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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Wisconsin In Situ Penning (WISP) gauge: A versatile neutral pressure gauge to measure partial pressures in strong magnetic fields

A new type of in-vessel Penning gauge, the Wisconsin In Situ Penning (WISP) gauge, has been developed and successfully operated in the Wendelstein 7-X (W7-X) island divertor baffle and vacuum vessel. The capacity of the quantitative measurements of the neutral reservoir for light impurities, in particular, helium, is important for tokamaks as well as stellarator divertors in order to avoid fuel dilution and radiative energy loss. Penning gauges assisted by spectroscopy are a powerful tool to obtain the total neutral pressure as well as fractional neutral pressures of specific impurities. The WISP gauge is a miniaturized Penning gauge arrangement, which exploits the ambient magnetic field of magnetic confinement fusion experiments to establish the Penning discharge. Then, in situ spectroscopy is conducted to separate the fractional neutral pressures of hydrogen, helium, and possibly also other impurities. The WISP probe head was qualified using the magnetic field of the Magnetized Dusty Plasma Experiment at Auburn University between 0.25 T and 3.5 T [E. Thomas et al ., J. Plasma Phys. 81, 345810206 (2015)]. The in-depth quantitative evaluation for hydrogen and helium will be shown as well as an exploration of nitrogen, argon, and neon. A power law scaling between current I and pressure p, I = f (Gas,V) p n⁽Gas, B⁾ , was shown. The factor f is gas and anode potential dependent, while n is gas and magnetic field strength dependent. Pressure measurements from 0.1 mbar and down to 1 × 10 -5 mbar were achieved, demonstrating a reliable operating range for relevant pressure levels in the divertor and main vessel regions in current and future fusion devices, with a time resolution of up to 1 kHz. The lowest achievable pressure measurement increases with an increase in B and can be shifted with the anode potential V . At W7-X, the WISP probe head was mounted on an immersion tube setup that passes through the cryostat and places the probe head close to the plasma. Two probe heads were positioned in different divertor pump gaps, top and bottom, and one close to the plasma on the midplane in one module. The gauges were in situ calibrated together with the ASDEX pressure gauges [G. Haas and H.-S. Bosch, Vacuum 51, 39 (1998)]. Data were taken during the entire operation phase 1.2b, and measurements were coherent with other neutral gas pressure gauges. For the spectroscopic partial pressure measurements, channels of a spectroscopic detection system based on photo-multipliers, a so-called filterscope [R. J. Colchin et al ., Rev. Sci. Instrum. 74, 2068 (2003)], provided by the Oak Ridge National Lab were used.

47 OTHER INSTRUMENTATION↗
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 OSTI2022

Argon metastable density and temperature of a 43 GHz microplasma

Argon (1s 5 ) metastable density and translational gas temperature are experimentally measured using laser diode absorption spectroscopy within a 43 GHz microplasma. The plasma is initiated and sustained within a photonic crystal constructed from a rectangular array of alumina rods, each 1 mm in diameter. This configuration generates stable microplasma from 10 to 600 Torr using millimeter wave power from 100 to 1200 mW. The metastable density is in the order of 10 18 m –3 at low pressure. However, Ar(1s 5 ) density decreases to undetectable levels with increasing pressure and wave power. The gas temperature is extracted from the Lorentzian line shape of the absorption profile at 811.53 nm. The gas temperature increases from approximately 400 K at low pressure to 2000 K at 320 Torr (427 mbar, 4.27 × 10 4 Pa). Furthermore, these data are compared with previous results and suggest that the microplasma has a dense core of electrons that depletes the metastable density at high gas pressure and wave power.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Magnetic field imaging in a laboratory plasma

In laboratory plasmas, arrays of probes have typically been used to measure the evolution of the magnetic field topology. Here, we present initial image-based measurements of the magnetic topology in a low-temperature plasma using a purely optical diagnostic. Laser induced fluorescence measurements of neutral velocity distribution functions are made using a fast camera, imaging the Zeeman splitting of σ-peaks in neutral argon. The separation of σ-peaks provides spatially resolved magnetic field magnitude measurements with a detection threshold on the order of 10 G

36 MATERIALS SCIENCE↗
DOE OSTI2020

Temperature distributions and gradients in laser-heated plasmas relevant to magnetized liner inertial fusion

