Plasma Electric Field Research Articles

Equatorial spread-F (ESF) and vertical winds

The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hierarchy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric field distributions perpendicular to the Earth’s magnetic field. Large scale irregularities are generated by a primary plasma instability that develops in electric fields and plasma densities. Other secondary instabilities then develop and generate irregularities at several scale sizes that often produce a plasma ‘hole’ or ‘bubble’ that rises up with high E× B velocities. The ESF/plasma bubble phenomenon has been studied extensively with experimental techniques and modeling, which revealed important features. In the bottom side F-layer, near sunset, when the vertical density gradient steepens as the layer is supported by the horizontal (North–South) Earth’s magnetic field lines against the omnipresent Earth’s gravitational acceleration ( g), the plasma conditions can give rise to Rayleigh–Taylor (RT) type instability. But the observed day to day variability of the ESF occurrence suggested that other agencies may also be involved in generating the instability. Sekar and Raghavarao (1987) with linear theory, and Raghavarao, Sekar and Suhasini (1992), with non-linear numerical modeling, suggested that vertical downward (upward) winds in the ambient gas have the potential to cause (inhibit) the ESF/bubble phenomenon. The presence of downward winds near the equator was reported earlier. In this paper, we show evidence for the presence of downward winds collocated with irregularities in electric fields and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite. The observations reported here are also consistent with the notion that the build-up of the equatorial ionization anomaly (EIA) prior to local sunset is important for the ESF instability.

Measurements of flows in the DIII-D divertor by Mach probes

First measurements of the Mach number of the background plasma in the DIII-D divertor are presented in conjunction with temperature T e and density n e using a fast scanning probe array. To validate the probe measurements, we compared our T e, n e and J sat data to Thomson scattering data and found good overall agreement in attached discharges and some discrepancy for T e and n e in detached discharges. The discrepancy is mostly due to the effect of large fluctuations present during detached plasmas on the probe characteristic; the particle flux is accurately measured in every case. A composite 2-D map of measured flows is presented for an ELMing H-mode discharge and we focus on some of the details. We have also documented the temperature, density and Mach number in the private flux region of the divertor and the vicinity of the X-point, which are important transition regions that have been little studied or modeled. Background parallel plasma flows and electric fields in the divertor region show a complex structure.

Measurement of electric fields in a plasma with the aid of the coherent four-wave mixing polarisation technique

A method for the measurement of a homogeneous static electric field in an isotropic homogeneous plasma by the coherent four-wave mixing (FWM) techniques is proposed and theoretically substantiated. The influence of plasma microfields on the intensity and polarisation of the FWM signal is investigated.

The motional Stark effect: Overview and future development (invited)

As an atomic beam propagates across a magnetic field, an electric field is generated, E=V×B. This results in a spectral splitting and polarization of the emitted radiation, known as the Stark effect. The direction of polarization of the emitted radiation provides a measure of the local magnetic field direction or pitch angle. In addition to the electric field generated by the neutral beam, intrinsic electric fields present in the plasma affect the direction of polarization. This has been exploited to measure both the plasma electric field and the magnetic field. Future development of the motional Stark effect utilizing laser induced fluorescence to access much lower magnetic fields will be discussed.

Comprehensive modelling network for dc glow discharges in argon

A set of three-dimensional models has been developed for dc glow discharges in argon. A combination of Monte Carlo, fluid and collisional-radiative models is used to describe the various species present in the glow discharge plasma, i.e. argon atoms in the ground state and in various excited levels, fast argon atoms, argon ions, electrons, sputtered copper atoms and the corresponding copper ions, both in the ground state and in various excited levels. Typical results of the models are current-voltage-pressure characteristics, the densities, energies and fluxes of the various plasma species, the potential and electric field distribution, information about collision processes in the plasma and about sputtering at the cathode, optical emission intensities etc. These results are compared with experimental data as much as possible, in order to check the validity of the models. The satisfactory agreement shows that the models present already a realistic picture of the glow discharge.

