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Abstracts are presented on various aspects of plasma physics, including theoretical discussions and ionospheric plasmas. The topics considered cover Alfven waves, magnetized plasmas, plasma diffusion, Poynting flux measurements, electric fields, the magnetosphere, auroras, and plasma convection.
Temperature distribution and radiation flux measurements in uranium plasma reactor with reflecting walls
Results are presented from the Polar Ionospheric Irregularities Experiment (PIIE), conducted from Sondrestrom, Greenland, on March 15, 1985, designed for an investigation of processes which lead to the generation of small-scale (less than 1 km) ionospheric irregularities within polar-cap F-layer auroras. An instrumented rocket was launched into a polar cap F layer aurora to measure energetic electron flux, plasma, and electric circuit parameters of a sun-aligned arc, coordinated with simultaneous measurements from the Sondrestrom incoherent scatter radar and the AFGL Airborne Ionospheric Observatory. Results indicated the existence of two different generation mechanisms on the dawnside and duskside of the arc. On the duskside, parameters are suggestive of an interchange process, while on the dawnside, fluctuation parameters are consistent with a velocity shear instability.
Interplanetary travel requires propulsion systems that can provide high specific impulse (Isp), while also having sufficient thrust to rapidly accelerate large payloads. One such propulsion system is the Variable Specific Impulse Magneto-plasma Rocket (VASIMR), which creates, heats, and ejects plasma to provide variable thrust and Isp, designed to optimally meet the mission requirements. The fraction of the total energy invested in creating the plasma, as compared to the plasma's total kinetic energy, is an important factor in determining the overall system efficiency. In VASIMR, this 'frozen flow loss' is appreciable when at high thrust, but negligible at high Isp. The loss applies to other electric thrusters as well. If some of this energy could be recovered through recombination processes, and reinjected as neutral kinetic energy, the efficiency of VASIMR, in its low Isp/high thrust mode may be improved. An experiment is being conducted to investigate the possibility of recovering some of the energy used to create the plasma by studying the flow characteristics of the charged and neutral particles in the exhaust of the thruster. This paper will cover the measurements of momentum flux and heat flux in the exhaust of the VASIMR test facility using helium as the propellant where the heat flux is comprised of both kinetic and plasma recombination energy. The flux measurements also assist in diagnosing and verifying the plasma conditions in the existing experiment.
A study of the plasma and electric-field measurements of the PVO in the Venus ionosheath near the terminator region is presented. A drastic decrease of the particle-flux intensity measured with the plasma instrument is encountered in the inner ionosheath before the inbound crossing of the ionopause. The outer boundary of the region of weak plasma fluxes is consistent with the presence of a rarefaction wave as reported previously from the Mariner 5 (Bridge et at., 1967), Venera (Verigin et al., 1978) and PVO (Spenner et al., 1980) plasma measurements in the Venus wake. Near that boundary there are, in addition, appreciable changes in the electric field signals detected with the PVO electric-field detectors. These observations suggest the existence of local plasma-wave activity associated with turbulent flow conditions in that region.
Electromagnetic thrusters typically use electric and magnetic fields to accelerate and exhaust plasma through interactions with the charged particles in the plasma. The energy required to create the plasma, i.e. ionization energy, is potential energy between the electron and ion. This potential energy is typically lost since it is not recovered as the plasma is exhausted and is known as frozen flow loss. If the frozen flow energy is a small fraction of the total plasma energy, this frozen flow loss may be negligible. However, if the frozen flow energy is a major fraction of the total plasma energy, this loss can severely reduce the energy efficiency of the thruster. Recovery and utilization of this frozen flow energy can improve the energy efficiency of a thruster during low specific impulse operating regimes when the ionization energy is a large fraction of the total plasma energy. This paper quantifies the recovery of the frozen flow energy, i.e. recombination energy, via the process of surface recombination for helium. To accomplish this task the momentum flux and heat flux of the plasma flow were measured and compared to calculated values from electrostatic probe data. This information was used to deduce the contribution of recombination energy to the total heat flux on a flat plate as well as to characterize the plasma conditions. Helium propellant was investigated initially due to its high ionization potential and hence available recombination energy.
