Changes In Plasma Density Research Articles

Alfven field line resonances at low latitudes (L = 1.5)

The nature of the spatial structure of resonant ULF waves at low latitudes has been studied as part of a joint project between the U.S. Geological Survey, the Institute of Physics of the Earth, Moscow, and the Kyrgyzian Institute of Seismology. Gradient analysis of data taken at a meridional array of three stations in Soviet Central Asia showed that Alfven field line resonances, in the Pc 3 bandwidth, do exist at L = 1.5. Resonant frequencies of 66‐84 mHz (12‐15 s) were measured. Resonance width and the radial gradient of Alfven frequency were determined from our experimental data. When compared with previous published determinations of the resonance width, the resonance width is observed to increase at lower latitudes. This is the result of an increase in ionospheric damping at lower latitudes. Ionospheric damping significantly effects both resonant frequencies and resonance widths. Initial analysis of the data showed that effects of geologic inhomogeneities between two stations can obscure resonant effects that are observed in ground‐based magnetometer data. A method was developed to address these geologic effects so that the response of the resonator can be seen in both amplitude and phase calculations. The cross‐phase spectrum was determined to be the most useful technique to identify the resonant frequency of the field line between two ground stations. The diurnal behavior of resonant frequency was examined using a cross‐phase analysis technique and is shown to agree with theoretical predictions at low latitudes. We can conclude that diurnal variations in resonant frequency are mainly due to diurnal changes in plasma density along the oscillating field line.

Nature and rate of vascular refilling during hemodialysis and ultrafiltration

The change of blood volume, of blood and plasma density (rho b, rho p) following a short ultrafiltration pulse (duration: 20 min; mean rate -35 ml/min) within the first hour of hemodialysis was analyzed in 13 hemodynamically stable patients (30 single measurements). Protein concentration of refilling volume (7 g/liter) was calculated from its density (1009.25 +/- 3.7 kg/m3, at 20 degrees C) and from the linear relationship between plasma density and protein concentration (cp) of uremic plasma samples (rho p = 1007.46 + 0.2422 x cp). The filtration coefficient (Lp,calc) determined from a relation derived from Starling's hypothesis was 5.6 +/- 1.4 ml/(min.mm Hg.50 kg lean body mass); N = 13, mean +/- SD, minimum 3.2, maximum 8.0. A model describing the dynamics of blood and plasma volume was developed. It was fit to on-line measurements of relative blood volume changes by variation of the filtration coefficient and of initial blood volume (Lp,fit, Vb,fit). The linear regression between Vb,fit and blood volume determined from anthropometry (Vb,calc) was highly significant (r = 0.79, N = 30, P < 0.001). Compared to Vb,calc, Vb,fit was typically increased by 21 +/- 11%, reflecting a fluid overload at the beginning of the treatment. Lp,fit was not different from Lp,calc. Lp,fit significantly increased with blood volume excess. Due to the small but definite protein content of refilling volume, the model accounts for increased blood volume recovery and occasional overshoot of blood and plasma volumes following ultrafiltration.

Modeling of core-wall particle dynamics in Tore Supra pumping experiments

Particle control experiments in Tore Supra have shown that the change in core plasma density with pumping at the outboard pump limiter amounts only to a small fraction of the total number of particles extracted by the outboard pump limiter system. To develop a model for the core-wall particle exchange at the level of individual basic physical processes, the particle exchange between the core plasma and the wall has been modeled in detail for a series of Tore Supra discharges. Core and scrape-off layer particle balance and wall diffusion calculations were performed using a radial transport code and a 1D wall diffusion code. Core-wall particle exchange for the case of the outboard pump limiter is found to be mediated by the localized charge exchange flux in the near limiter region. The wall particle efflux required for detailed balance does not match that calculated to be available from the near limiter wall region unless some local heating of the wall is assumed.

