Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “PLASMA POTENTIAL”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Onset of normal and inverse homoclinic bifurcation in a double plasma system near a plasma fireball

Plasma fireballs are generated due to a localized discharge and appear as a luminous glow with a sharp boundary, which suggests the presence of a localized electric field such as electrical sheath or double layer structure. The present work reports the observation of normal and inverse homoclinic bifurcation phenomena in plasma oscillations that are excited in the presence of fireball in a double plasma device. The controlling parameters for these observations are the ratio of target to source chamber (n{sub T}/n{sub S}) densities and applied electrode voltage. Homoclinic bifurcation is noticed in the plasma potential fluctuations as the system evolves from narrow to long time period oscillations and vice versa with the change of control parameter. The dynamical transition in plasma fireball is demonstrated by spectral analysis, recurrence quantification analysis (RQA), and statistical measures, viz., skewness and kurtosis. The increasing trend of normalized variance reflects that enhancing n{sub T}/n{sub S} induces irregularity in plasma dynamics. The exponential growth of the time period is strongly indicative of homoclinic bifurcation in the system. The gradual decrease of skewness and increase of kurtosis with the increase of n{sub T}/n{sub S} also reflect growing complexity in the system. The visual change of recurrence plot and gradual enhancement of RQA variables DET, L{sub max}, and ENT reflects the bifurcation behavior in the dynamics. The combination of RQA and spectral analysis is a clear evidence that homoclinic bifurcation occurs due to the presence of plasma fireball with different density ratios. However, inverse bifurcation takes place due to the change of fireball voltage. Some of the features observed in the experiment are consistent with a model that describes the dynamics of ionization instabilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Power measurements and coupler optimization in inductive discharges

The power absorbed by the plasma is one of the key parameters which defines processes in any plasma source. This power, however, can be very different from the power at the rf power source output or the coupler terminals, which has been used in many publications to characterize the plasma. This article describes how to find the power absorbed by the plasma and the power lost in the coupler and matcher network for inductively coupled plasmas. In addition, several practical coupler configurations to reduce the coupler coil loss and minimize the rf plasma potential are discussed. Here, we propose an effective and simple method to achieve that by the coupler coil splitting and insertion of the resonating capacitor in the middle of the coil. Our experimental data demonstrate this approach having superior coupler efficiency and substantially lower rf plasma potential.

47 OTHER INSTRUMENTATION↗
NASA NTRS2003

Testing and Analysis of NEXT Ion Engine Discharge Cathode Assembly Wear

Experimental and analytical investigations were conducted to predict the wear of the discharge cathode keeper in the NASA Evolutionary Xenon Thruster. The ion current to the keeper was found to be highly dependent upon the beam current, and the average beam current density was nearly identical to that of the NSTAR thruster for comparable beam current density. The ion current distribution was highly peaked toward the keeper orifice. A deterministic wear assessment predicted keeper orifice erosion to the same diameter as the cathode tube after processing 375 kg of xenon. A rough estimate of discharge cathode assembly life limit due to sputtering indicated that the current design exceeds the qualification goal of 405 kg. Probabilistic wear analysis showed that the plasma potential and the sputter yield contributed most to the uncertainty in the wear assessment. It was recommended that fundamental experimental and modeling efforts focus on accurately describing the plasma potential and the sputtering yield.

Domonkos, Matthew T.↗
NASA NTRS2019

Near Field Probe Measurements in the Plume of a NEXT Ion Thruster

Due to the extremely long lifetime and ground testing complications, such as facility backpressure effects, modeling is a necessary tool for validating mission lifetime requirements. Both NASA's Glenn Research Center and Jet Propulsion Laboratory have developed lifetime validation models for the NEXT ion thruster. The largest uncertainties in these models are due to unknown plasma properties, such as plasma potential, that occur very near to the grid. Previous studies have made measurements in the mid to far field. Here, we make very near field measurements of the plasma properties in the ion beam within 0.1 thruster radii of the grid using various probes. Emissive and triple Langmuir probes are swept through the plume to spatially resolve plasma potential, electron temperature and ion density very near to the grid. The goal is to provide refined inputs to the lifetime modeling efforts and increase the accuracy of the models.

