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NASA NTRS2003

Beam Tests of the Balloon-Borne ATIC Experiment

The Advanced Thin Ionization Calorimeter (ATIC) balloon-borne experiment is designed to perform cosmic-ray elemental spectra measurement from 50 GeV to 100 TeV for nuclei from hydrogen to iron. These measurements are expected to provide crucial hints about some of the most fundamental questions in astroparticle physics today. ATTIC'S design centers on an 18 radiation length (X(sub Omnicron)) deep bismuth germanate (BGO) calorimeter, preceded by a 0.75 lambda(sub int) graphite target. In September 1999 the ATIC detector was exposed to high-energy beams at CERN's SPS accelerator, within the framework of the development program for the Advanced Cosmic-ray Composition Experiment for the Space Station (ACCESS). In December 2000 - January 2001, ATIC flew on the first of a series of long duration balloon (LDB) flights from McMurdo Station, Antarctica. We present here results from the 1999 beam-tests, including energy resolutions for electrons and protons at several beam energies from 100 GeV to 375 GeV, as well as signal linearity and collection efficiency estimates. We show how these results compare with expectations based on simulations, and their expected impacts on mission performance.

Ganel, O.↗

NASA NTRS1978

Heating of the ambient ionosphere by an artificially injected electron beam

An electrostatic analyzer on the electron accelerator of the Electron Echo 2 experiment showed that the electrons of the background plasma were heated to 10,000 K or more within 8 ms of the start of gun pulses. The degree of heating was dependent on the orientation of the rocket with respect to the magnetic field but was not measurably dependent on ambient electron density, neutral atmosphere density, or on the pitch angle at which the 40-keV electron beam was injected. This heating was also accompanied by an increase of plasma density. No evidence was found for an ion-free region around the rocket during gun pulses. These observations show that significant amounts of ionization are taking place around the rocket. During part of the flight a two-temperature electron distribution was found. It is believed that the high-temperature part of these distributions represents secondaries produced by the beam.

Cartwright, D. G.↗

NASA NTRS1992

Plasma effects of active ion beam injections in the ionosphere at rocket altitudes

Data from ARCS rocket ion beam injection experiments are primarily discussed. There are three results from this series of active experiments that are of particular interest in space plasma physics. These are the transverse acceleration of ambient ions in the large beam volume, the scattering of beam ions near the release payload, and the possible acceleration of electrons very close to the plasma generator which produce intense high frequency waves. The ability of 100 ma ion beam injections into the upper E and F regions of the ionosphere to produce these phenomena appear to be related solely to the process by which the plasma release payload and the ion beam are neutralized. Since the electrons in the plasma release do not convect with the plasma ions, the neutralization of both the payload and beam must be accomplished by large field-aligned currents (milliamperes/square meter) which are very unstable to wave growth of various modes.

Arnoldy, R. L.↗

NASA NTRS1975

15 cm cusped magnetic field mercury ion thruster research

The importance of achieving a uniform current density in the exhaust beam of an electrostatic ion thruster is discussed in terms of thrust level and accelerator grid lifetime. A neutral residence time approach is used to propose a magnetic field geometry which should produce a highly uniform beam current density. The discharge chamber length to diameter ratio is shown to be an important optimization parameter and experimental evaluation of the cusped field thruster over a wide range of this parameter is presented. Beam profile measurements 5 cm downstream of the accelerator grid indicate a beam profile flatness parameter which is 25% greater than the SERT II value. Flatness parameters extrapolated to the plane of the accelerator grid are demonstrated to be as high as 0.9.

Beattie, J. R.↗

NASA NTRS2013

Current Flow and Pair Creation at Low Altitude in Rotation-Powered Pulsars' Force-Free Magnetospheres: Space Charge Limited Flow

