various origins (e.g., impact glacial, tectonic) and may shed light on some climatic paradoxes, such as inferred Proterozoic glaciations at low paleolatitudes.

Rampino, M. R.↗