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Results for “Plasma properties and parameters”

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

Extended magnetohydrodynamics simulations of thin-foil Z-pinch implosions with comparison to experiments

Cylindrical foil liners, with foil thicknesses on the order of 400 nm, are often used in university-scale Z-pinch experiments (~1 MA in 100 ns) to study physics relevant to inertial confinement fusion efforts on larger-scale facilities (e.g., the MagLIF effort on the 25-MA Z facility at Sandia National Laboratories). The use of ultrathin foil liners typically requires a central support rod to maintain the structural integrity of the liner target assembly prior to implosion. The radius of this support rod sets a limit on the maximum convergence ratio achievable for the implosion. In recent experiments with a support rod and a preimposed axial magnetic field, helical instability structures in the imploding foil plasma were found to persist as the foil plasma stagnated on the rod and subsequently expanded away from the rod. We have now used the 3D extended magnetohydrodynamics simulation code PERSEUS (which includes Hall physics) to study these experiments. The results suggest that it is the support rod which is responsible for the helical structures persisting beyond stagnation. Furthermore, we find that as the radius of the support rod decreases (i.e., as the convergence ratio increases), the integrity and persistence of the helical modes diminish. In the limit with no support rod, we find that the structure of the final stagnation column is governed by the structure of the central precursor plasma column. Furthermore, these simulation results and their comparisons to experiment are presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Trajectory of a flying plasma mirror traversing a target with density gradient

It has been proposed that laser-induced relativistic plasma mirror can accelerate if the plasma has a properly tailored density profile. Such accelerating plasma mirrors can serve as analog black holes to investigate Hawking evaporation and the associated information loss paradox. Here we reexamine the underlying dynamics of mirror motion in a graded-density plasma to provide an explicit trajectory as a function of the plasma density and its gradient. Specifically, a decreasing plasma density profile (down-ramp) along the direction of laser propagation would in general accelerate the mirror. In particular, a constant-plus-exponential density profile would generate the Davies–Fulling trajectory with a well-defined analog Hawking temperature, which is sensitive to the plasma density gradient but not to the density itself. Furthermore, we show that without invoking nano-fabricated thin-films, a much lower density gas target at, for example, ~1 × 10 17 cm -3 , would be able to induce an analog Hawking temperature, k B T H ~3.1 × 10 -2 eV, in the far-infrared region. We hope that this would help to better realize the experiment proposed by Chen and Mourou.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Nonlinear kinetic simulation study of the ion–ion streaming instability in single- and multi-ion species plasmas

The nonlinear evolution of the ion–ion streaming instability (IISI) is studied using numerical techniques novel to this problem that afford direct insight into the evolution of the particle distributions of each species. Here, during the linear phase of the instability, we demonstrate quantitative agreement with linear kinetic theory. Subsequently, the electrostatic field generated by the IISI causes ring-like velocity distributions of ions to form that are both heated and slowed to varying degrees relative to their initial flows. Due to variation in the trapping conditions for ion species of differing charge-to-mass ratio, when flows of multiple species interact, the nonlinear evolution of each species can be starkly different: we show a case where a lighter ion species is completely stopped by a heavier ion species via the IISI alone (i.e., without collisions) and, for the first time, demonstrate how the IISI can introduce a relative flow between ion species that initially have the same flow velocities, thereby separating them.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Analytical model for quasi-linear flow response to resonant magnetic perturbation in resistive-inertial and viscous-resistive regimes

In this work, a quasi-linear model for plasma flow response to the resonant magnetic perturbation (RMP) in a tokamak has been rigorously developed in the resistive-inertial (RI) and viscousresistive (VR) regimes purely from the two-field reduced MHD model. Models for plasma response to RMP are commonly composed of equations for the resonant magnetic field response (i.e. the magnetic island) and the torque balance of plasma flow. However, in previous plasma response models, the magnetic island and the torque balance equations are often derived separately from reduced MHD and full MHD equations, respectively. By contrast, in this work we derive both the magnetic island response and the torque balance equations in a quasi-linear model for plasma flow response entirely from a set of two-field reduced MHD equations. Such a quasi-linear model can recover previous plasma flow response models within certain limits and approximations. Furthermore, the physical origins of quasi-linear forces and moments in the flow response equation are also accurately calculated and clarified self-consistently.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Numerical study of coronal plasma jet formation