We present two-dimensional temperature measurements of magnetized and unmagnetized plasma experiments performed at Z relevant to the preheat stage in magnetized liner inertial fusion. The deuterium gas fill was doped with a trace amount of argon for spectroscopy purposes, and time-integrated spatially resolved spectra and narrow-band images were collected in both experiments. The spectrum and image data were included in two separate multiobjective analysis methods to extract the electron temperature spatial distribution T e ( r , z ) . The results indicate that the magnetic field increases T e , the axial extent of the laser heating, and the magnitude of the radial temperature gradients. Comparisons with simulations reveal that the simulations overpredict the extent of the laser heating and underpredict the temperature. Temperature gradient scale lengths extracted from the measurements also permit an assessment of the importance of nonlocal heat transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Simulation of the impact of particle recycling on the plasma in MPS-LD device based on the BOUT++ LPD module

A linear plasma device (LPD) module has been developed under the BOUT++ framework to simulate plasma transport in the MPS-LD. However, previously, the LPD module used a simplistic neutral particle model that only includes particle density and velocity, which prevents the full understanding of the plasma-neutrals interactions. In this work, we further optimize the neutral model by using a more complete neutral fluid model containing the continuity equation, momentum equation, and energy equation. The reactions such as charge exchange, excitation, and radiation collisions are included. Since the neutral particle source is mainly provided by particle recycling from the target, a particle recycling model is employed, which includes both fast reflection and slow thermal release. The upgraded LPD module is applied to simulate the argon (Ar) discharge experiment of MPS-LD, and the benchmark against experiment measurement and SOLPS-ITER simulation results are presented. Good agreements are obtained, showing the validation of the upgraded module. After that, the impact of particle recycling on Ar plasma is investigated. It is found that a higher recycling coefficient (R) promotes the achievement of high-density plasma at the target. In conclusion, the recycled Ar atoms change target plasma pressure as well as plasma-neutral collisions, which both contribute to plasma momentum loss, thus promoting the rollover of ion flux to the target.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Alfvénic modes excited by the kink instability in PHASMA

Magnetic flux ropes have been successfully created with plasma guns in the newly commissioned PHAse Space MApping (PHASMA) experiment. The flux ropes exhibit the expected m = 1 kink instability. The observed threshold current for the onset of this kink instability is half of the Kruskal–Shafranov current limit, consistent with predictions for the non-line tied boundary condition of PHASMA. The helicity, paramagnetism, and growth rate of the observed magnetic fluctuations are also consistent with kink instability predictions. The observed fluctuation frequency appears to be a superposition of a real frequency due to a Doppler shift of the kink mode arising from plasma flow (~2kHz) and a contribution from a wave mode (~5 kHz). The dispersion of the wave mode is consistent with an Alfvén wave. Distinct from most previ-ous laboratory studies of flux ropes, the working gas in PHASMA is argon. Thus, the ion cyclotron frequency in PHASMA is quite low and the frequency of the Alfvénic mode plateaus at ~0.5 of the ion gyro frequency with increasing background magnetic field strength.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Shear Strength and Release of Large Cryogenic Pellets from the Barrel of a Shattered Pellet Injector for Disruption Mitigation

Shattered pellet injection (SPI) has been chosen as the baseline disruption mitigation system on ITER due to its ability to rapidly inject material deep into the plasma to greatly increase the plasma density and radiate the thermal energy. SPI utilizes a mechanical punch or high-pressure gas to release and accelerate a pellet that has been cryogenically desublimated in the barrel of a pipe gun. Various material injection combinations could possibly be implemented during different phases of a disruption event to radiate plasma energy, reduce electromagnetic loads on machine components, avoid the formation of runaway electrons, or to dissipate runaway electrons that form. Each injection phase could possibly utilize combinations of deuterium, neon, or argon. In this paper we outline experimental measurements of pellet material shear strength at SPI operating temperatures to understand the force needed to release SPI pellets. Deuterium, neon, argon, and deuterium-neon mixture pellets with diameters of 8.5, 12.5, and 15.7 mm are formed at a range of relevant gas pressures and temperatures and dislodged from the cold zone with a slow-moving piston driven by a motor. The slow-moving piston is kept above the triple point temperature of the material while the pellet is forming, then cooled to below the triple point temperature before contacting the pellet to minimize any thermal conduction to the pellet. The piston incorporates a load cell to measure the force applied when the pellet breaks away from the cold zone in the barrel.The ability of the gas and punch methods to exceed the shear strength of the studied pellet materials for release has been analyzed. High pressure gas delivered by fast opening valves produce pressure shock to the pellet due to supersonic expansion of the propellant gas. Pressure (and therefore, force) oscillations are present due to transverse density propagation throughout the breech volume. Mechanical punches deliver an impact force through a high-kinetic energy impact. The effect of the mechanical shock on the pellet has been explored and will be presented in this paper. Scaling to larger ITER size SPI pellets will be described.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Computing the Paschen curve for argon with speed-limited particle-in-cell simulation

