Variation Of Ion Density Research Articles

EUVS: An instrument to be flown on the GOES spacecraft

Changes in Solar Extreme Ultra Violet (EUV) radiation produce some of the largest variations in neutral and ion densities of the upper atmosphere and ionosphere. Because EUV observations are difficult and often not continuous, proxies are typically used to represent the solar EUV flux. These proxies are becoming a significant source of error in the specification of the upper atmosphere. In order to improve our ability to track and model the neutral and ion densities between 80 and 600 km altitude, it is now critical to measure the actual solar EUV flux. We will be flying EUV Sensors (EUVS) on the Geostationary Operational Environmental Satellites (GOES), which are part of the United States National Oceanic and Atmospheric Administration (NOAA) satellite fleet. In this paper we will describe the GOES EUVS instrument.

Core ion flux bursts within solitary kinetic Alfvén waves

The Freja cold plasma analyzer (CPA) makes high time resolution measurements of the angle‐ and energy‐integrated core ion velocity distribution (< 20 eV) with spatial resolutions of ∼10 m. In a preliminary study, Knudsen et al. [1994] reported that in the most intense core ion heating regions, integral ion flux becomes bursty on timescales of tens of milliseconds, implying spatial scales of hundreds of meters. The present study demonstrates that these flux bursts are associated with solitary kinetic Alfvén waves (SKAW), and we show examples of a one‐to‐one correspondence between SKAW and ion flux bursts. In principle, the measured flux bursts can result from variations in ion temperature, drift, density, composition, or changes in the functional form of the velocity distribution (e.g., Maxwellian versus conical), or some combination thereof. By modeling the dependence of integral flux on various combinations of these parameters, we calculate the amount of variation needed to explain ion flux bursts, which can have amplitudes in excess of 109 cm−2s−1. This value is comparable in magnitude to the ram flux of cold ions in a 103 cm−3 plasma and is also comparable to the peak ion flux in ion outflow regions measured on previous spacecraft missions. In most cases, flux bursts can be explained either by localized heating to several eV or by bulk drifts of the order of 1–2 km/s; however, it is not possible to distinguish between these two possibilities from the CPA data alone. Plasma wave data show that SKAW are associated with broadband ELF waves extending from dc up to and beyond the proton cyclotron frequency ƒH+. Wave power above ƒH+ and below the oxygen cyclotron frequency ƒO+ is localized within SKAW and could in principle lead to localized ion heating. The region of the spectrum between ƒH+ and ƒO+ appears less localized and could produce a more uniform background of heated ions, depending on the relative contributions to total heating of the different spectral regimes.

Time-resolved study of the negative ion density in a volume H− ion source by photodetachment

Time-resolved measurements of the negative ion density and of other plasma parameters in a pure hydrogen Tandem source have been effected using the laser photodetachment technique. The study was made for hydrogen filling pressures in the range 7–11 mTorr and an arc current of 120 A. We found a strong time variation in negative ion density was present during the discharge pulse (0.5 s). The maximum negative ion density was observed at the beginning of the pulse, at the lowest filling pressure studied, followed by a rapid decrease. An improvement of the current distribution during the pulse was obtained by increasing the filling pressure. The electron density is highest at 7 mTorr and stays almost constant during the pulse. The increase in filling pressure leads to electron density values lower by a factor of 4 which are constant during the discharge pulse. The highest relative negative ion density during the pulse is obtained at 9 mTorr. The time variation of the negative ion density does not follow that of the electron density at the same discharge conditions. This indicates that the negative ion density is affected not only by the electron density, but also by the neutral particle density, as assumed by the mechanism of volume production of H− ions in a plasma. We attribute the time variation of the negative ion density during the discharge pulse to the ion pumping of hydrogen.

Effects of plasma parameters on viscosity

A numerical code is developed to study the effect of ion density and temperature (as well as ion pressure and temperature gradients) on the amplitude of the local maxima of the poloidal viscosity with respect to the poloidal Mach number using the model developed by Shaing (1993). The sum of the poloidal viscosity and ion-neutral collisions is determined from the dependence of the plasma radial current on the radial electric field in a biased electrode experiment. The experimental results in the interchangeable module stellarator agree qualitatively with the predictions of the numerical calculation regarding the effects of variation of ion density and temperature on viscosity.

