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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 OSTI2026

Benchmark for two-dimensional large scale coherent structures in partially magnetized E × B plasmas—community collaboration & lessons learned

Low-temperature plasmas (LTPs) are essential to both fundamental scientific research and critical industrial applications. As in many areas of science, numerical simulations have become a vital tool for uncovering new physical phenomena and guiding technological development. Code benchmarking remains crucial for verifying implementations and evaluating performance. This work continues the Landmark benchmark initiative, a series specifically designed to support the verification of LTP codes. In this study, seventeen simulation codes from a collaborative community of nineteen international institutions modeled a partially magnetized E × B Penning discharge. The emergence of large scale coherent structures, or rotating plasma spokes, endows this configuration with an enormous range of time scales, making it particularly challenging to simulate. The codes showed excellent agreement on the rotation frequency of the spoke as well as key plasma properties, including time-averaged ion density, plasma potential, and electron temperature profiles. Achieving this level of agreement came with challenges, and we share lessons learned on how to conduct future benchmarking campaigns. Comparing code implementations, computational hardware, and simulation runtimes also revealed interesting trends, which are summarized with the aim of guiding future plasma simulation software development.

benchmarking↗
NASA NTRS1984

Spokes in Saturn's rings - Dynamical and reflectance properties

The results and implications of dynamical and reflectance studies of spokes in the Voyager 2 low-resolution ring movie are presented. The spoke of greater interest (Spoke 1) can be traced from its first appearance on the morning and to its terminal dispersion on the evening side. Orbital velocities, reflectances, and reflectance profiles are reported. The behavior of Spoke 1 conforms to the plasma cloud model of spoke formation and evolution proposed by Goertz and Morfill (1983). A second spoke observed in the ring movie provides corroborating support for the observed behavior of Spoke 1.

Eplee, R. E., Jr.↗
DOE OSTI2025

Anticathode effect on multimodal azimuthal oscillations in electron beam generated E×B plasma

Electron beam (e-beam) generated plasmas with applied crossed electric and magnetic (⁠E x B⁠) fields are promising for low-damage (gentle) material processing. However, these plasmas can be subject to the formation of plasma non-uniformities propagating in the ⁠E x B direction. These rotating plasma structures (or “spokes”) enhance the transport of charged species across the magnetic field, which can harm the gentle processing capability of the plasma. In this work, we investigate the role of electrostatically active boundaries on the spoke formation by incorporating a variable bias conducting boundary (known as an anticathode) placed on the axially opposite side of the cathode. Our findings indicate azimuthal mode suppression occurs when the anticathode is electron collecting. Furthermore, we show selective azimuthal mode suppression by biasing the anticathode to an intermediate potential between the cathode and anode potentials. In conclusion, these findings suggest a link between the axial electron confinement in the e-beam generated plasma and azimuthally propagating plasma structure formation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

Radial growth of an extended spoke in Saturn's B ring

An analysis is reported of the pattern of radial growth of an extended spoke observed in the Voyager 2 low-resolution Saturn ring 'movie'. The feature is atypical in that it orbits Saturn at the corotational rate for 1-1/2 hours after the onset of its formation and then undergoes a 40-min acceleration to sustained Keplerian velocities. A correlation between the dynamical phases and the radial growth modes of the spoke is observed, one that seems consistent with the plasma cloud model of spoke formation and evolution proposed by Goertz and Morfill (1983), taken in the limit of high charge density.

Eplee, R. E., Jr.↗
NASA NTRS1983

A model for the formation of spokes in Saturn's rings

Evidence is mounting which implies that the generation, evolution, and motion of spokes require a dense plasma near the Saturn ring plane. It is presently suggested that spokes consist of charged, micron-sized dust particles which are elevated from the rings by radially moving, dense plasma columns due to meteor impacts on the ring. Electrostatic wall sheaths at the ring, and ring charging by electric fields sufficiently strong to overcome the gravitational force on small dust particles, arise from the dense plasma, which also increases the probability of dust particle excess electronic charge. The radial motion of the plasma column is due to an azimuthal polarization electric field due to the relative motion between the corotating plasma and the negatively charged dust particles moving at Keplerian speeds.