Nonself-sustained arc discharge in anode material vapors

The basic characteristics of the nonself-sustained arc discharge in a vapor of the anode material are studied. The influence of thermoemission parameters of the cathode on volt-ampere characteristics of the discharge is described. It is established that in a free mode of the discharge cathode operation, when the discharge current I/sub D/ is lower than the current of the thermoelectron emission from the discharge cathode I/sub C/, the deposition rate of the films q is directly proportional to the discharge current I/sub D/. In the compelled mode of the cathode operation, when I/sub D/>I/sub C/, q/spl sim/W/sub D//sup 2/, where W/sub D/=I/sub D/U/sub D/ with U/sub D/ being the discharge voltage. It is shown that the magnetic field increases the plasma density and changes the density profile from n(x)-1/x/sup 2/ to n(x)-1/x with x being a distance along the flow. The motion of created plasma flow is shown to have a noncollisional character with constant electron temperature of 5-7 eV along the flow. The values of plasma potential and electric field in the flow are determined; the values of cathodic and anodic potential drops in the discharge are evaluated. The angular distributions of ion and neutral fluxes in the created plasma flow are described. It is shown that the plasma flows parameters depend substantially on the working material. With use of crossed electric and magnetic fields, the flow ionization coefficient was enhanced up to 85% for the discharge in Ti vapors, and 35% for the discharge in Cu vapors.

Rocket measurements of high‐altitude spread F irregularities at the magnetic dip equator

We present the first rocket measurements of equatorial spread F irregularities at altitudes significantly above 600 km, in what is believed to be the inertially dominated regime of spread F. On October 14, 1994, at 1955 LT (2255 UT) a Black Brant X rocket instrumented to measure plasma density and electric fields was launched from Alcântara, Brazil (2° 18.75'S, 44°22.3'W, dip angle 1.5° at 300 km altitude), into active topside spread F plumes. On upleg the payload remained continuously in these plumes up to an altitude of 822 km. Power spectra of electron density fluctuations show characteristic dual‐power law behavior with spectral indices of −1.7 at frequencies below 60 Hz and −5 at frequencies above 60 Hz. Shocklike coherent structures characterized the density waveforms. The electric field fluctuations had spectral indices ranging from −3 to −4. We observe no evidence of altitude variation in the spectral indices or in the density fluctuation waveforms. Over the entire altitude range of the observations, 350–822 km, the data remain in agreement with previous measurements of collisional spread F below 600 km.

98/04117 Flame aerosol synthesis of ceramic powders

Under typical discharge conditions, atoms or molecules excited by electrons have a unique velocity component along the plasma electric field direction. This component results in Doppler shifts that can be observed and measured by a high resolution Fourier transform spectrometer. We show theoretically that if the upper state of a given transition suffers radiation trapping, its Doppler shift will be reduced by a factor dependent upon the average number of trapping cycles a photon of that frequency experiences. We further show how the diminution of the Doppler shift can be related to the volume and the density of the trapping species. These results can be used to measure the unknown concentration of a trapping species provided a “standard” is available. A typical application of this technique would be the measurement of the concentration of a metastable discharge species, e.g., He, using the trapping of its known ground state density as a standard.

Determination of concentrations via the diminution of Doppler shifts by radiation trapping

Under typical discharge conditions, atoms or molecules excited by electrons have a unique velocity component along the plasma electric field direction. This component results in Doppler shifts that can be observed and measured by a high resolution Fourier transform spectrometer. We show theoretically that if the upper state of a given transition suffers radiation trapping, its Doppler shift will be reduced by a factor dependent upon the average number of trapping cycles a photon of that frequency experiences. We further show how the diminution of the Doppler shift can be related to the volume and the density of the trapping species. These results can be used to measure the unknown concentration of a trapping species provided a “standard” is available. A typical application of this technique would be the measurement of the concentration of a metastable discharge species, e.g., He, using the trapping of its known ground state density as a standard.

Plasma Electric Field Measurements as a Diagnostic of Neutral Sheets in Prominences

AbstractMeasurements of plasma electric fields offer, in principle, a direct test for the presence of neutral sheets in prominences. Macroscopic electric field intensities of order 1–10 V cm−1 are an essential element of MHD models of prominences containing neutral sheets. These fields should be detectable with our electrograph techniques using the high Paschen-series lines in the NIR, and the 15–9 and 16–9 transitions of H I around 10.5 μ. We discuss the upper limits of 1 V cm−1 we have achieved so far, and their implication for our ability to distinguish prominence models of the ideal-MHD type (e.g., Kippenhahn-Schluter 1957), from those fundamentally different models (e.g., Kuperus and Tandberg-Hanssen 1967, Martens and Kuin 1989) in which neutral sheets play a central role.

