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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

The Effect of Anode Configuration on the Plasma Plume of a 25-A class Hollow Cathode

The effect of anode configuration on the plasma plume properties of a 25-A class hollow cathode are experimentally characterized. At a nominal operating condition of 20.8 A and 14.7 sccm of xenon, the cathode was operated with two axially-segmented anodes of different diameters at varying distance from the cathode. A fast, reciprocating motion stage was used to rapidly insert an ion saturation probe and an emissive probe on cathode centerline, and from those measurements, spatial profiles of plasma density, plasma potential, and electron temperature were calculated. For both anode diameters, maximum plasma potential and electron temperature increased with increasing anode distance. However, the 64-mm diameter anode was found to be significantly more sensitive to position, with measurements indicating that parameters changed continuously with position instead of an instantaneous mode change at a certain critical distance. The 254-mm diameter, in contrast, showed much more similar plasma property profiles across the range of anode positions tested and much smaller changes per unit increase in distance. These results are discussed in the context of assessing anode design for component-level cathode tests.

space propulsion↗
NASA NTRS1979

Plasma-sprayed dual density ceramic turbine seal system

Dual density, plasma sprayed ceramic coating systems were investigated for possible application as abradable turbine tip seal systems in small gas turbine engines. Abradability, erosion resistance, internal leakage, and microstructural characterization were investigated for polyester and cenosphere filled zirconium oxide composites. Results indicate the polyester system is more abradable but displays significantly less erosion resistance than the cenosphere system. It is also stated that the absence of significant blade tip damage during abradability testing of both systems suggests additional effort may result in a more nearly optimum balance of abradability and erosion resistance.

Clingman, D. L.↗
NASA NTRS1995

Analytical model for the density distribution in the Io plasma torus

An analytical model is developed for the diffusive equilibrium plasma density distribution in the Io plasma torus. The model has been employed successfully to follow the ray path of plasma waves in the multi-ion Jovian magnetosphere; it would also be valuable for other studies of the Io torus that require a smooth and continuous description of the plasma density and its gradients. Validity of the analytical treatment requires that the temperature of thermal electrons be much lower than the ion temperature and that superthermal electrons be much less abundant than the thermal electrons; these two conditions are satisfied in the warm outer region of the Io torus from L = 6 to L = 10. The analytical solutions agree well with exact numerical calculations for the most dense portion of the Io torus within 30 deg of the equator.

Mei, YI↗
NASA NTRS1976

Preliminary scaling laws for plasma current, ion kinetic temperature, and plasma number density in the NASA Lewis bumpy torus plasma

Parametric variation of independent variables which may affect the characteristics of bumpy torus plasma have identified those which have a significant effect on the plasma current, ion kinetic temperature, and plasma number density, and those which do not. Empirical power law correlations of the plasma current, and the ion kinetic temperature and number density were obtained as functions of potential applied to the midplane electrode rings, the background neutral gas pressure, and the magnetic field strength. Additional parameters studied included the type of gas, the polarity of the midplane electrode rings, the mode of plasma operation, and the method of measuring the plasma number density. No significant departures from the scaling laws appear to occur at the highest ion kinetic temperatures or number densities obtained to date.

Roth, J. R.↗
NASA NTRS1976

Preliminary scaling laws for plasma current, ion kinetic temperature, and plasma number density in the NASA Lewis Bumpy Torus plasma

Parametric variation of independent variables which may affect the characteristics of the NASA Lewis Bumpy Torus plasma have identified those which have a significant effect on the plasma current, ion kinetic temperature, and plasma number density, and those which do not. Empirical power-law correlations of the plasma current, and the ion kinetic temperature and number density were obtained as functions of the potential applied to the midplane electrode rings, the background neutral gas pressure, and the magnetic field strength. Additional parameters studied include the type of gas, the polarity of the midplane electrode rings (and hence the direction of the radial electric field), the mode of plasma operation, and the method of measuring the plasma number density. No significant departures from the scaling laws appear to occur at the highest ion kinetic temperatures or number densities obtained to date.