A novel microwave plasma cavity assembly for atomic emission spectrometry

The design approach that leads to a simple new cavity structure for a microwave discharge used for analytical chemistry is outlined. The design and construction of a totally new and efficient microwave cavity assembly, namely an integrated microwave generator/cavity combination, operated at 2.45 GHz, and preliminary operating conditions that exceed certain operational capabilities of the existing Beenakker and surfatron arrangements are described. The feasibility of operating such a microwave plasma cavity arrangement to form plasma discharges in argon, helium, nitrogen, air and oxygen at atmospheric pressure for atomic emission spectrometry has been demonstrated. The generation of an oxygen microwave induced plasma (MIP) discharge with conventional tubes used routinely in the argon or helium MIP suggests its application in spectrochemical analysis. The introduction of wet aerosols, because of the excellent resistance to detuning caused by changes in plasma density, can be accomplished by using direct solution nebulization with no desolvation system and with a typical inductively coupled plasma nebulizer-spray chamber.

Rotational discontinuities in anisotropic plasmas

The kinetic structure of rotational discontinuities (RDs) in anisotropic plasmas with T⟂/T‖ &gt; 1 is investigated by using a one‐dimensional electromagnetic hybrid code. To form the RD, a new approach is used where the plasma is injected from one boundary and reflected from the other, resulting in the generation of a traveling fast shock and an RD. Unlike the previously used methods, no a priori assumptions are made regarding the initial structure (i.e. width or sense of rotation) of the rotational discontinuity. The results show that across the RD both the magnetic field strength and direction, as well as the plasma density change. Given that such a change can also be associated with an intermediate shock, the Rankine‐Hugoniot relations are used to confirm that the observed structures are indeed RDs. is found that the thickness of RDs is a few ion inertial lengths and is independent of the rotation angle. Also, the preferred sense of rotation is in the electron sense; however, RDs with a rotation angle larger than 180° are found to be unstable, changing their rotation to a stable ion sense.

Signatures of transient boundary layer processes observed with Viking

Transient penetration of plasma with magnetosheath origin is frequently observed with the hot plasma experiment on board the Viking satellite at auroral latitudes in the dayside magnetosphere. The injected magnetosheath ions exhibit a characteristic pitch angle/energy dispersion pattern, earlier reported for solar wind ions accessing the magnetosphere in the cusp regions. In contrast to the continuous plasma entry in the cusp, the events discussed here show temporal features which suggest a connection to transient processes at or in the vicinity of the magnetospheric boundary. A single event study confirms previously published observations that the injected ions flow essentially tailward with a velocity comparable to the magnetosheath flow and that the energy spectra inferred for the source population resemble magnetosheath spectra. Those ion injection structures, which were resolved by the Viking mass spectrometer, consist of protons. Based on a statistical study, it is found that these events are predominantly observed around 0800 and 1600 MLT, in a region populated both by ring current/plasma sheet particles and by particles whose source is the magnetosheath plasma. Magnetic field line tracing based on the Tsyganenko magnetic field model yields a scatter of the source locations around the mid‐latitude region of the magnetospheric boundary. The probability for these events to occur is highest when the interplanetary magnetic field (IMF) is confined to the ecliptic plane. The event occurrence frequency shows a dawn‐dusk asymmetry depending on the azimuthal direction of the IMF. The occurrence frequency is independent of the solar wind velocity but increases with increasing solar wind pressure. For radially directed IMF the probability for observing the events is generally higher than for azimuthally directed IMF. The difference is especially pronounced during times when the solar wind pressure is comparatively low. Some of the most prominent events are obviously associated with significant changes in solar wind plasma density. The connection of the events to transient impulsive solar wind/magnetosphere interaction processes, such as transient reconnection (FTE), impulsive plasma transfer, Kelvin Helmholtz instabilities, and solar wind pressure pulses, is discussed. A relation with transient reconnection can be excluded.

A search for evidence of the evolution of rotational discontinuities in the solar wind from nonlinear Alfvén waves

The nonlinear theory of the steepening of Alfvén waves into solitons and then into rotational discontinuities (RDs) predicts correlations between the β of the plasma, the sense of polarization of the discontinuity, and changes of magnetic field strength and plasma density across the discontinuity. The predicted correlations have been searched for in ISEE 3 field and plasma data, but no statistically significant evidence for the evolution of RDs from Alfvén solitons could be found. Approximately equal numbers of right‐ and left‐handed discontinuities were observed. Some of the rotations approached 180°, but none exceeded that value. Nearly all the RDs in the 33‐day data set were propagating outward from the Sun, which implies that sunward propagating Alfvénic fluctuations produced by stream interactions in the interplanetary medium seldom steepen into discontinuities before reaching 1 AU. The only unambiguously sunward propagating RD in the data set showed all of the expected relations for a steepened Alfvén soliton. Interplanetary processes, such as changing wave speeds and β and refraction of the discontinuities, may have changed the properties of the RDs created close to the Sun to such an extent that any original correlations are no longer detectable at 1 AU. Another possible explanation of the basically negative result is that some of the RDs produced near the Sun may have steepened from fast or slow Korteweg‐deVries solitons.

