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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Ion-acoustic-like waves excited by the reflected ions at the earth's bow shock

Some model distributions based on recent observations and simulations of the plasma at the earth's bow shock are found to be unstable to obliquely propagating electrostatic instabilities. The model distributions consist of either an ion beam or an ion velocity ring accompanied by a bi-Maxwellian background ion distribution and a flattop electron distribution. Ion anisotropies and nonthermal electrons are capable of significantly lowering the threshold of the ion beam instability. The generated waves share many properties in common with the ion acoustic waves that have been observed at the earth's bow shock. The results also indicate the importance of an anisotropy in the background ion velocity distribution in identifying sources of the ion-acoustic-like waves observed in the ion foreshock and the solar wind.

Akimoto, K.↗
NASA NTRS2001

Ion Energy and Ion Flux Distributions of CF4/Ar/O2 Inductively Coupled Plasmas in a GEC Cell

Knowledge of ion kinetics in plasma processing gas mixtures, such as CF4:Ar:O2, is important for understanding plasma assisted etching and deposition of materials. Ion energies and ion fluxes were measured in this mixture for 80:10:10, 60:20:20, and 40:30:30 mixture ratios in the pressure range of 10-50 mTorr, and at 200 and 300 W of RF power. Ions from plasma, sampled through a 10 micron orifice in the center of the lower plane electrode, were energy and mass analyzed by a combination of electrostatic energy and quadrupole mass filters. CFx(+) (x = 1 - 3), F2(+), F(+), C(+) from CF4, Ar(+) from Ar, and O2(+) and O(+) from O2, and by-product ions SiFx(+)(x = 1 - 3) from etching of quartz coupling window, COFx(+)(x = 1 - 3), CO(+), CO2(+), and OF(+) were detected. In all conditions ion flux decreases with increase of pressure but increase with increase of RF power. Ar(+) signal decreases with increase of pressure while CF3(+), which is the dominant ion at all conditions, increases with increase in pressure. The loss mechanism for Ar(+) and increase of CF3(+) is due to large cross section for Ar(+) + CF4 yields Ar + CF3(+) + F. Ion energies, which range from 15-25 eV depending on plasma operating conditions, are nearly Gaussian. By-product ion signals are higher at lower pressures indicating stronger plasma interaction with quartz window.

Rao, M. V. V. S.↗
NASA NTRS2018

3D Simulations of Ion Thruster Accelerator Grid Erosion Accounting for Charge Exchange Ion Space Charge

Accelerator (accel) grid sputtering by ions formed through charge-exchange (CEX) reactions between beam ions and residual neutral gas is a critical life-limiting mechanism for gridded ion thrusters. The three-dimensional ion optics code CEX3D is designed to simulate this grid erosion for a single beamlet, with a particular emphasis on non-axisymmetric features such as the "pits and grooves" erosion commonly observed on the accel grid downstream face in two-grid thrusters. The treatment of CEX ions in the code was recently upgraded with a new particle-in-cell (PIC) module to account for the influence of these ions' space charge on the electrostatic potential downstream of the grids. In order to achieve reasonable computation times while resolving the Debye length near the grids and avoiding gross violations of the Courant-Friedrichs-Lewy (CFL) condition, macroparticle velocities in the PIC calculation are limited through a rescaling procedure that preserves ion trajectories and space charge density. The code accounts for beam divergence, finite momentum transfer in CEX collisions, and radial losses of CEX ions from the beam; these effects are important for determining the CEX ion flux to the accel grid because the calculated potential downstream of the grids can become very flat. The upgraded code has been used to simulate operation of NASA's Evolutionary Xenon Thruster (NEXT) during the 51 kHr Long Duration Test - a selection of results is presented and compared with experimental data.

Chaplin, Vernon H.↗
NASA NTRS2006

Production of High Energy Ions Near an Ion Thruster Discharge Hollow Cathode

Several researchers have measured ions leaving ion thruster discharge chambers with energies far greater than measured discharge chamber potentials. Presented in this paper is a new mechanism for the generation of high energy ions and a comparison with measured ion spectra. The source of high energy ions has been a puzzle because they not only have energies in excess of measured steady state potentials, but as reported by Goebel et. al. [1], their flux is independent of the amplitude of time dependent plasma fluctuations. The mechanism relies on the charge exchange neutralization of xenon ions accelerated radially into the potential trough in front of the discharge cathode. Previous researchers [2] have identified the importance of charge exchange in this region as a mechanism for protecting discharge cathode surfaces from ion bombardment. This paper is the first to identify how charge exchange in this region can lead to ion energy enhancement.