A new scenario for solar flare eruption in the coronal holes is analyzed by using MHD stability concepts for a spheromak configuration. The stability properties of a spheromak partially embedded into a conducting surface are studied using three dimensional MHD simulations. In agreement with the analytical theory, a large degree of line-tying stabilizes the spheromak's tilt instability, while the elongation has a destabilizing effect. High-resolution nonlinear simulations also demonstrate current sheet formation at the upper surface of the spheromak, where the tilted magnetic field of the spheromak reconnects with the background magnetic field. The calculated stability threshold and the observed magnetic reconnection support a model of coronal jet eruptions where a dome-like magnetic structure grows through flux emergence on the solar surface, tilts, reconnects, and erupts. Countering the effect from elongation, line-tying strongly stabilizes a spheromak growing from a flux-emergence process, suggesting that to accelerate the onset of eruptive coronal jets, there must be magnetic reconnection at the bottom of the spheromak to detach the structure from the solar surface.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Electron density measurement using a partially covered hairpin resonator in an inductively coupled plasma

Hairpin probes are used to determine electron densities via measuring the shift of the resonant frequency of the probe structure when immersed in a plasma. This manuscript presents new developments in hairpin probe hardware and theory that have enabled measurements in a high electron density plasma, up to approximately 10 12 cm -3 , corresponding to a plasma frequency of about 9 GHz. Hardware developments include the use of both quarter-wavelength and three-quarter-wavelength partially covered hairpin probes in a transmission mode together with an easily reproducible implementation of the associated microwave electronics using commercial off-the-shelf components. The three-quarter-wavelength structure is operated at its second harmonic with the purpose of measuring higher electron densities. New theory developments for interpreting the probe measurements include the use of a transmission line model to find an accurate relationship between the resonant frequency of the probe and the electron density, including effects of partially covering the probes with epoxy. Measurements are taken in an inductively coupled plasma sustained in argon at pressures below 50 mTorr. Results are compared with Langmuir probe and interferometry measurements.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Driven resonant current amplification in self-organized plasma configurations with uniform λ and plasma pressure confinement

Driven resonant current amplification in uniform λ=$^{μoJ}_{B}$, self-organized plasma configurations with plasma pressure confinement is studied. This work extends previous analyses of force-free states (J ∥ B) to equilibria with plasma pressure gradients (∇P = J × B ≠ 0). An analytical treatment shows that in equilibria with uniform λ and a plasma pressure gradient that varies linearly with poloidal magnetic flux, driven resonances are either downshifted or upshifted from their force-free values. Additionally, a modification of off resonance current amplification is observed in pressure confining equilibria. Computational results add support to analytical predictions, and also include driven resonance behavior with equilibrium profiles not considered analytically. Furthermore, these results suggest not only persistence, but also modification of driven resonant current amplification in sustained spheromak configurations with plasma pressure confinement when compared to force-free states. By providing a basis for the existence of driven, high current amplification, pressure confining equilibria, this work provides insight into the accessibility of sustained spheromak states of most interest for potential fusion energy applications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Laser absorption measurements of electron density in nanosecond-scale atmospheric pressure pulsed plasmas

We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as n e = 7 x 10 19 cm -3 , and decay to 1/e of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[ 5 S o ]-3p[ 5 P] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Multi-fluid and kinetic models of partially ionized magnetic reconnection

Magnetic reconnection in partially ionized plasmas is a ubiquitous and important phenomenon in both laboratory and astrophysical systems. Here, simulations of partially ionized magnetic reconnection with well-matched initial conditions are performed using both multi-fluid and fully-kinetic approaches. Despite similar initial conditions, the time-dependent evolution differs between the two models. In multi-fluid models, the reconnection rate locally obeys either a decoupled Sweet–Parker scaling, where neutrals are unimportant, or a fully coupled Sweet–Parker scaling, where neutrals and ions are strongly coupled, depending on the resistivity. In contrast, kinetic models show a faster reconnection rate that is proportional to the fully-coupled, bulk Alfvén speed, $v^*_A$. In this work, these differences are interpreted as the result of operating in different collisional regimes. Multi-fluid simulations are found to maintain $ν_{ni}L/v^*_A$ ≳1, where $ν_{ni}$ is the neutral–ion collision frequency and L is the time-dependent current sheet half-length. This strongly couples neutrals to the reconnection outflow, while kinetic simulations evolve to allow $ν_{ni}L/v^*_A$ <1, decoupling neutrals from the reconnection outflow. Differences in the way reconnection is triggered may explain these discrepancies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Unabsorbed light beamlets for diagnosing coronal density profiles and absorption nonuniformity in direct-drive implosions on OMEGA

Laser light scattered from a target is the most-direct measurement for diagnosing laser absorption in a direct-drive implosion. The 3ωGOI (gated optical imager) beamlets diagnostic images unabsorbed light from all 60 OMEGA beams as distinct “beamlet” spots for each beam. The implosion can be diagnosed using the position and intensity of these beamlets. The position of each beamlet in the image is determined by refraction and can be used to fit the coronal plasma density profile of the implosion. The inferred plasma density profiles are comparable to profiles predicted by the 1-D hydrodynamics code LILAC but suggest the predictions underestimate the density farther out in the corona. The intensity of light in each spot depends on the cumulative effects of absorption and cross-beam energy transfer along the beamlet’s path through the corona. Furthermore, the measured variation in intensity and polarization between similar spots indicates that absorption during OMEGA implosions is less uniform than previously known.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Talbot-Lau x-ray deflectometer: Refraction-based HEDP imaging diagnostic