Global ionosphere‐polar wind system during changing magnetic activity

A time‐dependent, three‐dimensional, multi‐ion model of the global ionosphere‐polar wind system was used to study the system's response to an idealized geomagnetic storm for different seasonal and solar cycle conditions. The model covered the altitude range from 90 to 9000 km for latitudes greater than 50° magnetic in the northern hemisphere. The geomagnetic storm contained a 1‐hour growth phase, a 1‐hour main phase, and a 4‐hour decay phase. Four storm simulations were conducted, corresponding to winter and summer solstices at both solar maximum and minimum. The simulations indicated the following: (1) O+ upflows typically occur in the cusp and auroral zone at all local times, and downflows occur in the polar cap. However, during increasing magnetic activity, O+ upflows can occur in the polar cap, (2) The O+ upflows are typically the strongest where both Te and Ti are elevated, which generally occurs in the cusp at the location of the dayside convection throat, (3) The upward H+ and O+ velocities increase with Te, which results in both seasonal and day‐night asymmetries in the ion velocities, (4) During “increasing” magnetic activity, O+ is the dominant ion at all altitudes throughout the polar region, (5) For solar minimum winter, there is an H+ “blowout” throughout the polar region shortly after the storm commences (negative storm effect), and then the H+ density slowly recovers. The O+ behavior is opposite to this. There is an increase in the O+ density above 1000 km during the storm's peak (positive storm effect), and then it decreases as the storm subsides, and (6) For solar maximum summer, the O+ and H+ temporal morphologies are in phase; but the ion density variations at high altitudes are opposite to those at low altitudes. During the storm's peak, the H+ and O+ densities increase at high altitudes (positive storm effect) and decrease at low altitudes (negative storm effect).

The design and application of electron cyclotron resonance discharges

During the past ten years electron cyclotron resonance (ECR) plasma-processing technology has matured into a diverse assortment of ECR plasma reactor and plasma source design concepts and has been extensively applied to numerous low-pressure plasma processing applications. This paper reviews the substantial progress made in the design and application of ECR plasma technology in recent years. Five representative ECR reactor/source designs from large-area 450-cm/sup 2/ discharges to compact plasma sources inserted into molecular-beam epitaxy (MBE) machines are described in detail. The performance of these ECR devices is evaluated by computing performance figures of merit from the available experimental data. These calculations are then compared with the behavior as predicted from a global model of the discharge. This comparison suggests that global plasma models can be employed as an approximate method for ECR reactor design. More extensive diagnostics and numerical models that investigate the spatial variation of ion density and ion energy distributions are also presented. Several illustrative ECR plasma-processing applications are discussed. These include submicron etching of silicon, etching of III-V and II-VI electronic and photonic devices, and the epitaxial growth of GaN. The variety and the sophistication of these applications demonstrate that low-pressure high-density ECR plasma processing technology has evolved into a very useful, versatile group of plasma-processing machines.

Measurement of sheath characteristics in the presence of convection and ionization

A flame plasma whose electron and (or) ion density can be varied over several orders of magnitude is constrained to flow perpendicular to a planar grid Langmuir probe. The probe is biased negative to the plasma, and the current–voltage characteristics and the thickness of the ion sheath formed at the probe are measured. The level of the electron and (or) ion density is set within a range at which the probe current due to thermal ionization throughout the sheath is comparable with the current of ions convected into the sheath. The experimental results are compared with the predictions of a recent theoretical paper that calculates the effect of recombination upon the characteristics of planar, cylindrical, and spherical probes with boundary layer sheaths. The theoretical predictions and experimental results for an idealized planar configuration show good agreement over wide ranges of variation of probe bias and plasma electron and (or) ion density. This verification of the theoretical planar electrode – perpendicular-flow model, which is the basis for all three boundary layer relations, is seen as providing strong backing for these relations, which have application to ionization measurements in various forms of recombinant plasma.

Critical Ionization Velocity (CIV) experiment XANI onboard the INTERCOSMOS 24 — “ACTIVE” satellite

Satellite INTERCOSMOS-24 “ACTIVE” was launched on September 30 1989, with an initial apogee 2500 km, perigee 530 km and an orbital inclination of 82.2°. Under “ACTIVE” project, a wide number of plasma diagnostics instruments were especially designed to study the ionospheric response to high power VLF transmitter emissions, by means of satellite-subsatellite system during this mission. As a separate part of this project, a Critical Ionization Velocity (CIV) experiment called XANI (Xenon ANomalous Ionization) had been carried out to study neutral gas/plasma interaction processes in the ionosphere using neutral Xe gas injection. Seven Xe gas release experiments had been realized at daytime high latitude ionosphere. In this paper an Ion Drift Meter (IDM) and Retarding Potential Analyzer (RPA) data concerning transverse ion drift velocity vector measurements and total ion density variations during gas release experiment are shown. Two examples of the RPA data, taken after the beginning of Xe injection, show some prompt ionization at the ionization front about 30% in comparison with the background ion concentration. Horizontal ion drift of few hundred meters per second, transversal to the satellite velocity vector, had been observed at the ionization front.