Goertz, C. K.↗
NASA NTRS1972

Ion heating mechanism in a modified Penning discharge

Ions with Maxwellian energy distributions and kinetic temperatures ranging from 20 eV to 7 keV have been observed in a modified Penning discharge operating in the steady state. Investigation of the plasma revealed two distinct spoke-like concentrations of charge rotating with different velocities in the sheath between the plasma and the anode ring. The faster spoke consists of electrons rotating with the E/B drift velocity, where E is the electric field and B is the magnetic field strength. The slow spoke consists of ions, the thermal velocity of which is observed to be proportional to the spoke velocity. The experimental data are consistent with a model whereby the ion drift velocity in this spoke, corresponding to kilovolt ion energies, is Maxwellianized by strong electrostatic turbulence in the sheath. Theoretical expressions are derived for the frequency of the electron and ion spoke rotation, for the ion kinetic temperature, and for the ion heating efficiency as functions of the discharge parameters. These expressions are shown to be consistent with extensive experimental data.

Roth, J. R.↗
NASA NTRS1971

Origin of hot ions observed in a modified Penning discharge.

Ions with a Maxwellian energy distribution and kinetic temperatures ranging from below 100 eV to several keV have been observed in a steady-state modified Penning discharge. Observations in the plasma, with capacitive probes at several azimuthal locations, are consistent with the existence of two distinct spokes rotating with different velocities in the sheath between the plasma and the anode ring. The faster (0.5-10 MHz) spoke consists of electrons rotating with the E/B drift velocity. The slow (0.1-1.0 MHz) spoke consists of ions, the measured thermal velocity of which is directly proportional to the spoke velocity. The interaction of the two spokes is apparently responsible for the observed electrostatic ?turbulence' and ion thermalization. The anode sheath thickness is smaller than the ion gyrodiameter in this plasma. Thus the ions are in the electric field of the sheath for only a fraction of their orbit, and their E/B drift (spoke) velocity is smaller than that of the electrons.

Roth, J. R.↗
NASA NTRS1971

Origin of hot ions observed in a modified Penning discharge

Ions with a Maxwellian energy distriubtion and kinetic temperatures ranging from below 100 eV to several keV are observed in a steady state modified Penning discharge. Observations in the plasma, with capacitive probes at several azimuthal locations, are consistent with the existence of two distinct spokes rotating with different velocities in the sheath between the plasma and the anode ring. The faster (0.5 to 10 MHz) spoke consists of electrons rotating with the E/B drift velocity. The slow (0.1 to 1.0 MHz) spoke consists of ions whose measured thermal velocity is directly proportional to the spoke velocity. The interaction of the two spokes is apparently responsible for the observed electrostatic turbulence and ion thermalization. The anode sheath thickness is smaller than the ion gyrodiameter in this plasma. Thus the ions are in the electric field of the sheath for only a fraction of their orbit, and their E/B drift (spoke) velocity is smaller than that of the electrons.

Roth, J. R.↗
DOE OSTI2024

Investigating instabilities in magnetized low-pressure capacitively coupled RF plasma using particle-in-cell (PIC) simulations

The effect of a uniform magnetic field on particle transport in low-pressure radio frequency (RF) capacitively coupled plasma (CCP) has been studied using a particle-in-cell model. Three distinct regimes of plasma behavior can be identified as a function of the magnetic field. In the first regime at low magnetic fields, asymmetric plasma profiles are observed within the CCP chamber due to the effect of $\overrightarrow{E}$ × $\overrightarrow{B}$ drift. As the magnetic field increases, instabilities develop and form self-organized spoke-shaped structures that are distinctly seen within the bulk plasma closer to the sheath. In this second regime, the spoke-shaped coherent structures rotate inside the plasma chamber in the -$\overrightarrow{E}$ × $\overrightarrow{B}$ direction, where $\overrightarrow{E}$ × $\overrightarrow{B}$ are the DC electric and magnetic field vectors, respectively, and the DC electric field exists in the sheath and pre-sheath regions. The spoke rotation frequency is in the megahertz range. As the magnetic field strength increases further, the rotating coherent spokes continue to exist near the sheath. The coherent structures are, however, accompanied by new small-scale incoherent structures originating and moving within the bulk plasma region away from the sheath. This is the third regime of plasma behavior. The threshold values of the magnetic field between these regimes were found not to vary with changing plasma reactor geometry (e.g., area ratio between ground and powered electrodes) or the use of an external capacitor between the RF-powered electrode and the RF source. The threshold values of the magnetic field between these regimes shift toward higher values with increasing gas pressure. Analysis of the results indicates that the rotating structures are due to the lower hybrid instability driven by density gradients and electron-neutral collisions. This paper provides guidance on the upper limit of the magnetic field for instability-free operation in low-pressure CCP-based semiconductor deposition and etch systems that use the external magnetic field for plasma uniformity control.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2013

Azimuthal spoke propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E × B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E × B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Polk, James E.↗
NASA NTRS2013

Azimuthal Spoke Propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E x B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E x B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Cylindrical Hall Thruster (CHT)↗
NASA NTRS1973

Origin of hot ions observed in a modified Penning discharge.