Arthur, Neil↗
NASA NTRS2019

Near Field Probe Measurements in the Plume of a NEXT Ion Thruster

Due to the extremely long lifetime and ground testing complications, such as facility backpressure effects, modeling is a necessary tool for validating mission lifetime requirements. Both NASA's Glenn Research Center and Jet Propulsion Laboratory have developed lifetime validation models for the NEXT ion thruster. The largest uncertaintiesin these models are due to unknown plasma properties, such as fields plasma potential, that occur very near to the grid. Previous studies have made measurements in the mid to far field. Here, we make very near field measurements of the plasma properties in the ion beam within 0.1 thruster radii of the grid using various probes. Emissive and triple Lang-muir probes are swept through the plume to spatially resolve plasma potential, electron temperature and ion density very near to the grid. The goal is to provide refined inputs to the lifetime modeling efforts and increase the accuracy of the models.

Arthur, Neil A.↗
DOE OSTI2013

Spherical ion oscillations in a positive polarity gridded inertial-electrostatic confinement device

A pulsed, positive polarity gridded inertial electrostatic confinement device has been investigated experimentally, using a differential emissive probe and potential traces as primary diagnostics. Large amplitude oscillations in the plasma current and plasma potential were observed within a microsecond of the discharge onset, which are indicative of coherent ion oscillations about a temporarily confined excess of recirculating electron space charge. The magnitude of the depth of the potential well in the established virtual cathode was determined using a differential emissive Langmuir probe, which correlated well to the potential well inferred from the ion oscillation frequency for both hydrogen and argon experiments. It was found that the timescale for ion oscillation dispersion is strongly dependent on the neutral gas density, and weakly dependent on the peak anode voltage. The cessation of the oscillations was found to be due to charge exchange processes converting ions to high velocity neutrals, causing the abrupt de-coherence of the oscillations through an avalanche dispersion in phase space.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Negative ion production and beam extraction processes in a large ion source (invited)

Recent research results on negative-ion-rich plasmas in a large negative ion source have been reviewed. Spatial density and flow distributions of negative hydrogen ions (H{sup −}) and positive hydrogen ions together with those of electrons are investigated with a 4-pin probe and a photodetachment (PD) signal of a Langmuir probe. The PD signal is converted to local H{sup −} density from signal calibration to a scanning cavity ring down PD measurement. Introduction of Cs changes the slope of plasma potential local distribution depending upon the plasma grid bias. A higher electron density H{sub 2} plasma locally shields the bias potential and behaves like a metallic free electron gas. On the other hand, the bias and extraction electric fields penetrate in a Cs-seeded electronegative plasma even when the electron density is similar. Electrons are transported by the penetrated electric fields from the driver region along and across the filter and electron deflection magnetic fields. Plasma ions exhibited a completely different response against the penetration of electric fields.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1988

Temporal evolution of collisionless sheaths

Experimental measurements have been performed to determine the temporal evolution of Langmuir sheaths near an electrode to which a negative step bias is applied in a collisionless argon plasma. The plasma was produced by a hot-filament discharge in a multidipole device. Plasma potential data were obtained using emissive probes with two different techniques: time-resolved sampling and time-averaged techniques. The sheath is found to initially form close to the electrode, to extend to a maximum separation, and to contract to a steady-state value. The time scale required to reach a steady state is close to the time scale of the presheath relaxation. Characteristics of sheaths in RF plasmas are also measured using a parallel-plate plasma capacitor. It is observed that the plasma potential profile has significant variation with frequency, even for frequencies as low as 1 kHz which are far below the ion plasma frequency (about 1 MHz).