ion propulsion↗
NASA NTRS1983

Ion-beam-driven electrostatic ion cyclotron instabilities

Results are presented of a particle simulation study of the electrostatic ion-cyclotron (EIC) instability driven by a parallel ion beam. The results of this simulation study demonstrate the nonlinear consequences of nonresonant EIC waves destabilized by an ion beam parallel to the magnetic field for the case of a large beam velocity. As a consequence of the instability, it is shown that the beam ions are heated strongly in the perpendicular direction and suffer a strong anomalous friction via EIC waves which leads to the beam slowing down. Simulation results indicate that the anomalous slowing down of beam ions by EIC waves is much larger than that from the classical electron-drag, and the perpendicular collision frequency measured from perpendicular beam heating is as large as that from Bohm diffusion. It is concluded that the ion beam driven EIC wave is a viable mechanism for the transfer of ion parallel beam energy to the ion perpendicular energy.

Okuda, H.↗
NASA NTRS1984

Bands of ions and angular V's - A conjugate manifestation of ionospheric ion acceleration

Data from the hot plasma instruments on Dynamics Explorer 1 and 2 spacecraft have been used to study the injection, drift, and subsequent precipitation of suprathermal positive ions in the auroral zone. The observation at both high and low altitudes of electron inverted 'V' events in the boundary plasma sheet (BPS) and of ion 'bands' (energy decreasing with decreasing latitude) in the adjacent central plasma sheet (CPS) leads to the following ion injection model: upward-moving energetic ion beams are injected onto BPS magnetic field lines by the electrostatic potential drops associated with electron inverted V's. As the ion beams move toward the equator and into the conjugate hemisphere they are convected to lower latitudes and into the CPS. The energy-latitude dependence of the ion bands, coupled with concurrent ion convection measurements, indicate that the ion distributions are primarily O(+), in agreement with their postulated ionospheric source.

Winningham, J. D.↗
NASA NTRS1986

The acceleration of ions and electrons by electromagnetic ion cyclotron waves

Test particle calculations of the interaction of electrons and ions with obliquely propagating electromagnetic ion cyclotron waves are presented. Such waves have frequencies below the ion cyclotron frequency. The test particle calculations show that field-aligned electrons can be accelerated parallel to the magnetic field by such waves and that ions can be accelerated perpendicular to the magnetic field to form ion conics. Ions can be accelerated much more effectively when the wave frequency is half the ion gyrofrequency. The results are applicable to acceleration of ions and electrons in the low-Beta plasma of the auroral zone.

Temerin, M.↗
NASA NTRS1988

Effect of mid-altitude ion heating on ion outflow at polar latitudes

The effect of ion heating on polar ion outflow, when either the parallel or perpendicular (or both) ion temperatures at the exobase are elevated above values typical for the ionosphere, was investigated using a modified semikinetic steady-state model of Barakat and Schunk (1983) that allowed for anisotropic ion heating at the exobase and eliminated lower boundary potential jumps. In addition, the relative impact of the ion heating vs electron heating on the oxygen escape fluxes was investigated by examining the flux of O(+) ions for various combinations of electron and ion temperatures. It is demonstrated that the O(+) escape flux can be increased, to levels as high as were obtained by Barakat and Schunk (1983) with the electron temperatures of 10,000 K, by raising, instead, the ion temperatures (but to values considerably less than the 100,000 K observed by Moore et al., 1986).

Li, Peng↗
NASA NTRS1988

Ion-cyclotron wave heating of heavy ions in the equatorial magnetosphere - A numerical simulation theory

A 1-2/2 dimensional hybrid numerical simulation code is used to study the heating of cold H(+) ions and heavy ions by electromagnetic ion-cyclotron waves (ICWs) in the ring current region of the equatorial magnetosphere. Consideration is given to a plasma consisting of electrons, hot H(+) ions, and cold heavy ions in which the ICWs are driven by the temperature anisotropy of the hot protons. For large-amplitude ICWs, it is found that the cold H(+) ions are preferentially heated over the heavy ions although the cold H(+) ions are heated by a three-step process.

Chen, M. W.↗
NASA NTRS1993

The multi-ion, multi-event test of ion cyclotron resonance heating

The multi-ion, multi-event study of ion cyclotron resonance heating was funded to study ion energization through ion cyclotron resonance with low frequency broadband electromagnetic turbulence. The initial work on the ion cyclotron resonance heating (ICRH) of oxygen ions was presented in Crew et al. Crew and his co-authors developed a two-parameter representation of selected oxygen conic distributions and modeled the conic formation in terms of resonance heating. The first year seeks to extend the work of Crew and his co-authors by testing the applicability of the ICRH mechanism to helium ion conic distributions, using data obtained from the Energetic Ion Composition Spectrometer and the Plasma Wave Instrument on Dynamics Explorer 1.