Probe measurements of TFTR edge plasmas at neutral beam heating powers up to 20 MW

This paper presents radial profiles in the scrape-off layer of neutral beam heated plasmas in TFTR at powers up to 20 MW. The measurements have been made with, among other, a novel “fast probe” which moves quickly through the scrape-off during the discharges to provide radial profiles of electron density, electron temperature, and power flux parallel to field lines. The dependence of the edge on changes in plasma current and density during beam heating has been measured. A number of heating scenarios have been investigated, including current ramping, power ramping, and variation in beam injection direction. Scrape-off scale lengths of density, temperature and power during ohmic and beam phases are found to be similar: λ n = 4.5 cm, λ T = 5 cm, λ P = 2.5 cm . The power to the ion side is consistently a factor 2–4 higher than that to the electron side, and there are differences in scale-length of up to 1 cm between the two sides. At low core electron densities, the edge electron density during counter-injection is found to exceed that during co-injection by more than a factor of ten.

Heavy ion beam‐ionosphere interactions: Charging and neutralizing the payload

The argon release controlled studies (ARCS 1‐3) rocket flights carried ion generators to altitudes of 400‐500 km in the nighttime auroral ionosphere. Three distinct electrical charging and neutralization processes were seen on the payloads during gun operation: steady or dc vehicle charging, brief charging at gun turn‐on, and extended oscillatory sequences. Many of the unexpected consequences of gun firings are attributed to these payload charging and neutralization processes. Electrical charging is regulated by the rate at which low‐energy electrons escape from the generator, which in turn is dependent on magnetic field geometry. Each ion generator produced a dipolar magnetic field which merged with the Earth's field near the rocket. The resulting local magnetic field guided electrons back to the rocket for certain gun orientations, thereby inhibiting neutralization. Transient charging was attributed to the formation of an electron cloud around at least some vehicles, while dc charging altered the rocket's surroundings until the electron escape rate balanced the ion beam flux. We concluded that during oscillatory events the entire environment of a payload could alternate between hot electron and cold electron configurations at very high rates, possibly exceeding 10 kHz. These changes in the plasma environment did not produce substantial electric field perturbations at the dc or ac high impedance electric field sensors, so were not seen in data from typical wave detectors. However, changes in plasma density and temperature produced dramatic effects on low impedance electric current sensors such as Langmuir probes. These observations illustrate the importance of carrying both low and high impedance detectors on vehicles involved in active experiments. A number of possible future experiments are suggested to test and extend our understanding of the proposed charging and neutralization mechanisms.

Characteristics of plasma flow from laser irradiated planar thin foil targets

Filamentary structures or jetting in the plasma flow generated from laser irradiated thin foil targets have been observed using an optical probe technique. Laser wavelength, pulse duration, and intensity range used in the experiments were 1.06 μm, 5 ns, and 5×1012 W/cm2, respectively. A 0.53-μm-wavelength optical pulse of 50-μJ energy and 1-ns duration was used as an optical probe. Optical shadowgrams showed that the filamentary structures or jets in the plasma flow were strongest in the region about 300 μm away from the target surface, and in time, much later than the peak of the main laser pulse. It was also observed that jetting does not affect the stability of the ablatively accelerated thin foils. Jetting was found to be stronger for targets with a higher atomic number. A time-resolved interferometric study of the plasma produced from targets with high atomic numbers also showed the existence of an abrupt change in fringe shift or plasma density in the region of these jets.