Modeling of argon direct current glow discharges and comparison with experiment: how good is the agreement?

A set of three-dimensional models was developed for the different plasma species in a glow discharge, and these models were combined, to obtain an overall picture of a direct current glow discharge in argon. Typical results of these models include the electrical current flowing through the discharge as a function of pressure and voltage, the density profiles and flux energy distributions of the plasma species, the potential and electric field distributions in the plasma, information on collision processes, the crater profiles due to sputtering at the cathode and optical emission spectra due to radiative decay of excited atoms. The calculated data were compared with experimental observations to check the validity of the models, and the reliability of both modeling and experimental results is discussed.

Model for noncollisional heating in inductively coupled plasma processing sources

Low pressure (<10 mTorr) inductively coupled plasma sources are being developed for etching and deposition of semiconductors and metals. In models for these devices, plasma dynamics are typically coupled to the electromagnetic fields through Ohm’s law, which implies that collisional heating is the dominant power transfer mechanism. In this article, we describe an algorithm to couple plasma dynamics to electromagnetic field propagation which self-consistently includes noncollisional heating effects as well. The algorithm makes use of kinetic information available from an electron Monte Carlo simulation to compute plasma currents that are then used in computation of the electromagnetic field. Results for plasma density and electric field amplitude are presented as a function of power and pressure, and are compared to results from models that consider only collisional heating. We find that noncollisional heating effects are important at pressures of less than 10–20 mTorr, a range that depends both on gas mixture and geometry. Noncollisional heating effects allow the wave to couple more efficiently to the plasma. As a result, the electric field amplitude required to deposit a given amount of power in the plasma is smaller than that needed when only collisional heating is considered. For a constant power deposition, this generally leads to lower plasma densities.

The effect of Er on motional-Stark effect measurements of q, a new technique for measuring Er, and a test of the neoclassical Er

Previous analysis of motional-Stark Effect (MSE) data to measure the q-profile ignored contributions from the plasma electric field. The MSE measurements are shown to be sensitive to the electric field and require significant corrections for plasmas with large rotation velocities or pressure gradients. MSE measurements from rotating plasmas on the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 2, 2176 (1975)] confirm the significance of these corrections and verify their magnitude. Several attractive configurations are considered for future MSE-based diagnostics for measuring the plasma radial electric field. MSE data from TFTR are analyzed to determine the change in the radial electric field between two plasmas. The measured electric field quantitatively agrees with the predictions of neoclassical theory. These results confirm the utility of a MSE electric field measurement.

Computer simulation of argon-nitrogen and argon-oxygen inductively coupled plasmas

The properties of mixed-gas Ar-N 2 and Ar-O 2 inductively coupled plasmas (ICPs) are obtained by computer simulation. The results from the simulations are compared with existing experimental data. The reasons for the deviations are discussed, and inferences are drawn to connect the simulated results to recommendations for practical analytical measurements using ICP atomic emission and ICP mass spectrometries. The effect of the concentration of N 2 and O 2 in the outer gas flow (0%–100%) and injector gas flow (0%–20%) and the influence of the active power (800–1500 W) on the distribution of the plasma temperatures, electric and magnetic fields and tangential velocity are investigated. In general, mixed-gas plasmas move closer to the state of local thermodynamic equilibrium (LTE) as the concentrations of the molecular gases (N 2 and O 2) in the outer gas flow are increased. In contrast with the experimental results, the LTE model predicts that mixed-gas plasmas have maximum temperatures (9900–10100 K) comparable with Ar ICPs (10 600 K). The predicted temperature of the mixed-gas plasma is reduced as the concentration of the molecular gas is increased. The temperatures of Ar-O 2 ICPs are estimated to be higher than those of Ar-N 2 plasmas, except when the outer gas flow contains 100% N 2 or O 2.