Upon inclusion of collisions, the speed-limited particle-in-cell (SLPIC) simulation method successfully computed the Paschen curve for argon. The 1D3V simulations modeled an electron cascade across an argon-filled capacitor, including electron-neutral ionization, electron-neutral elastic collisions, electron-neutral excitation, and ion-induced secondary electron emission. In electrical breakdown, the timescale difference between ion and electron motion makes traditional PIC methods computationally slow. To decrease this timescale difference and speed up computation, here we used SLPIC, a time-domain algorithm that limits the speed of the fastest electrons in the simulation. The SLPIC algorithm facilitates a straightforward, fully kinetic treatment of dynamics and collisions. SLPIC was as accurate as PIC, but ran up to 200 times faster. SLPIC accurately computed the Paschen curve for argon over three orders of magnitude in pressure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Measurements of ion-electron energy-transfer cross section in high-energy-density plasmas

Here, we report on measurements of the ion-electron energy-transfer cross section utilizing low-velocity ion stopping in high-energy-density plasmas at the OMEGA laser facility. These measurements utilize a technique that leverages the close relationship between low-velocity ion stopping and ion-electron equilibration. Shock-driven implosions of capsules filled with D 3 He gas doped with a trace amount of argon are used to generate densities and temperatures in ranges from 1 × 10 23 to 2 × 10 24 cm –3 and from 1.4 to 2.5 keV, respectively. The energy loss of 1-MeV DD tritons and 3.7-MeV D 3 He alphas that have velocities lower than the average velocity of the thermal electrons is measured. The energy loss of these ions is used to determine the ion-electron energy-transfer cross section, which is found to be in excellent agreement with quantum-mechanical calculations in the first Born approximation. This result provides an experimental constraint on ion-electron energy transfer in high-energy-density plasmas, which impacts the modeling of alpha heating in inertial confinement fusion implosions, magnetic-field advection in stellar atmospheres, and energy balance in supernova shocks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Similarity properties in capacitive radio frequency plasmas with nonlinear collision processes

Abstract Similarity laws (SL) are essential for correlating the characteristics of plasmas at different dimensional scales, which have been validated for radio frequency (rf) discharges at low pressure but under limited conditions. In this work, we evaluate the effects of nonlinear collisions (e.g. stepwise ionization) on the similarity properties in capacitive rf discharges in argon across a wide range of pressure regimes via fully kinetic particle-in-cell simulations. The SL scalings of fundamental discharge parameters, e.g. the electron power absorption and electron energy probability function, are examined with and without nonlinear collisions, respectively. Without the nonlinear collisions, the similarity scalings are found to be rigorously valid. When the nonlinear collisions are considered, the similarity properties in rf discharges still exist approximately, which indicates that the violations caused by the nonlinear collisions are still minor in the studied cases. The reasons for the effectiveness of SL scalings with nonlinear collision processes are also discussed. The results from this study confirmed the validity of similarity transformations with more complex reaction kinetics, which may promote the applicability of the SL scalings for the design and fabrications of plasma devices.

Physics↗
DOE OSTI2024

Neutron and photon in-vivo materials characterization at the evolving plasma-material interface in plasma-burning fusion environments

This report includes the first set of MOSS data collected in the context of fusion-relevant plasma-material interactions. The data collected show that the surface curvature and stress irreversibly change during low-energy deuterium ion irradiation. This holds for both tungsten thin films, bulk tungsten, and single-crystal silicon. Meanwhile, argon irradiation did not produce this same effect. The surface morphology of the exposed samples was virtually identical regardless of gas species. There were, however, some surface chemistry differences between argon-exposed and deuterium-exposed samples, mainly concerning the oxide states of the W4f XPS signal. The changes in surface chemistry could be due to hydrogen dynamics on the tungsten surfaces, though further investigation will be required with in-situ irradiation and analysis. The changes in surface stress could also not be explained by temperature alone, as the temperature reached during the ion irradiations was too low to induce non-elastic strain. With this in mind, deuterium retention in the materials could change the surface curvature and stress. To confirm this, further work will be needed to quantify the change in curvature and relate it to ion flux, fluence, and deuterium retention. Unfortunately, when this work was conducted, the experimental setup for thermal desorption spectroscopy was not operating correctly, and that work could not be included in this dissertation. Despite experimental difficulties, this work shows the first results of a promising technique to study deuterium dynamics in tungsten-based reconstituted thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Shock physics and shadowgraphic measurements of laser-produced cerium plasmas

Shadowgraphic measurements are combined with theory on gas-dynamics to investigate the shock physics associated with nanosecond laser ablation of cerium metal targets. Time-resolved shadowgraphic imaging is performed to measure the propagation and attenuation of the laser-induced shockwave through air and argon atmospheres at various background pressures, where stronger shockwaves characterized by higher propagation velocities are observed for higher ablation laser irradiances and lower pressures. The Rankine-Hugoniot relations are also employed to estimate the pressure, temperature, density, and flow velocity of the shock-heated gas located immediately behind the shock front, predicting larger pressure ratios and higher temperatures for stronger laser-induced shockwaves.