Roth, J. R.↗
DOE OSTI2026

Effect of Density Irregularities on Radio Frequency Wave Propagation in Ionospheric Plasmas

Density irregularities play a vital role in determining how radio frequency (RF) waves travel through plasmas. In the Earth’s ionosphere, these density irregularities also impact radio communication. In this study, we conduct a detailed numerical analysis of RF wave propagation in small-scale ionospheric density irregularities using the advanced Petra-M code. We focus specifically on high-frequency (HF) waves, ranging from 3 to 30 MHz, which are essential for military, amateur radio operators, and emergency communications. By introducing density structures, such as equatorial plasma bubbles derived from fluid simulations, we demonstrate that HF waves can scatter in multiple directions when they encounter these irregularities. Additionally, we observe significant mode conversion, where incoming electromagnetic waves transform into electrostatic modes within the density gradient layer. This shows that smaller density irregularities can greatly weaken signals or cause complete signal loss for receivers, emphasizing the need for increased awareness and innovative solutions in radio communication transmission.

Kim, Eun-Hwa [Princeton Plasma Physics Laboratory ↗
DOE OSTI2024

Toroidal Alfvén mode instability driven by plasma current in low-density Ohmic plasmas of the spherical tori

Due to intrinsically low magnetic fields, at low density the drift speed of the current-carrying electrons in spherical tokamaks can exceed fraction of the Alfvén speed sufficient for the excitation of the Alfvén gap modes. A particular case of the toroidal mode, observed during minor disruptions in Ohmic shots on SUNIST [Liu et al., Phys. Plasmas 23, 120706 (2016)], is considered in the present communication. Due to the negligible effect of the electron pressure gradient, the growth rate scales linearly with the drift speed, with slope inversely proportional to electron thermal velocity. In the absence of continuum damping, the threshold value of the drift speed for TAE excitation is independent of electron temperature.

Marchenko, V. S. (ORCID:0000000261480533)↗
NASA NTRS2004

Ground-Based Simulation of Low-Earth Orbit Plasma Conditions: Plasma Generation and Characterization

A 16-cm diameter plasma source operated on argon is described that is capable of producing a plasma environment that closely simulates the low Earth orbit (LEO) conditions experienced by satellites in the altitude range between 300 to 500 km. The plasma source uses a transverse-field magnetic filter, and has been successful in producing low electron temperature plasmas that contain streaming ion populations. Both of these characteristics are important because the plasma in LEO is relatively cold (e.g., Te approx. 0.1 eV) and the ram energy of the ions due to the motion of the satellite relative to the LEO plasma is high (e.g., 7,800 m/s which corresponds to approx. 5 eV for O+ ions). Plasma source operational conditions of flow rate and discharge power are presented that allow the electron temperature to be adjusted over a range from 0.14 to 0.4 eV. The expanding plasma flow field downstream of the source contains both low-energy, charge-exchange ions and streaming ions with energies that are adjustable over a range from 4 eV to 6 eV. At low flow rates and low facility pressures, the streaming ion component of the ion population comprises over 90% of the total plasma density. In the work described herein, a large area retarding potential analyzer was used to measure both electron and ion energy distribution functions in the low density, expanding plasma produced downstream of the plasma source. The benefits of using this type of plasma diagnostic tool in easily perturbed, low-density plasma are identified, and techniques are also discussed that can be used to perform real-time measurements of electron temperature. Finally, recommendations are made that may enable lower electron temperatures to be produced while simultaneously decreasing the plasma source flow rate below 1 to 2 sccm.

Williams, John D.↗
NASA NTRS1981

The generation mechanism for magnetosheath lion roars

The origin of lion roars, intense electromagnetic whistler-mode waves which occur throughout the magnetosheath, is investigated based on particle and field data obtained on the ISEE satellites. Analysis of the measured magnetic fields, plasma densities and plasma wave intensities during periods of lion roar emission reveals the lion roar bursts to be correlated with decreases in magnetic field intensity, while the plasma density exhibits a strong anticorrelation with the magnetic field variability during the bursts and the total plasma pressure remains essentially constant. Results indicate that the waves can originate by the cyclotron resonant instability with anisotropic magnetosheath electrons whenever the magnetic energy per particle falls to values comparable with the electron thermal energy. Variations in the magnetic energy appear to be associated with the hydromagnetic mirror instability which in turn is excited by a pressure anisotropy in the magnetosheath ion population.