Electrodynamic coupling effects in the thermosphere/ionosphere system

Thermospheric winds have a strong influence on the dynamics and electrodynamics of the ionosphere, and in return, ionospheric electrodynamic processes feed back onto the dynamics of the thermosphere. Observations of geomagnetic variations and ionospheric electric fields can provide valuable information about the distribution and variability of upper atmospheric winds systems, including atmospheric tides and planetary waves that propagate into the ionosphere from the middle atmosphere. The observations can lead to inferences about the generation and propagation conditions of those global waves in the lower and middle atmosphere, and hence provide information concerning changes in the state of the global atmosphere. Realistic simulation modeling of ionospheric electrodynamic effects must take into account the interactions between neutral and plasma dynamics, as is done in the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model. At low latitudes, the mutual coupling among neutral dynamics, ionospheric plasma dynamics, electrical conductivities, electric fields, and currents strongly affects the behavior of the thermosphere and ionosphere. At high latitudes, thermospheric and ionospheric phenomena are also strongly coupled to magnetospheric electrodynamics.

Use of fluorescence spectroscopy of CS for measurements of electric fields in a discharge plasma

It is demonstrated that the use of LIF spectroscopy of CS molecules provides sensitive measurements of plasma electric fields with high spatial resolution. The experiment was performed in a helium glow discharge which was produced in a discharge vessel applying a voltage to two gold-plated electrodes (60 mm in diameter, 40 mm spacing). The CS molecule admixture was generated by a rf discharge in a gas flow system outside the discharge vessel. The plasma parameters in the negative glow discharge were obtained by probe measurements. The electron density was about and the electron temperature was 0.2 eV on the discharge axis for , and helium gas pressure . The measured electric field strength was in the range of and its spatial distribution in the cathode fall region is in good agreement with cathode fall theory.

Calculated profiles of H I lines of interest for solar plasma electric field measurements

We present calculated Stark-polarized line profiles for a number of H i lines observed in the visible and infrared emission spectrum of solar prominences and other limb activity. For use in measurements of possible electric fields in these structures, we also calculate curves giving the difference in line width between the 1/2 (I ± Q) profiles as a function of electric-field intensity. Our calculations take into account magnetic fields in these structures, and incorporate typical observed values of Doppler broadening. These calculations explicitly consider the H i fine structure neglected in previous work, and thus are more accurate in the range of low to intermediate electric-field intensity likely to be encountered in solar plasmas (E < 103 V cm−1). Our results enable us to compare behavior when E and B are parallel, or perpendicular. We draw particular attention to the high electric-field sensitivity of the transitions between high levels such as 12–8 and 15–9 in H i, observed in prominences at wavelengths around 11μ. Their sensitivity is roughly an order of magnitude larger than that of the high Paschen-series lines used in solar plasma electric field studies so far.

VLF waves in the magnetosphere

The study of VLF waves at ground based stations is an important source of information on particles trapped in the magnetosphere. By various techniques it is also possible to measure plasma densities, electric fields and monitor energetic particle injection. By studying the propagation of waves beneath the ionosphere it is possible to study particle precipitation from the magnetosphere. In this paper we summarise some of the techniques and results obtained from the study of VLF waves at the South African research station in Antarctica.

Formation of oscillating perpendicular electric field by AC limiter biasing on HYBTOK-II tokamak

It will be necessary to establish the divertor that will withstand the spatially peaked thermal loads for future long pulse or steady state fusion reactors. In this research, formation of the oscillating electric field perpendicular to the magnetic field in a tokamak scrape-offlayer and its application to the broadening of the divertor heat load are discussed. An AC biasing voltage between 5 to 100 kHz is applied to poloidal limiters and time responses of the plasma potential and electric field in the edge region is studied. The oscillating electric field both in the radial and poloidal directions is excited by AC limiter biasing and significant penetration of the electric field into a core region was observed. The radial excursion of the charged particle due to the oscillating E p × B t drift in the radial direction is estimated to be as large as the SOL width.

A high frequency probe for absolute measurements of electric fields in plasmas

At high frequencies (1 GHz), it is difficult to maintain a sufficiently high probe impedance to allow measurements of voltage differences using a double probe. A new type of double probe with terminated coaxial cables (50 Omega ) is proposed and tested. The low impedance implies that the two probe wires are essentially short-circuited and only the probe current is measured. The relation between the displacement current due to surface charges and the electric field is derived and tested in a known field. The current measured in a plasma can be identified with the displacement current when the current due to plasma electrons is damped by an appropriate DC bias. Perturbations of the plasma by the probe field can also be checked by varying the DC potential of the probe.