Persoon, Ann M.↗
NASA NTRS1993

Multi-ion, multi-event test of ion cyclotron resonance heating

The multi-ion, multi-event study of ion cyclotron resonance heating has been funded to study ion energization through ion cyclotron resonance with low frequency broadband electromagnetic turbulence. The modeling algorithm for the ion cyclotron resonance heating (ICRH) of oxygen ions was presented in Crew et al. (1990). Crew and his co-authors developed a two-parameter representation of selected oxygen conic distributions and modelled the conic formation in terms of resonance heating. The first year of this study seeks to extend the work of Crew and his co-authors by testing the applicability of the ICRH mechanism to helium ion conic distributions, using data obtained from the Energetic Ion Composition Spectrometer and the Plasma Wave Instrument on Dynamics Explorer 1.

Persoon, Ann M.↗
NASA NTRS2007

Impact of Ring Current Ions on Electromagnetic Ion Cyclotron Wave Dispersion Relation

Effect of the ring current ions in the real part of electromagnetic ion Cyclotron wave dispersion relation is studied on global scale. Recent Cluster observations by Engebretson et al. showed that although the temperature anisotropy of is energetic (> 10 keV) ring current protons was high during the entire 22 November 2003 perigee pass, electromagnetic ion cyclotron waves were observed only in conjunction with intensification of the ion fluxes below 1 keV by over an order of magnitude. To study the effect of the ring current ions on the wave dispersive properties and the corresponding global wave redistribution, we use a self-consistent model of interacting ring current and electromagnetic ion cyclotron waves, and simulate the May 1998 storm. The main findings of our simulation can be summarized as follows: First, the plasma density enhancement in the night MLT sector during the main and recovery storm phases is mostly caused by injection of suprathermal plasma sheet H + (approximately < 1 keV), which dominate the thermal plasma density. Second, during the recovery storm phases, the ring current modification of the wave dispersion relation leads to a qualitative change of the wave patterns in the postmidnight-dawn sector for L > 4.75. This "new" wave activity is well organized by outward edges of dense suprathermal ring current spots, and the waves are not observed if the ring current ions are not included in the real part of dispersion relation. Third, the most intense wave-induced ring current precipitation is located in the night MLT sector and caused by modification of the wave dispersion relation. The strongest precipitating fluxes of about 8 X 10(exp 6)/ (cm(exp 2) - s X st) are found near L=5.75, MLT=2 during the early recovery phase on 4 May. Finally, the nightside precipitation is more intense than the dayside fluxes, even if there are less intense waves, because the convection field moves ring current ions into the loss cone on the nightside, but drives them out of the loss cone on the dayside. So convection and wave scattering reinforce each other in the nightside, but interfere in the dayside sector.

Khazanov, G. V.↗
NASA NTRS2010

The Biological Effectiveness of Silicon Ions is Significantly Higher than Iron Ions for the Induction of Chromosome Damage in Human Lymphocytes

Chromosome aberrations were measured in human peripheral blood lymphocytes after in vitro exposure to Si-28-ions with energies ranging from 90 to 600 MeV/u, or Fe-56-ions with energies ranging from 200 to 5,000 MeV/u. The LET of the various Fe beams in this study ranged from 145 to 440 keV/micron and the LET Si ions ranged from 48 to 158 keV/micron. Doses delivered were in the 10 to 200 cGy range. Dose response curves for chromosome exchanges in cells at first division after exposure, measured using fluorescence in situ hybridization (FISH) with whole chromosome probes, were fitted with linear or linear-quadratic functions. The relative biological effectiveness (RBE) was estimated from the initial slope of the dose response curve for chromosome damage with respect to gamma-rays. The estimates of RBE(sub max) values for total chromosome exchanges ranged from 4.4+/-0.4 to 31.5+/-2.6 for Fe ions, and 11.8+/-1.0 to 42.2+/-3.3 for Si ions. The highest RBE(sub max) value for Fe ions was obtained with the 600 Mev/u beam and 170 MeV/u beam produced the highest RBE(sub max) value for Si ions. For both ions the RBE(sub max) values increased with LET, reaching a maximum at about 180 keV/micron for Fe and about 100 keV/micron for Si, and decreased with further increase in LET.