We report the results of an investigation of particle acceleration and electron-positron plasma generation at low altitude in the polar magnetic flux tubes of rotation-powered pulsars, when the stellar surface is free to emit whatever charges and currents are demanded by the force-free magnetosphere. We apply a new 1D hybrid plasma simulation code to the dynamical problem, using Particle-in-Cell methods for the dynamics of the charged particles, including a determination of the collective electrostatic fluctuations in the plasma, combined with a Monte Carlo treatment of the high-energy gamma-rays that mediate the formation of the electron-positron pairs.We assume the electric current flowing through the pair creation zone is fixed by the much higher inductance magnetosphere, and adopt the results of force-free magnetosphere models to provide the currents which must be carried by the accelerator. The models are spatially one dimensional, and designed to explore the physics, although of practical relevance to young, high-voltage pulsars. We observe novel behaviour (a) When the current density j is less than the Goldreich-Julian value (0 < j/j(sub GJ) < 1), space charge limited acceleration of the current carrying beam is mild, with the full Goldreich-Julian charge density comprising the charge densities of the beam and a cloud of electrically trapped particles with the same sign of charge as the beam. The voltage drops are of the order of mc(sup 2)/e, and pair creation is absent. (b) When the current density exceeds the Goldreich-Julian value (j/j(sub GJ) > 1), the system develops high voltage drops (TV or greater), causing emission of curvature gamma-rays and intense bursts of pair creation. The bursts exhibit limit cycle behaviour, with characteristic time-scales somewhat longer than the relativistic fly-by time over distances comparable to the polar cap diameter (microseconds). (c) In return current regions, where j/j(sub GJ) < 0, the system develops similar bursts of pair creation. These discharges are similar to those encountered in previous calculations by Timokhin of pair creation when the surface has a high work function and cannot freely emit charge. In cases (b) and (c), the intermittently generated pairs allow the system to simultaneously carry the magnetospherically prescribed currents and adjust the charge density and average electric field to force-free conditions. We also elucidate the conditions for pair creating beam flow to be steady (stationary with small fluctuations in the rotating frame), finding that such steady flows can occupy only a small fraction of the current density parameter space exhibited by the force-free magnetospheric model. The generic polar flow dynamics and pair creation are strongly time dependent. The model has an essential difference from almost all previous quantitative studies, in that we sought the accelerating voltage (with pair creation, when the voltage drops are sufficiently large; without, when they are small) as a function of the applied current.

Timokhin, A. N.↗

NASA NTRS1987

The theory of ionospheric focused heating

Ionospheric modification by high power radio waves and by chemical releases are combined in a theoretical study of ionospheric focused heating. The release of materials which promote electron-ion recombination creates a hole in the bottomside ionosphere. The ionospheric hole focuses high power radio waves from a ground-based transmitter to give a 20 dB or greater enhancement in power density. The intense radio beam excites atomic oxygen by collisions with accelerated electrons. Airglow from the excited oxygen provides a visible trace of the focused beam. The large increase in the intensity of the radio beam stimulates new wave-plasma interactions. Numerical simulations show that the threshold for the two-plasmon decay instability is exceeded. The interaction of the pump electromagnetic wave with the backward plasmon produces a scattered electromagnetic wave at 3/2 the pump frequency. The scattered wave provides a unique signature of the two-plasmon decay process for ground-based detection.

Bernhardt, P. A.↗

NASA NTRS1990

Ion Beam Heating in the Auroral Zone

Recent satellite observations at high altitudes (greater than 5000 km) in the auroral zone have shown the existence of hybrid or bimodal ion beam distributions that are evidence of both parallel and perpendicular ion acceleration. Acceleration parallel to the magnetic field is most likely due to quasi-static electric fields (double layers) which can create outflowing ion beams; since ions of different mass will have different drift speeds due to this acceleration, a plasma configuration unstable to the ion-ion two-stream acoustic mode develops. When the net drift velocity (U) between the two ion species is greater than the sound speed (C(sub 0)), the ion-ion instability has maximum growth at oblique wave propagation. To study the nonlinear effects of the ion-ion instability in terms of plasma heating, a numerical simulation parametric study has been performed. It was found that the parallel acceleration that forms the ion beams occurs on a time scale faster than ion- ion wave growth at low drifts; thus ion-ion wave growth is expected to occur primarily for higher drift speeds (U greater than C(sub 0)) which results in strong oblique heating of the ions (both hydrogen and oxygen) forming elevated ion conics (sometimes called 'bowl' distributions). Also, strong parallel electron heating in the direction of the ion beams can occur, and electrons near the top of the acceleration region may attain a net upward drift along with the elevated ion conics. Variation of the oxygen density greatly affects the ion heating due to the ion-ion instability; as the oxygen density decreases, oxygen heating increases, in agreement with observations (Collin et al., 1987). Ion-ion electrostatic wave properties and the plasma heating that results over a wide range of auroral zone parameters are included.

Schriver, David↗

NASA NTRS1976

Fifteen cm mercury ion thruster research, 1976

Improvements in 15 cm diameter, SERT II, mercury ion thruster performance effected by the use of SHAG optics at 33 V discharge voltage were discussed. At a 200 eV/ion discharge power, 90 percent propellant utilization and 660 mA beam current condition a doubly-to-singly charged ion current ratio of about 4 percent was measured. Performance of the 15 cm multipole mercury thruster (optimized for length and the point of electron injection) was compared to that of divergent (SERT II) and cusped field designs and found to be comparable. The need for a magnetic baffle in the multipole thruster was identified and the preferred point of electron injection was at the upstream end of the discharge chamber. Results of preliminary tests on the effects of discharge voltage and total accelerating voltage on perveance and beam divergence characteristics of two grid ion optics were examined. Experimental data showing the effect of target temperature on sputtering rates in a mercury discharge environment were presented and a deficiency in the tests procedure was identified.