42 ENGINEERING↗
DOE OSTI2024

Effects of Plume Targeted Cooling on Residual Stress in Controlled Atmosphere Plasma Sprayed Coatings

Thermal spray processes can benefit from cooling to maintain substrate temper, reduce processing times, and manage thermally induced residual stresses. “Plume quenching” is a plume-targeted cooling technique which has been shown to reduce substrate temperatures by redirection of hot plume gases using a lateral argon curtain injected into the plume, while limiting interaction with the substrate or affecting coating properties. Here, this study explores the use of this technique for residual stress management by reducing the thermally driven component in nickel and tantalum coatings on titanium and aluminum substrates. The in-situ residual stress profiles were measured for all substrate and coating pairings during spraying and cooling, and the deposition and thermal stresses recorded. For substrate and coating pairings where the predominant component of residual stress was thermal (driven by a large difference in coefficient of thermal expansion, Δα, between coating and substrate), plume quenching reduced both the thermal stress and the final stress state of the coating. This was seen primarily in tantalum on aluminum coatings where the Δα was -17 × 10 -6 /°C, and thermal stress was reduced by 7.5% and 22.4% for the plume quenching rates of 50 and 100 slpm, respectively.

42 ENGINEERING↗
DOE OSTI2024

Argon metastable density and temperature of a 94 GHz microplasma

Laser diode absorption spectroscopy is used to experimentally measure Ar(1s 5 ) metastable density and translational gas temperature within a 94 GHz microplasma. A square two-dimensional photonic crystal (PhC) at this resonance frequency serves to ignite and sustain the plasma from 20 to 200 Torr (2.7 × 10 3 –2.7 × 10 4 Pa) by using millimeter wave power from 300 to 1000 mW. Metastable density within the plasma is estimated from the absorption line shape of the laser traversing the PhC. The metastable density reaches an order of 10 19 m –3 at lower pressure and decreases as pressure increases. From the Lorentzian line shape of the absorption profile at 811.53 nm, the gas temperature is extracted and found to increase from 500 K at 20 Torr to 1300 K at 200 Torr. Furthermore, these data are analyzed and compared with a zero-dimensional plasma model and with previous experimental plasma results at 43 GHz.

42 ENGINEERING↗
DOE OSTI2022

Understanding how minority relativistic electron populations may dominate charge state balance and radiative cooling of a post-thermal quench tokamak plasma

Minority relativistic electron populations can occur in a range of complex plasmas. Of specific interest is when runaway electrons form among the presence of high-atomic-number ion species in a tokamak plasma discharge. It has been recently demonstrated that ion charge state distributions and radiation losses at low bulk electron temperatures can be dominated by relativistic electrons, even though their density is orders of magnitude lower. Furthermore, this was attributed to the relativistic enhancement of electron impact inelastic cross sections. In this work, we provide a closer inspection of the atomic physics underpinning this effect. We also demonstrate the consequences of runaway enhanced scattering on post-disruption tokamak fusion discharges with neon and argon impurities present. Effects on charge state distributions, radiation and spectral characteristics, and reduced-order modeling considerations are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

The importance of laser wavelength for driving inertial confinement fusion targets. II. Target design

We describe details of radiation-hydrodynamics simulations of directly driven targets for inertial confinement fusion using laser drivers with different laser wavelengths. Of particular interest here are comparisons of frequency-tripled glass (laser wavelength 351 nm) lasers with the argon fluoride (193 nm) and krypton fluoride (248 nm) excimer lasers and the effects that these laser wavelengths have on the target designs. We explore the effect these drivers have on the compromise involved between lowering laser plasma instabilities (LPIs) or hydrodynamic instabilities while providing high gains and seek to quantify this trade-off. Short-wavelength drivers have significant advantages, primarily in using less power and energy to drive targets. Additionally, they expand the allowed operating regime that is constrained by LPI avoidance and the production of higher pressures needed for more hydrodynamically stable targets. Potential disadvantages to shorter drive wavelengths, such as increased imprint, are examined and found to be unimportant.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