One year variations in the near Earth solar wind ion density and bulk flow velocity

One year periodic variations in the near Earth solar wind ion density and bulk flow velocity are reported. The variations show an inverse relationship between the ion velocity and density. The peak strength of the observed density variation ranges from 50–100% over the background. These variations imply either large scale mass loading inside the Earth's orbit or intrinsic solar modulations. Analyses of both near Earth and Pioneer Venus Orbiter spacecraft data provide a comparison at two different heliocentric distances. Several explanations for these variations are discussed.

The Use of Langmuir Probe Measurements to Study Reaction Kinetics in Remote Plasma-Enhanced Chemical Vapor Deposition of Silicon

The reaction kinetics of Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) have been studied using Langmuir probe measurements of plasma density and plasma potential combined with growth rate data as a function of r-f power, chamber pressure, and substrate bias. An observed increase in growth rate for negative substrate bias suggests that argon ions drive the reaction. The variation of ion density, and plasma potential with r-f power suggests that the flux of argon ions, not their kinetic energy is responsible for increased growth rates at higher r-f powers.

Variations of helium ion density from theoretical considerations

A theoretical model is proposed to determine the variations of helium ions in the F region. It is based on equations of momentum continuity for H{su+}, O{su+} and He{su+}, integrating these along the magnetic lines of force. By day a starting value of N(He{su+} can be determined within the altitude range 400 \\dv 600 km when using the ratio of the densities of He and N{in2} from MSIS \\3- 86. The height profile of He{su+} then depends on that of O{su+}. Empirical expressions are given for the starting value and for the dependence of the He{su+} profile on the O{su+} density. At night the uncertainties increase and the theoretical model needs information about the {if{frsol|{rmN}({rmHe}{su+})/{rmN}({rmO}{su+})}} ratio as well as about the He{su+} downward flux at the upper boundary ( 1000 km).

Optimized eight-inch extraction system for reactive ion beam etching

A methodology of ion extraction system optimization is proposed, taking into account the influence of the ion density variation in plasma at the plasma sheath. Software tools for extraction grids design and machining have been developed, allowing compensation for the current, divergence, and deflection of a single beamlets. Simulation was used to predict the properties of the three-grid single beamlet. The measurement of the radial ion saturation current density distribution in plasma using a double Langmuir probe for an axial magnetic field up to 16 mT, vacuum chamber pressure ranging from 4×10−5 to 2×10−3 and rf power from 100 to 900 W has been performed. These distributions were used to calculate the grid compensation functions. An extraction system was optimized for the reference magnetic field in the air coil B=7.20 mT, vacuum chamber pressure p=2.0×10−4 mbar, rf power Prf=500 W, and net acceleration voltage Vnet=300 V with argon as a working gas. The ion current density distributions for optimized and nonoptimized extraction systems have been measured and compared with predictions of the model. A maximum detectable variation in the ion current density across the 8 in. target can be reduced to < 3%.

Undulation of a magnetized electron beam by a periodic ion channel

Electron-beam dynamics are calculated for an electron beam propagating parallel to a uniform magnetic field through a periodic ion density channel. Results show that resonant excitation of transverse electron oscillations occurs when the wavelength of the ion density variation equals the axial distance over which an electron cyclotron orbit occurs. This indicates that a periodic ion channel behaves as an electrostatic undulator with magnetoresonance. It is found that periodic ion densities of 1011–1012 cm−3 are required to achieve transverse electron velocities comparable to those of a typical magnetostatic undulator.

Radial oscillations and the ion hose instability of an electron beam propagating in a periodic ion channel

Recently, the propagation of an electron beam through an ion channel with periodically varying ion density has been proposed as a method of transporting or modulating an electron beam. A theoretical treatment indicates that, in the absence of an external magnetic field, cylindrically symmetric radial oscillations of the beam electrons are excited by the channel, with resonant excitation occurring when the wavelength of the ion density variation equals the betatron wavelength. An analysis of the ion focusing regime (IFR) ion hose instability indicates that the e-folding length of this instability increases when the variations of the electron density are increased. In the absence of an external magnetic field and damping, moderate electron density variations (caused, for example, by a periodic ion channel or the nonrigidity of the beam and channel) result in a wavelength and e-folding length on the order of the beam-averaged betatron wavelength.