Talbot-Lau x-ray interferometry has been implemented to map electron density gradients in High Energy Density Physics (HEDP) experiments. X-ray backlighter targets have been evaluated for Talbot-Lau X-ray Deflectometry (TXD). Cu foils, wires, and sphere targets have been irradiated by 10–150 J, 8–30 ps laser pulses, while two pulsed-power generators (∼350 kA, 350 ns and ∼200 kA, 150 ns) have driven Cu wire, hybrid, and laser-cut x-pinches. A plasma ablation front generated by the Omega EP laser was imaged for the first time through TXD for densities >1023 cm−3. Backlighter optimization in combination with x-ray CCD, image plates, and x-ray film has been assessed in terms of spatial resolution and interferometer contrast for accurate plasma characterization through TXD in pulsed-power and high-intensity laser environments. The results obtained thus far demonstrate the potential of TXD as a powerful diagnostic for HEDP.

Valdivia, M. P.↗
DOE OSTI2021

Fusion gain from cylindrical liner-driven implosions of field reversed configurations

MagLIF experiments [M.R. Gomez et al., Phys. Plasmas 22, 056306 (2015)] on Z have demonstrated the basic principles of Magneto-Inertial Fusion (MIF) for wall confined plasmas. Other MIF schemes have been proposed based on the liner implosion of closed field magnetically confined plasmas such as Field Reversed Configurations (FRCs) [T. P. Intrator et al., Phys. Plasmas 15, 042505 (2008)]. We present a semi-analytical model of liner driven FRC implosions that predicts the fusion gain of such systems. The model predicts a fusion gain near unity for an FRC imploded by a liner driven by the Z Machine. We show that FRCs could be formed and imploded at the Z facility using the AutoMag liner concept [S. A. Slutz et al., Phys. Plasmas 24, 012704 (2017)]. Furthermore, an initial bias magnetic field can be supplied by the external magnets used in MagLIF experiments. The reverse field is then supplied by an AutoMag liner, which has helical conducting paths imbedded in an insulating substance. Experiments [Shipley et al., Phys. Plasmas 26, 052705 (2019)] have demonstrated that AutoMag can generate magnetic fields greater than 30 Tesla inside of the liner. We have performed 2D Radiation MHD simulations of the formation and implosion of an FRC, which are in good agreement with the analytical model. The FRC formation process could be studied on small pulsed power machines delivering about 1 MA.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Reflectivity and spectrum of relativistic flying plasma mirrors

Flying plasma mirrors induced by intense lasers via laser wake field acceleration scheme have been proposed as a promising way to generate few-cycle EUV or x-ray lasers. In addition, if such a relativistic plasma mirror can accelerate, then it would serve as an analog black hole to investigate the information loss paradox associated with the black hole Hawking evaporation. Among these applications, the reflectivity, which is usually frequency-dependent, would affect the outgoing photon spectrum and, therefore, impact on the analysis of the physics under investigation. In this paper, these two issues are investigated analytically and numerically with one-dimensional particle-in-cell simulations. Based on our simulation results, we propose a new model that provides a better estimate of the reflectivity than those studied previously. Moreover, we found that the peak frequency of the reflected spectrum of a Gaussian incident wave deviates from the expected value, 4γ 2 ω, due to the dependence of reflectivity on the frequency of the incident wave.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Influence of magnetic angle on the E × B drift in a magnetic presheath

In this work, we provide experimental measurements of the E X B flow above a non-magnetic, absorbing boundary in a magnetized plasma. Measurements are taken as a function of the angle between the background magnetic field and the boundary normal. The measurements are compared to computational predictions for oblique magnetic fields based on Chodura's model. Ion flow measurements were obtained with laser induced fluorescence and the presheath potential structure was measured with an emissive probe. The ions were found to accelerate to nearly 30% of the sound speed parallel to the boundary at oblique angles of the magnetic field (~80°) and the ion speed at the electrostatic sheath edge was found to decrease with angle. The edge of the magnetic presheath has been experimentally determined based on the appearance of the E X B drift and is found to be independent of the angle of the magnetic field. We also demonstrate that laser induced fluorescence is suitable as a non-perturbative diagnostic to measure the electric field in a magnetized sheath.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