Fast shocks at the edges of hot diamagnetic cavities upstream from the Earth's bow shock

Recently, several interesting events, described as hot expanding diamagnetic cavities, have been observed upstream from the earth's bow shock using the ISEE 1 and 2 spacecraft. It has been suggested that fast shocks may form at the edges of some of these events because of the rapid expansion of the cavities. We have examined plasma density, temperature, velocity, and total field changes across the edges of several events and find these changes to be qualitatively consistent with the presence of shocks there. The presence of flat‐topped electron distributions and occasional electron beams at and downstream from the edges provides additional evidence for shocks. Plasma wave observations also show shocklike electrostatic noise at the edges of several events. We conclude that the edges of diamagnetic cavity events are often shocks, with a range of shock strengths similar to that observed in the interplanetary medium. The range of shock strengths may be the result of different convection and/or expansion speeds of the cavities; these convection and/or expansion speeds in turn may ultimately be related to the age of the cavities.

Taming the surfatron

Several years ago, a surface-wave-induced plasma device known as a “surfatron” appeared as a promising new source for atomic emission spectrometry. Since that time little further development of the surfatron has been published. We have therefore sought to re-evaluate the surfatron by (a) reviewing the subject of surface-wave induced plasmas, (b) describing in detail the construction and operation of the surfatron, (c) studying the tuning capabilities of the system and (d) examining the spatial emission properties of the surfatron. The surfatron possesses a number of advantages over other kinds of microwave-induced plasma devices such as the Beenakker-type resonant cavity. Significantly, surfatron-sustained plasmas were produced readily in either argon or helium and were easily tuned. Moreover, the helium plasma showed resilience to the introduction of nebulized solutions and also possessed an annular shape. The surfatron showed much greater resistance to detuning caused by changes in plasma density than did the more conventional resonant-cavity system and required no external tuning elements.

Integral stochastic alpha particle losses in a tokamak reactor

The authors consider stochastic alpha-particle losses due to the toroidal magnetic field ripple in a tokamak. The fraction of alpha-particle energy losses is calculated for the whole cycle from generation to thermalization. To determine this quantity they solve a two-dimensional (velocity module – angle) kinetic problem describing Coulomb collisions between alpha particles and plasma electrons and ions as well as the stochastic loss of these particles in the τ-approximation. Calculations show that the losses are not very sensitive to changes in plasma temperature and density but depend strongly on the ripple. For 0.3% ripple on the outer circumference of the chamber the fraction of alpha-particle energy losses does not exceed 1%, but for a 1.2% ripple the losses are already 8–10%, while for a further increase in ripple they can rise as high as 50%.

Plasma convection in the poloidal limiter shadow of a tokamak

Plasma flow in the poloidal limiter shadow of a tokamak is examined. It is shown that, as a result of contact with a limiter, a large poloidal electric field is created in the plasma. Plasma drift in this field can be considered as anomalous diffusion. The characteristic dimension of the change in plasma density is found to be of the order of the ion cyclotron radius calculated for a poloidal magnetic field. The plasma flow is non-ambipolar and the electric current closes its circuit via the limiter. The results obtained are confirmed by experiments on various tokamaks.

Effects of wall conditioning on plasma parameters, impurities and hydrogen recycling in TEXTOR

A recent series of experiments on TEXTOR offered a unique opportunity to measure the effects of changing wall conditions (surface impurities, temperature, and pretreatment) on plasma parameters, impurities, and hydrogen recycling. The TEXTOR liner is Inconel 625. The liner was baked to 150 or 300°C. It was cleaned by (1) a radio-frequency supported glow discharge (RG) or (2) an electron cyclotron resonant (ECR) plasma. Methane was added to the RG discharge, for one experiment, to increase the carbon concentration and lower the oxygen levels on the wall. Changing wall surface compositions caused the optical light intensities from the major plasma impurities, carbon, and oxygen, to vary by a factor of 2 to 5 and hydrogen recycle coefficients to vary by more than an order of magnitude. Except during dirty wall conditions and during the carbon addition experiments, changes in plasma density and current, electron temperature and energy confinement times were relatively small.