Thorne, R. M.↗
DOE OSTI2025

Electron cyclotron emission measurements and modeling in optically semi-transparent plasma on the HSX stellarator

On the Helically Symmetric Experiment (HSX), electron cyclotron emission (ECE) at the second harmonic of the electron gyrofrequency is measured with a 16-channel heterodyne receiver. An ECE radiometer diagnostic has been calibrated with a broadband frequency source, allowing independent measurements of the electron temperature. The antenna pattern has a significant impact on the measurements, and modeling its effect is necessary to produce an ECE radiation temperature spectrum. Measurements, calibration results, and modeling of the ECE data are presented in this paper. At a relatively high plasma density, HSX plasmas are thermal, and the radiation temperature measured by the radiometer is in good agreement with the electron temperature measured by a Thomson scattering diagnostic. At a relatively low plasma density, the deviation from a Maxwellian distribution is large, and asymmetry in the radiation temperature profile is measured. These measurements are supported by single-pass and multi-pass ray tracing calculations of the electron cyclotron emission using the TRAVIS ray tracing code. Thermal and nonthermal electron populations exist in HSX, and the effect of these populations on the asymmetry in the measured emission is presented.

Cyclotron radiation↗
NASA NTRS1983

Comparison of spherical double probe electric field measurements with plasma bulk flows in plasmas having densities less than 1 cm-3

For a three-hour period in the magnetotail over which plasma density varied from less than 0.1 to about 1/cu cm, comparisons of ISEE-1 spherical double probe (dawn to dusk) electric field measurements and ISEE-2 plasma flows (converted to electric fields) show the zero lag cross correlation coefficient between 768 second averages of the two data sets to have been 0.93. A statistical relative uncertainty between pairs of points in the two data sets, estimated by only including counting statistics in the plasma measurement and time variations of the observed electric field over the measurement interval, is able to account for at least 75 percent of the deviations between the two data sets. In agreement with simple Langmuir probe theory, it has been found that the spacecraft potential measured over the three-hour interval by the double probe instrument varied as the log of the product of the plasma density and the square root of the electron temperature.

Mozer, F. S.↗
NASA NTRS1991

High resolution measurements of density structures in the Jovian plasma sheet

A recent effort to digitize the plasma density by using the low frequency cutoff of trapped continuum radiation in the vicinity of the Jovian plasma sheet has revealed the existence of sharply defined density structures in the plasma sheet. These structures typically have a plasma density which is relatively constant but of order 50 percent greater or less than in the surrounding plasma. At the boundaries of these structures, the transitions from low to high density occur on time scales of about ten seconds, which correspond to spatial dimensions on the order of a few ion Larmor radii. The structures themselves last for intervals from less than a minute to more than five minutes, corresponding to size scales from a fraction of a Jovian radius to more than a Jovian radius, depending of the velocity of the structure relative to the spacecraft. In view of the importance of near corotation plasma flows, these structures are likely to be limited in both the longitudinal and radial dimensions and, therefore, could represent flux tubes with greatly varying plasma content. These observations are presented as among the first to directly address the theoretically proposed interchange instability.

Ansher, J. A.↗
NASA NTRS1992

High resolution measurements of density structures in the Jovian plasma sheet

A recent effort to digitize the plasma density by using the low frequency cutoff of trapped continuum radiation in the vicinity of the Jovian plasma sheet has revealed the existence of sharply defined density structures in the plasma sheet. These structures typically have a plasma density which is relatively constant but of order 50 percent greater or less than in the surrounding plasma. At the boundaries of these structures, the transitions from low to high density occur on time scales of about ten seconds, which correspond to spatial dimensions on the order of a few ion Larmor radii. The structures themselves last for intervals from less than a minute to more than five minutes, corresponding to size scales from a fraction of a Jovian radius to more than a Jovian radius, depending on the velocity of the structure relative to the spacecraft. In view of the importance of near corotation plasma flows, these structures are likely to be limited in both the longitudinal and radial dimensions and, therefore, could represent flux tubes with greatly varying plasma content. These observations are presented as among the first to directly address the theoretically proposed interchange instability.