Cho, M. H.↗
DOE OSTI2012

Improved analysis techniques for cylindrical and spherical double probes

A versatile double Langmuir probe technique has been developed by incorporating analytical fits to Laframboise's numerical results for ion current collection by biased electrodes of various sizes relative to the local electron Debye length. Application of these fits to the double probe circuit has produced a set of coupled equations that express the potential of each electrode relative to the plasma potential as well as the resulting probe current as a function of applied probe voltage. These equations can be readily solved via standard numerical techniques in order to determine electron temperature and plasma density from probe current and voltage measurements. Because this method self-consistently accounts for the effects of sheath expansion, it can be readily applied to plasmas with a wide range of densities and low ion temperature (T{sub i}/T{sub e} Much-Less-Than 1) without requiring probe dimensions to be asymptotically large or small with respect to the electron Debye length. The presented approach has been successfully applied to experimental measurements obtained in the plume of a low-power Hall thruster, which produced a quasineutral, flowing xenon plasma during operation at 200 W on xenon. The measured plasma densities and electron temperatures were in the range of 1 Multiplication-Sign 10{sup 12}-1 Multiplication-Sign 10{sup 17} m{sup -3} and 0.5-5.0 eV, respectively. The estimated measurement uncertainty is +6%/-34% in density and +/-30% in electron temperature.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1992

A first-principles model for orificed hollow cathode operation

A theoretical model describing orificed hollow cathode discharge is presented. The approach adopted is based on a purely analytical formulation founded on first principles. The present model predicts the emission surface temperature and plasma properties such as electron temperature, number densities and plasma potential. In general, good agreements between theory and experiment are obtained. Comparison of the results with the available related experimental data shows a maximum difference of 10 percent in emission surface temperature, 20 percent in electron temperature and 35 percent in plasma potential. In case of the variation of the electron number density with the discharge current a maximum discrepancy of 36 percent is obtained. However, in the case of the variation with the cathode internal pressure, the predicted electron number density is higher than the experimental data by a maximum factor of 2.

Salhi, A.↗
DOE OSTI2015

PLASMA DIAGNOSTIC POTENTIAL OF 2p4f IN N{sup +}—ACCURATE WAVELENGTHS AND OSCILLATOR STRENGTHS

Radiative emission lines from nitrogen and its ions are often observed in nebula spectra, where the N{sup 2+} abundance can be inferred from lines of the 2p4f configuration. In addition, intensity ratios between lines of the 2p3p-2p3s and 2p4f-2p3d transition arrays can serve as temperature diagnostics. To aid abundance determinations and plasma diagnostics, wavelengths and oscillator strengths were calculated with high precision for electric dipole (E1) transitions from levels in the 2p4f configuration of N{sup +}. Electron correlation and relativistic effects, including the Breit interaction, were systematically taken into account within the framework of the multiconfiguration Dirac-Hartree-Fock method. Except for the 2p4f-2p4d transitions with quite large wavelengths and the two-electron-one-photon 2p4f-2s2p {sup 3} transitions, the uncertainties of the present calculations were controlled to within 3% and 5% for wavelengths and oscillator strengths, respectively. We also compared our results with other theoretical and experimental values when available. Discrepancies were found between our calculations and previous calculations due to the neglect of relativistic effects in the latter.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Plasma/Wall interaction of an insulated material by laser-induced fluorescence diagnostic

We present Argon Ion Velocity Distribution Function (IVDF) in the vicinity of an insulated BNSiO{sub 2} ceramic and a glass plate in a non magnetized plasma by laser-induced fluorescence diagnostic. Results show the rather surprising self-consistent formation of a positive or inverse sheath in the two cases. The positive plasma potential repels ions from the insulated wall and is not explained by any sheath theory. Electron secondary emission of the ceramic can be a good candidate to explain these results. (authors)

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Effects of wall electrodes on Hall effect thruster plasma

This paper investigates the physical mechanisms that cause beneficial and detrimental performance effect observed to date in Hall effect thrusters with wall electrodes. It is determined that the wall electrode sheath can reduce ion losses to the wall if positioned near the anode (outside the dense region of the plasma) such that an ion-repelling sheath is able to form. The ability of the wall electrode to form an ion-repelling sheath is inversely proportional to the current drawn—if the wall electrode becomes the dominant sink for the thruster discharge current, increases in wall electrode bias result in increased local plasma potential rather than an ion-repelling sheath. A single-fluid electron flow model gives results that mimic the observed potential structures and the current-sharing fractions between the anode and wall electrodes, showing that potential gradients in the presheath and bulk plasma come at the expense of current draw to the wall electrodes. Secondary electron emission from the wall electrodes (or lack thereof) is inferred to have a larger effect if the electrodes are positioned near the exit plane than if positioned near the anode, due to the difference in energy deposition from the plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Cross-field electron transport induced by a rotating spoke in a cylindrical Hall thruster