OGO-E plasma spectrometers for measuring total flux below 12 kev and energy /spectral density/ of plasma
The performance of a low-power cylindrical Hall thruster, which more readily lends itself to miniaturization and low-power operation than a conventional (annular) Hall thruster, was measured using a planar plasma probe and a thrust stand. The field in the cylindrical thruster was produced using permanent magnets, promising a power reduction over previous cylindrical thruster iterations that employed electromagnets to generate the required magnetic field topology. Two sets of ring-shaped permanent magnets are used, and two different field configurations can be produced by reorienting the poles of one magnet relative to the other. A plasma probe measuring ion flux in the plume is used to estimate the current utilization for the two magnetic configurations. The measurements indicate that electron transport is impeded much more effectively in one configuration, implying a higher thrust efficiency. Preliminary thruster performance measurements on this configuration were obtained over a power range of 100-250 W. The thrust levels over this power range were 3.5-6.5 mN, with anode efficiencies and specific impulses spanning 14-19% and 875- 1425 s, respectively. The magnetic field in the thruster was lower for the thrust measurements than the plasma probe measurements due to heating and weakening of the permanent magnets, reducing the maximum field strength from 2 kG to roughly 750-800 G. The discharge current levels observed during thrust stand testing were anomalously high compared to those levels measured in previous experiments with this thruster.
The performance of a low-power cylindrical Hall thruster, which more readily lends itself to miniaturization and low-power operation than a conventional (annular) Hall thruster, was measured using a planar plasma probe and a thrust stand. The field in the cylindrical thruster was produced using permanent magnets, promising a power reduction over previous cylindrical thruster iterations that employed electromagnets to generate the required magnetic field topology. Two sets of ring-shaped permanent magnets are used, and two different field configurations can be produced by reorienting the poles of one magnet relative to the other. A plasma probe measuring ion flux in the plume is used to estimate the current utilization for the two magnetic topologies. The measurements indicate that electron transport is impeded much more effectively in one configuration, implying higher thrust efficiency. Thruster performance measurements on this configuration were obtained over a power range of 70-350 W and with the cathode orifice located at three different axial positions relative to the thruster exit plane. The thrust levels over this power range were 1.25-6.5 mN, with anode efficiencies and specific impulses spanning 4-21% and 400-1950 s, respectively. The anode efficiency of the permanent-magnet thruster compares favorable with the efficiency of the electromagnet thruster when the power consumed by the electromagnets is taken into account.
A 50 cm ion thruster is being developed to operate at >65 percent total efficiency at 11 kW, 2700 s Isp and over 25 kW, 4500 s Isp at a total efficiency of >75 percent. The engine is being developed to address the need for a multimode system that can provide a range of thrust-to- power to service national and commercial near-earth onboard propulsion needs such as station-keeping and orbit transfer. Operating characteristics of the 50 cm ion thruster were measured under simulated beam extraction. The discharge current distribution at the various magnet rings was measured over a range of operating conditions. The relationship between the anode current distribution and the resulting plasma uniformity and ion flux measured at the thruster exit plane is discussed. The thermal envelope will also be investigated through the monitoring of magnet temperatures over the range of discharge powers investigated. Discharge losses as a function of propellant utilization was also characterized at multiple simulated beam currents. Bulk plasma conditions such as electron temperature and electron density near engine centerline was measured over a range of operating conditions using an internal Langmuir probe. Sensitivity of discharge performance to chamber length is also discussed. This data acquired from this discharge study will be used in the refinement of a throttle table in anticipation for eventual beam extraction testing.
The flow around the Io flux tube was studied by analyzing the eleven spectra taken by the Voyager 1 Plasma Science (PLS) experiment in its vicinity. The bulk plasma parameters were determined using a procedure that uses the full response function of the instrument and the data in all four PLS sensors. The mass density of the plasma in the vicinity of Io is found to be 22,500 + or - 2,500 amu/cu cm and its electron density is found to be 1500 + or - 200/cu cm. The Alfven speed was determined using three independent methods; the values obtained are consistent and taken together yield V sub A = 300 + or - 50 km/sec, corresponding to an Alfven Mach number of 0.19 + or - 0.02. For the flow pattern, good agreement was found with the model of Neubauer (1980), and it was concluded that the plasma flows around the flux tube with a pattern similar to the flow of an incompressible fluid around a long cylinder obstacle of radius 1.26 + or - 0.1 R sub Io.