Impurity generation during intense lower hybrid heating experiments on the Alcator C tokamak

Experiments are underway on the Alcator C tokamak with over 1 MW of RF power injected into the plasma at a frequency of 4.6 GHz to study both heating and current drive effects. During these studies, impurity generation from limiter structures has been observed. The RF induced impurity influx is a strongly nonlinear function of net injected power. For P RF < 500 kW, only small effects are seen. As P RF approaches 1 MW, however, sharp increases in impurity influxes and Z eff are observed. Three different limiter materials have been used during these studies: molybdenum, graphite, and silicon-carbide coated graphite. In each case, the materials of the limiter structure are seen to dominate the increased impurity influx. In a typical case, with P RF = 1.0 MW, n e = 1.3 × 10 14 cm −3 , and the SiC coated limiters, Z eff is seen to increase from 1.5 before the RF pulse to about 4 during the heating. At the same time, central T e increases from 2000 to 3000 eV and central T i from 1200 to 1800 eV. Similar effects are seen in both H 2 and D 2 working gas discharges. The contribution to impurity generation of nonthermal electrons, which are produced by the RF, is under investigation. Changes in edge plasma temperature and density, as well as the possibility that the particle transport is affected by the RF, are also being examined. Results of the experiments with the three different limiter materials are compared, and contributions of impurity radiation to the overall power balance are estimated.

Generation and propagation of an electromagnetic pulse in the trigger experiment and its possible role in electron acceleration

The dc electric field detector onboard the instrumented portion of the Trigger payload detected a large‐amplitude (∼200 mV/m) electric field pulse which was observed with a time delay consistent only with an electromagnetic wave. A model for this perturbation is constructed in this paper and the associated field‐aligned current calculated as a function of altitude. Near the explosion point, downward currents of 2500 µA/m² and upward currents of 90 µA/m² are indicated by the model. The lower upward current has the same direction as the field‐aligned current in discrete auroral arcs. Taking into account the attenuation of the wave and the changes in Alfven speed and plasma density, the associated differential drift velocity between electrons and ions is found to peak at 750‐km altitude at a value one‐third of the electron thermal speed. This is well above the threshold for ion cyclotron wave turbulence and approaches the value necessary for production of electrostatic shocks or double layers. The differential velocity associated with the downward current well exceeds the electron thermal speed. The time delay for observation of accelerated electrons is consistent with the Alfven propagation time to 750‐km altitude. The initial explosion pulse had internal temporal variations just above the oxygen ion cyclotron frequency in agreement with other experiments of this type. The Poynting flux in the electromagnetic pulse was sufficiently intense to create the observed particle precipitation.

MHD simulation of a beat frequency heated plasma

The heating of a plasma in a solenoid, with a beat frequency harmonic which is excited at a frequency near to that of a Langmuir mode in a plasma, is examined. It is shown that at high temperatures the heating rate is very insensitive to changes in plasma density. The amount of energy that can be coupled to a plasma in a solenoid with this heating scheme is investigated by using a one-dimensional computer code which incorporates an exact solution of the relevant MHD equations. The absorption of energy from a high powered laser is shown to be significantly enhanced with this process.

Investigation of shock waves and tangential discontinuities of the cosmic plasma by the radio interference technique

The possibility of investigation of the cosmic plasma dynamics by the radio interference technique based on a finite time of radio wave propagation between the sounding and responding stations is shown. By locating the sounding station on a spacecraft the greatest contribution to the phase difference ΔΦ( t)or the phase difference growth rate Δƒ between the sounding and response signals are caused by disturbances in close proximity to the spacecraft. This method permits interplanetary shock waves and tangential discontinuities to be registered and the velocities and plasma density changes on their fronts to be determined. By using experimental data of ΔΦ( t) or Δƒ(t) one can also obtain information about plasma concentration jump, location and motion of bow shock wave and magnetopause and plasmapause. Available experimental data about different disturbances of cosmic plasma were analysed and the requirements on frequency stability of spacecraft-borne and groundbased radio equipment were estimated to register those disturbances. In most cases relative stability 10 −11–10 −13 provided by present atomic frequency standards is sufficient.

Beam instability in the case of sharp changes in plasma density

The authors investigate amplification of waves in the case when the beam propagates perpendicularly to the boundary at which plasma density changes abruptly. Expressions are found which link wave amplitudes on both sides of the boundary. In the general case of obliquely incident waves, the fluctuations of surface-charge density of the type of surface waves arise at the boundary. These fluctuations cannot be resonantly excited by the beam propagating perpendicularly to the boundary.