Positive Corona and Atmospheric Negative Ion Density under Repetitive Impulse Conditions

The variations in the negative ion density in the atmosphere around an electrode gap in which positive corona is produced during repetitive voltage impulses is discussed. A set of equations is solved numerically for the time variation of negative ion density; the influence of positive and negative, small and large ions is included, together with the natural rate of production of ions in the atmosphere. The results show that in repetitive testing the background density of negative ions is always lower than the natural density, and that it is higher in humid air than in dry air.

ICRF fusion reactivity enhancement in tokamaks

A systematic study of ICRF fusion reactivity enhancement has been conducted, using a new bounce-averaged two-dimensional Fokker-Planck code. Second-harmonic heating of deuterium in a 50–50 DT plasma is assumed, and the results are obtained as a function of background plasma density and temperature. An enhancement factor of ten is achieved at low Q (= fusion power/RF power), which is important for ion-tail diagnostics, but at Q = 0.5 the enhancement is ⪅ 2. Significant poloidal variations in ion density (up to 14%) and in fusion reactivity (by a factor up to 2.5) are found.

The distribution of topside spread F from in situ measurements by Defense Meteorological Satellite Program: F2 and F4

On the basis of 331 equatorial passes of two Defense Meteorological Satellite Program satellites at 840 km, we have determined that two distinct types of topside ion depletions occur in the evening sector during equinoctial periods. First, during the early evening (1854 LT) a smooth type of depletion occurs with over 80% probability of observation per pass. This feature is aligned parallel to the magnetic equator and is probably related to the equatorial fountain effect and general F region dynamics. Second, we have seen irregular depletions later in the evening (1936–2154 hours) which are the topside signatures of spread F. Their total probability of occurrence per pass increases from ∼20% at 1936 hours to ∼60% at 2154 hours. A statistical study of the entire data set has shown that spread F depletions occur only when the topside ion density surpasses a seasonally controlled threshold. Therefore longitudinal and local time variations of topside ion density are reflected in similar variations in the occurrence probability of spread F as reported both here and earlier. A detection model including orbit‐feature intersection geometry, the above mentioned threshold, and Poisson statistics has been empirically verified.

Probe experiment for measurement of plasma parameters and ion drift velocities in the ionospheric plasma onboard the Intercosmos-Bulgaria 1300 satellite

In agreement with the scientific program Bulgaria-1300, a complex of probe instruments was designed for the measurements equipment: ID-1, P-6, P-7 and DIET-2 to define the local plasma parameters. The instruments measure: electron temperature Te, electron density Ne, ion temperature Ti, ion density Ni, ion energy distribution f(E j), mass ion composition Mi, complete ion drift vector V D 0.1 to 5 km/s and ion density variations ΔN j N j within the range 0.50–100%.

Spatial and temporal electron and ion density variations within the focal region of an intense laser beam in a tenuous plasma

The recent availability of focussed laser beams with flux densities well in excess of 1015 watts/cm2 at 1.06 μm has led to experimental investigations of direct interaction between intense electromagnetic fields and charged particles. In this paper the relevant hydrodynamic two-fluid mode equations for a singly ionized tenuous plasma eveloped by an intense spatially varying electromagnetic field at the focus of a pulsed laser beam is solved by computer simulations for a cylindrically symmetric geometry. Radial, axial and temporal variations of electron and ion densities are given for different laser intensities and particle number densities. Fluctuations in electron density are found due to the bulk oscillation of electrons relative to what is essentially a static array of massive ions. The possibility of generating intense electromagnetic fields devoid of charged particles is discussed.

Results of conductivity, ion mobility and ion concentration measurements obtained with a parachuted Gerdien aspiration analyser probe in heights below 70 km

The results of conductivity, ion mobility and ion conductivity measurements performed below 70 km with a parachuted Gerdien aspiration analyser during winter and in the summer season are described. The mobility of the most abundant species of positive ions was found to be the same for summer and winter conditions. Observed increases of conductivity are therefore caused by increased carrier density. Increases of negative conductivity in heights above 40 km found in the winter seasons were probably caused by an increase of the number density of free electrons. A straight-forward relation between the number density of charged carriers and the air temperature was not found. Suggestions are made for the cause of the observed seasonal variation in ion density and in λ, the ratio of negative ions to electrons.