Data obtained with the Voyager Plasma Science Experiment (PSE) during a flyby of the Jovian moon Io are analyzed to quantify the bulk plasma parameters near the Io flux tube. The PSE contains four modulated grid Faraday cups, three pentagonally shaped and the other circular. The path of the spacecraft around Jupiter and past Io ensured that the maximum flux was passing perpendicular to the spacecraft instruments when Voyager was closest to Io (20,000 km). A total of 11 high resolution (M mode) plasma spectra were obtained, revealing 6 species of ions: H(+), O(+), O(2+), S(+), S(2+) and S(3+). All species had the same bulk velocity and convected maxwellian distributions; the heavy ions species were all the same temperature. The mass density of the plasma near Io was 20,000-25,000 amu/cu cm and the electron density was 1300-1700/cu cm. Calculations of the Alfven speed yielded a value ranging from 250-350 km/sec and an Alfven Mach no. of 0.17-0.21. The data were equivalent to that expected from the flow of an incompressible fluid around a long cylinder with a radius about 1.25 that of Io's.
Absolute fluxes and energy distributions of ions in inductively coupled plasmas of Ar, CHF3/Ar, and CHF3/Ar/O2 have been measured. These plasmas were generated in a Gaseous Electronics Conference (GEC) cell modified for inductive coupling at pressures 10-50 mTorr and 100-300 W of 13.56 MHz radio frequency (RF) power in various feedgas mixtures. In pure Ar plasmas, the Ar(+) flux increases linearly with pressure as well as RF-power. Total ion flux in CHF3 mixtures decreases with increase in pressure and also CHF3 concentration. Relative ion fluxes observed in the present studies are analyzed with the help of available cross sections for electron impact ionization and charge-exchange ion-molecule reactions. Measurements of plasma potential, electron and ion number densities, electron energy distribution function, and mean electron energy have also been made in the center of the plasma with a RF compensated Langmuir probe. Plasma potential values are compared with the mean ion energies determined from the measured ion energy distributions and are consistent. Electron temperature, plasma potential, and mean ion energy vary inversely with pressure, but increase with CHF3 content in the mixture.
We report mass spectrometric studies of energy distributions and absolute concentrations of ions generated in CF4/O2/Ar inductively coupled rf plasmas. The ions were collected through a 100 mm orifice in the grounded and water cooled lower electrode in a GEC cell configuration. The measurements were made at gas pressures in the 10-50 mTorr range and rf coil power in the 100-300 W range. The observed ions are CF3(+), CF2(+), CF(+), C(+), F(+), COF(+), CO(+), O2(+), and O(+). The relative abundance of these ions varies with pressure and rf power. The energy distribution and absolute concentrations are correlated with electron number density and floating plasma potential measured by a compensated Langmuir probe.
The DE 1/2 plasma measurements of the dayside polar cusp at high and low altitudes have been examined for six near-conjugate passes. The plasma moments of downward electrons are computed, including energy flux, near flux, number density, and average energy from plasma data from both the high-altitude plasma instrument and the low-altitude plasma instrument. The DE 1/2 plasma measurements at the peaks of energy flux are compared, and the downward energy flux expected at the DE 2 altitude is modelled according to the convergence of magnetic flux tubes and using the DE 1 plasma data.
Explorer x satellite measurements of nearby interplanetary plasma flux and its effects on geomagnetic field
In the solar wind and in middle latitude regions of the magnetosphere, spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: (1) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock, (2) that scale lengths for large changes in E at the subsolar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause, and (3) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels. Spike-like, non-oscillatory, fields lasting less than 0.2 seconds are occasionally seen at the bow shock and at the magnetopause and also intermittently appear in magnetosheath and plasma sheet regions under highly variable field conditions.
The relative contributions of electron precipitation and transport of dayside plasma to the maintenance of the Venus nightside ionosphere during the long Venusian night are investigated based on simultaneous Pioneer Venus Orbiter Retarding Potential Analyzer measurements of suprathermal electron fluxes and plasma densities. In about 20 orbits, the nightside integral electron flux of electrons with energies between 5 and 45 eV is observed to be relatively constant in time and altitude, while plasma density is observed to vary by a factor of 10 or more with no correlation with the electron flux. Ionization rates and ion density height profiles are computed for O(+) and O2(+) as a function of magnetic dip angle based on a typical electron spectrum, or a downward flux of O(+) ions. Comparison of the computed profiles with the measured median O(+) and O2(+) density profiles reveals that the measured profiles can only be reproduced by a downward flux of O(+) equal to about 10 to the 8th/sq cm per sec; suprathermal electron energy distributions produce O2(+) and O(+) levels only about half and one tenth those usually observed, respectively. It is thus concluded that transport of O(+) ions from the dayside Venus ionosphere is responsible for approximately 75% of the typical nightside ionization, with variations in O(+) transport mechanism responsible for most of the observed nightside density variations. The remaining ionization is attributed to suprathermal electrons, which contribute principally to the O2(+) peak.