Rotating spoke phenomena have been observed in a variety of Hall thruster and other E x B devices. It has been suggested that the spoke may be associated with the enhancement of the electron cross-field transport. In this paper, the current conducted across the magnetic field via a rotating spoke has been directly measured for the first time in the E x B discharge of a cylindrical Hall thruster. The spoke current was measured using a segmented anode. Synchronized measurements with a high speed camera and a four-segment anode allow observation of the current as a function of time and azimuthal position. Upwards of 50% of the total current is conducted through the spoke, which occupies a quarter of the Hall thruster channel area. To determine the transport mechanism, emissive and Langmuir probes were installed to measure fluctuating plasma potential, electron density, and temperature. A perturbed, azimuthal electric field and density are observed to oscillate in-phase with the rotating spoke. The resulting drift current is found to enhance electron transport with a magnitude equal to the spoke current to within margins of error.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Scanning ion sensitive probe for plasma profile measurements in the boundary of the Alcator C-Mod tokamak

A new Ion Sensitive Probe head has been created for the outer-midplane scanning probe system on the Alcator C-Mod tokamak. The new probe head contains three elements: an ion sensitive probe to measure ion temperature and plasma potential, a Langmuir probe to measure electron temperature, density, and floating potential, and a second Langmuir probe to measure ion saturation current and the density fluctuations arising from ''blob'' events. The ion sensitive probe current is normalized to this measurement to reduced deleterious effects of the strong fluctuations. Design of the high heat flux probe (>100 MW/m{sup 2}) and initial results are presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Enhanced transportation of energetic electrons in dual-frequency atmospheric microplasmas

A comparative study of electron kinetics between single-frequency (SF) microplasmas and their equivalent dual-frequency (DF) microplasmas with matching effective frequencies in atmospheric-pressure helium discharges was performed using particle-in-cell simulation with a Monte Carlo collision. The effective-frequency concept helps in analyzing DF microplasmas in a fashion similar to SF microplasmas with effective parameters. In this study, the plasma characteristics such as the plasma potential, density, and electron energy probability functions of the SF microplasma and its DF counterpart were almost the same. However, the oscillating sheath edge was pushed further into the electrode for a substantial fraction of the time and the sheath width decreased in DF microplasmas. As a result, the transportation of the energetic electrons ({epsilon} > 4 eV) usable for tailoring the surface chemistry in atmospheric microplasmas is enhanced in DF microplasmas as compared to SF microplasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Single discharge of the matrix source of negative hydrogen ions: Influence of the neutral particle dynamics

The study presents two-dimensional (2D) fluid-plasma-model description of a planar-coil inductively-driven discharge, considered as a single element of a matrix source of volume-produced negative hydrogen ions. Whereas the models developed up to now have been directed towards description of the charged particle behavior in the discharge, including that of the negative ions, this model stresses on the role of the neutral particle dynamics and of the surface processes in the formation of the discharge structure. The latter is discussed based on comparison of results obtained for discharges in a flowing gas and at a constant gas pressure as well as for different values of the coefficient of atom recombination on the walls. The conclusions are that the main plasma parameters – electron density and temperature and plasma potential – determining the gas discharge regime stay stable, regardless of changes in the redistribution of the densities of the neutral particles and of the positive ions. With regards to the volume production of the ions, which requires high density of (vibrationally excited) molecules, the impact on the degree of dissociation of the coefficient of atom recombination on the wall is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1984

Experimental observations of self-similar plasma expansion

The present investigation is concerned with measurements of the plasma potential profile of an expanding plasma, taking into account the demonstration of the self-similar behavior of such a plasma. The obtained experimental results are consistent with self-similar solutions reported by Crow et al. (1975). It is found that the quasi-neutrality condition breaks down early during the experiment. A consideration of the effect of charge separation is, therefore, required. Attention is given to the evolution of the potential profiles of the expanding plasma as a function of time, the accelerated ion fronts, and the sheath formation of the expanding plasma into a floating boundary.

Chan, C.↗