On the mechanism of ionization oscillations in Hall thrusters

Low-frequency ionization oscillations involving plasma and neutral density (breathing modes) are the most violent perturbations in Hall thrusters for electric propulsion. Because of its simplicity, the zero-dimensional (0D) predator–prey model of two nonlinearly coupled ordinary differential equations for plasma and neutral density has often been used for the characterization of such oscillations and scaling estimates. In this work, we investigate the properties of its continuum analog, the one-dimensional (1D) system of two nonlinearly coupled equations in partial derivatives (PDEs) for plasma and neutral density. This is a more general model, of which the standard 0D predator–prey model is a special limit case. We show that the 1D model is stable and does not show any oscillations for the boundary conditions relevant to Hall thrusters and the uniform ion velocity. We then propose a reduced 1D model based on two coupled PDEs for plasma and neutral densities that is unstable and exhibit oscillations if the ion velocity profile with the near-the-anode back-flow (toward the anode) region is used. Comparisons of the reduced model with the predictions of the full model that takes into account the self-consistent plasma response show that the main properties of the breathing mode are well captured. In particular, it is shown that the frequency of the breathing mode oscillations is weakly dependent on the final ion velocity but shows a strong correlation with the width of the ion back-flow region.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Scaling theory of three-dimensional magnetic reconnection spreading

We develop a first-principles scaling theory of the spreading of three-dimensional (3D) magnetic reconnection of finite extent in the out of plane direction. This theory addresses systems with or without an out of plane (guide) magnetic field, and with or without Hall physics. The theory reproduces known spreading speeds and directions with and without guide fields, unifying previous knowledge in a single theory. New results include: (1) Reconnection spreads in a particular direction if an x-line is induced at the interface between reconnecting and non-reconnecting regions, which is controlled by the out of plane gradient of the electric field in the outflow direction. (2) The spreading mechanism for anti-parallel collisionless reconnection is convection, as is known, but for guide field reconnection it is magnetic field bending. We confirm the theory using 3D two-fluid and resistive-magnetohydrodynamics simulations. (3) The theory explains why anti-parallel reconnection in resistive-magnetohydrodynamics does not spread. (4) The simulation domain aspect ratio, associated with the free magnetic energy, influences whether reconnection spreads or convects with a fixed x-line length. (5) We perform a simulation initiating anti-parallel collisionless reconnection with a pressure pulse instead of a magnetic perturbation, finding spreading is unchanged rather than spreading at the magnetosonic speed as previously suggested. The results provide a theoretical framework for understanding spreading beyond systems studied here, and are important for applications including two-ribbon solar flares and reconnection in Earth’s magnetosphere.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

RF sheath induced sputtering on Proto-MPEX. I. Sheath equivalent dielectric layer for modeling the RF sheath

The pulsed linear plasma device Prototype Material Plasma Exposure eXperiment (Proto-MPEX) uses a radio frequency (RF) helicon antenna with an aluminum nitride ceramic window for plasma production. The RF sheath created under the helicon antenna is sufficient to cause ion impact energies to be greater than the sputtering threshold of the AlN helicon window material and for impurities to be created. In this study, we investigate the RF sheath on the inner diameter of the helicon window and its impact on the impurity production rates in Proto-MPEX. Three models—a 3D COMSOL finite element RF model of the Proto-MPEX helicon region, a rectified DC sheath potential model, and the 3D Global Impurity TRansport code—are coupled together to study impurity production and transportation. This novel method of impurity generation and transport modeling spans length scales ranging from the sheath (millimeters or less) up to the full device (meters) and can be applied to other radio frequency sources and antennas in a wide range of plasma physics studies, including basic plasmas, low-temperature processing plasmas, plasma thrusters, and fusion plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Modeling of q 95 windows for the suppression of edge localized modes by resonant magnetic perturbations in the KSTAR tokamak

The EPEC code is employed to model the q95 windows for n = 2 and n = 1 resonant magnetic perturbation (RMP)-induced edge localized mode (ELM) suppression in typical KSTAR H-mode discharges. The plasma equilibria used in the study are derived by rescaling the experimental plasma equilibrium in KSTAR discharge #18594 measured at time t = 6450 ms. The n = 2 and n = 1 ELM suppression windows predicted by EPEC are comparatively narrow in q95 (i.e., Δq 95 ≃ 0.1) and are such that (assuming a relative pedestal pressure reduction of 15% is required to trigger ELM suppression), even in the middle of the windows, there is a threshold RMP coil current of about 1–3 kA/turn that must be exceeded before ELM suppression occurs. Furthermore, the n = 2 and n = 1 ELM suppression windows calculated by the EPEC code are consistent with the experimental observations and are also broadly similar to those recently calculated by the TM1 code [Hu et al., Phys. Plasmas 28, 052505 (2021)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