Ions with a Maxwellian energy distribution and kinetic temperatures ranging from below 100 eV to several keV have been observed in a steady-state modified Penning discharge. Observations in the plasma, with capacitive probes at several azimuthal locations, are consistent with the existence of two distinct spokes rotating with different velocities in the sheath between the plasma and the anode ring. The faster (0.3-10 MHz) spoke consists of electrons rotating with the E/B drift velocity. The slow (0.06-0.8 MHz) spoke consists of ions, the measured thermal velocity of which is directly proportional to the spoke velocity. The interaction of the two spokes is apparently responsible for the observed electrostatic turbulence and ion thermalization.

Roth, J. R.↗
NASA NTRS1974

Spoke wavenumbers and mode transitions in NASA-Lewis bumpy torus

Waveforms of fluctuations of plasma potential, resulting from rotating electron and ion charge concentrations, have been observed in the Lewis bumpy torus. Since these fluctuations are thought to be spoke rotations in the direction of the E x B drift, measuring the direction of spoke rotation should determine the direction of the electric fields over much of the plasma surface. Since inward pointing electric fields should enhance confinement of the hot ions in this device, knowledge of the direction of the spoke rotation could give valuable information on the mechanism of plasma confinement and radial transport. A Langmuir probe was inserted into the plasma to measure the floating potential of the plasma; hence the direction of the electric field could be checked another way.

Gerdin, G. A.↗
DOE OSTI2025

Mode transitions and spoke structures in E × B Penning discharge

Two-dimensional particle-in-cell simulations in the (radial-azimuthal) plane perpendicular to the axial direction of a cylindrical $E$ x $B$ Penning discharge are presented. The low-pressure discharge is self-consistently supported by plasma ionization from the electron beam injected axially, along the direction of the external magnetic field. It is shown that with the increasing strength of the external magnetic field, the discharge undergoes a sequence of transitions between several azimuthal modes. Azimuthal m > 1 spiral arm structures are excited at low magnetic field values as plasma confinement improves and the radial density profile becomes peaked. With a larger field, spiral arms with m > 1 are replaced by the m = 1 spoke mode, most clearly seen in plasma density. A transition from spiral arms to the spoke regime occurs when the plasma potential in the center changes from weakly positive (or zero) to negative. Further increase in the magnetic field results in a well-developed m = 1 spoke mode with additional small-scale higher-frequency m > 1 structures inside and around the spoke. It is shown that while ionization and collisions affect some characteristics of the observed fluctuations, the basic features of the spoke and m > 1 spiral structure remained similar without ionization. The role of energy conservation in small-scale high-frequency modes and spoke dynamics is discussed. It is demonstrated that in regimes with the m = 1 spoke mode, additional m = 4 harmonics of the ion and electron fluxes to the wall appear due to the square boundary. The frequency of the m = 1 mode is weakly affected by the geometry of the boundary.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1974

Spoke wavenumbers and mode transitions in the NASA Lewis bumpy torus

Waveforms of fluctuations of plasma potential, resulting from rotating electron and ion charge concentrations, were observed in a bumpy torus. Since these fluctuations were thought to be spoke rotations in the direction of the ExB drift, measuring the direction of spoke rotation should determine the direction of the electric fields over much of the plasma surface. A Langmuir probe was inserted into the plasma to measure the floating potential of the plasma, hence the direction of the electric field could be checked another way.

Gerdin, G. A.↗
NASA NTRS1989

Dusty plasmas in the solar system

The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for single grains the results have been long known, the reduction of the average charge on a grain by 'Debye screening' has only recently been discovered. This reduction can be important in the Jovian ring and in the rings of Uranus. The emerging field of gravitoelectrodynamics which deals with the motion of charged grains in a planetary magnetosphere is then reviewed. Important mechanisms for distributing grains in radial distance are due to stochastic fluctuations of the grain charge and a systematic variation due to motion through plasma gradients. The electrostatic levitation model for the formation of spokes is discussed, and it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings. Finally, collective effects in dusty plasmas are discussed which affect various waves, such as density waves in planetary rings and low-frequency plasma waves. The possibility of charged grains forming a Coulomb lattice is briefly described.

Goertz, C. K.↗