George, Kerry↗
NASA NTRS2015

Initial Thrust Measurements of Marshall's Ion-ioN Thruster

Electronegative ion thrusters are a variation of tradition gridded ion thruster technology differentiated by the production and acceleration of both positive and negative ions. Benefits of electronegative ion thrusters include the elimination of lifetime-limiting cathodes from the thruster architecture and the ability to generate appreciable thrust from both charge species. Following the continued development of electronegative ion thruster technology as exhibited by the PEGASES (Plasma Propulsion with Electronegative GASES) thruster, direct thrust measurements are required to push interest in electronegative ion thruster technology forward. For this work, direct thrust measurements of the MINT (Marshall's Ion-ioN Thruster) will be taken on a hanging pendulum thrust stand for propellant mixtures of Sulfur Hexafluoride and Argon at volumetric flow rates of 5-25 sccm at radio frequency power levels of 100-600 watts at a radio frequency of 13.56 MHz. Acceleration grid operation is operated using a square waveform bias of +/-300 volts at a frequency of 25 kHz.

Schloeder, Natalie R.↗
NASA NTRS2018

Numerical Investigation of Ion Transport in the MOMA Ion Mass Spectrometer

The Mars Organic Molecule Analyzer (MOMA) is a miniature ion trap mass spectrometer designed for the upcoming ExoMars Rover mission. The spectrometer uses laser desorption to ionize a Martian soil sample within an instrument internal clean zone maintained at ambient Martian pressure. A high-speed aperture valve transiently opens to allow ionized constituents, along with the ambient gas, to enter a vacuum cavity containing a linear ion trap mass spectrometer. The ambient clean zone and the vacuum cavity are connected via a few centimeter long aperture valve ion guide tube. In this paper, we present results from a recently completed numerical investigation of ion transport from the ion source across the ion guide. Specifically, we focus on collisional coupling between ions and the neutral molecules flowing into the vacuum cavity. The simulation domain contains the ambient region, and we consider the variation in ion conductance with ambient pressure. We also analyze the impact of a fixed potential bias applied to the aperture valve. Simulations are performed with a two-dimensional axisymmetric PIC / DSMC code Starfish. Numerical results are compared to experimental data.

Breida, Lubos↗
NASA NTRS1980

Ion heating by strong electrostatic ion cyclotron turbulence

A theory of the ion heating due to electrostatic ion cyclotron (EIC) waves in the auroral zone is presented. Due to the slowly convecting nature of the EIC mode, quasi-linear plateau formation cannot stabilize the waves, and growth occurs until the nonlinear mechanisms of ion resonance broadening and electron trapping provide saturation. The large amplitude and coherent nature of the resulting wave imply that quasi-linear theory provides only a lower limit to the ion heating. An upper bound on the heating rate is derived using a time-average model of ion dynamics in the coherent waves. The effects of ion heating in the presence of the magnetic gradient force and parallel electric fields are considered, with the result that perpendicular energies over 100 eV are easily attainable from a 1 eV source plasma. Perpendicular heating in the absence of a parallel electric field yields conical ion distributions, which in the presence of an electric field become field-aligned beams.

Lysak, R. L.↗
NASA NTRS1983

Evolution of ion distributions across the nearly perpendicular bow shock - Specularly and non-specularly reflected-gyrating ions

Data from ISEE 1 and 2 spacecraft were used to study the evolution of the ion distributions in the perpendicular terrestrial bow shock. The plasma data were taken during passage of the spacecraft downstream of and through the shock. Solar wind ions had velocities ranging from Mach 2-12.4, and reflected ions featured a relative density of 1-3 percent of the solar wind density at Mach 2 to 15-25 percent at Mach 8-12. Computer simulations have indicated that the ions provide essential dissipation at the shock and gyrate about the magnetic field lines in the plasma rest frame at a speed twice that of the normal incident solar wind flow. The ion density decreases by up to two orders of magnitude at the forward end of the foot of the shock profile, suggesting that the ions are reflected by the shock specularly, and may enhance downstream ion thermalization.

Sckopke, N.↗
NASA NTRS1985

Simulations of heavy ion heating by electromagnetic ion cyclotron waves driven by proton temperature anisotropies

Heating of heavy ions by the electromagnetic ion cyclotron (EMIC) waves, which are driven by proton temperature anisotropies, is studied by means of hybrid particle simulations. Initially, relaxation of the temperature anisotropies in the proton distribution and isotropic heating of the heavy ions are observed (phase I), followed by substantial perpendicular heating of the heavy ions (phase II). The heavy ions are distinctly gyrophase modulated by the EMIC waves. The isotropic heating in phase I is due to magnetic trapping by the excited proton cyclotron waves. The perpendicular heating in phase II is attributed to cyclotron resonance with the EMIC waves, which becomes possible by means of the preceding heating in phase I. Saturation of the EMIC instability is instead attributed to magnetic trapping of the majority ions: protons. When the proton anisotropy is very large, frequency shift (decrease) of the proton cyclotron waves to less than 1/2 Ohm(p) is observed. The present mechanism is not only relevant to He(+) heating in the dayside equator of the magnetosphere, but it also predicts hot He2(+) ions behind the earth's bow shock.

Tanaka, M.↗