Ansher, J. A.↗
NASA NTRS1985

Density sensitive X-ray line ratios in the Be I, B I, and Ne I isoelectronic sequences

The intensities of X-ray transitions in highly charged ions in the Be I, B I, and Ne I isoelectronic sequences have been calculated as functions of electron density. The intensities of the transitions from the 2s(n)2p(m)3p configuration, relative to the intensities from the 2s(n)2p(m)3s and 2s(n)p(m)3d configurations, are strong functions of electron density in high-density plasmas. The density sensitivity occurs at electron densities between 10 to the 16th/cu cm (for Si ions) and 10 to the 22nd/cu cm (for Kr ions). Opacity is unimportant for plasma dimensions that are characteristic of dense laser-produced plasmas. These X-ray line ratios represent a promising new density diagnostic for high-density plasmas.

Feldman, U.↗
NASA NTRS1987

Density fluctuations induced by nonlinear Alfven waves

The generation of plasma density fluctuations by nonlinear Alfven waves is considered. Of particular interest is the modification to the 'static approximation', describing the relationship between wave intensity and plasma density, when the Alfven wave packet is rapidly steepening. The study concentrates on a forced wave equation for the plasma density, which yields the static approximation in a select approximnation. An analytic solution to this equation is obtained for a reasonable model of a steeping wave packet. The principal results are: (1) the static approximation should give a surprisingly robust description of the coupling between Alfven wave intensity and plasma density; (2) departures of the plasma density perturbation from the static approximation result are determined by a parameter that is a simple function of the wave amplitude and plasma beta; (3) for rapidly evolving wave packets, the position of maximum density change is displaced from that of maximum wave intensity, and (4) the magnitude of the density fluctuation will not be much different than the static approximation estimate.

Spangler, Steven R.↗
NASA NTRS1995

Capillary Pressure Balance with Repeated Head-Up Tilt in Humans

We measured blood density (BD). plasma density (PD) and hematocrit (Ht) during repeated 70 deg head-up tilt (HUT) consisting of a pre-drink period with two supine (P and P3) and two HUT (P2 and P4) phases of 45 min each. At the end of P4 test subjects (N=8) drank 10 ml/kg body weight of isotonic (290 mOsm/kg) sodium chloride (ISO) or hypotonic (<10 mOsm/kg) unsweetened us (HYP), or nothing (control: CON). The following periods continued the supine (P5, P7) / upright (P6) sequence. Mass density of plasma and whole blood using a "mechanical oscillation" technique were measured from ear lobe blood. Tilting altered (p<0.001) BD, PD, and Ht in all consecutive Phases. BD and PD were different (p<0.05) between CON. HYP. and ISO Phases 6 and 7. The computed mass density of fluid (FD). which moved between intra. and extravascular compartments. averaged 1008 g/l and was the same in any conditions. In the non-drink treatment (P3, P5, P7), supine PV steadly decreased when compared to P1. Tilt-induced PV shifts ranged from 9.7 to 16.7% when compared to PV du. ring the respective previous Phases. After drinking, PV increased above CON values at the end of Phase 7 by 12.9% with ISO, and by 6.6% with HYP. Progressive hemoconcentration. occurred during non-drink supine; isotonic saline ingestion increased supine PV a control level but did not stop or reverse the decrease of upright hemoconcentration. Upright density values were not slowed with repeated upright tilting; decreased fluid loss occurred in consecutive upright periods. Upright capillary pressure balance was achieved with diminished plasma volume loss during repeated tilting.

Hinghofer-Szalkay, H. G.↗