Wilbur, P. J.↗

NASA NTRS2016

Growth and Saturation of Ion Acoustic Waves in Hall Thrusters

In this paper we present for the first time a conjecture that the electron transport in Hall thrusters is controlled by ion acoustic waves in two distinct ion populations. The first, and largest population, is the main beam, an ion fluid generated upstream of the Acceleration Region. These ions experience electrons with largest ExB drift, but because beam ions have been heated during the ionization process, Landau damping limits the growth of ion acoustic waves. The second population is composed of ions generated at low potentials farther downstream that don’t thermalize with the fast moving ion beam. The existence of this separate population has been recognized for decades as the source of the high angle ions in Hall thruster plumes. Because these ions aren’t heated and have velocities much different from main beam ions, the amplitude of ion acoustic waves in this population is not Landau damped and grows to saturation. Scattering from these waves reduces the Hall parameter in the near plume where slow ions make up a significant fraction of the population.

Katz, Ira↗

NASA NTRS1986

A comparison of plasma waves produced by ion accelerators in the F-region ionosphere

Ion beams injected into the ionosphere are known to produce waves related to the normal modes of the plasma. The spectra of plasma waves produced during four sounding rocket experiments are examined. The experimental conditions were somewhat different during each experiment. The accelerated ion was either Xe(+) or Ar(+) and the experimental geometry, described by the separation vector between the plasma wave receiver and the ion accelerator, was either parallel or perpendicular to the geomagnetic field.

Kintner, P. M.↗

NASA NTRS1979

The screen hole plasma sheath of an ion accelerator system

Results of the first probing of the screen hole sheath of an ion accelerator system are presented. The screen hole sheath, represented as a set of equipotential contours, extends over a large distance within the discharge plasma. Under no conditions examined does the sheath enter the screen hole. Edge hole defocusing of multiaperture accelerator systems is due primarily to local plasma density variations rather than adjacent screen hole sheath interactions. The sheath boundary is independent of screen-to-accelerator grid spacing when the grid set is operated at the minimum ion beam divergence condition. Significant ion focusing effects occur in the sheath adjacent to the screen grid webbing leading to increased ion source beam current efficiency with decreasing screen-to-accelerator grid separation and/or screen grid thickness.

Aston, G.↗

NASA NTRS2011

The Impact of Return-Current Losses on the Observed Emissions from Solar Flares

Electrons accelerated in solar flares are expected to drive a co-spatial return current in the ambient plasma when they escape the acceleration region. This return current maintains plasma neutrality and the stability of the beam of streaming electrons. The electric field that drives this return current also decelerates the energetic electrons in the beam. The corresponding energy loss experienced by the accelerated electrons can affect the observed properties of the X-ray and radio emissions from flares and the evolution of the thermal flare plasma. I will discuss the properties of the flare emissions expected in a classical, steady-state model. As part of this discussion, I will examine Gordon Emslie's 1980 conjecture that return-current losses result in a maximum brightness for the hard X-ray emission from flares.

Holman, Gordon D.↗

NASA NTRS1989

Plasma/Neutral-Beam Etching Apparatus

Energies of neutral particles controllable. Apparatus developed to produce intense beams of reactant atoms for simulating low-Earth-orbit oxygen erosion, for studying beam-gas collisions, and for etching semiconductor substrates. Neutral beam formed by neutralization and reflection of accelerated plasma on metal plate. Plasma ejected from coaxial plasma gun toward neutralizing plate, where turned into beam of atoms or molecules and aimed at substrate to be etched.

Langer, William↗

NASA NTRS1980

Artificial aurora conjugate to a rocket-borne electron accelerator

An accelerator intended to send electron beams upward along an L = 1.24 magnetic field line was flown from a rocket launched from Kauai, Hawaii, on October 15, 1972. Though the intent was to produce several hundred observable auroral streaks in the Southern Hemisphere, imaging instruments operated there aboard jet aircraft detected only a single aurora. Produced by a 0.155-A beam of energy 22.8 keV, the aurora was of expected brightness and had a diameter (210 + or - 50 m) somewhat larger than expected and an altitude (top 116 + or - 2 km; bottom 92 + or - 2 km) higher than expected.

Davis, T. N.↗

NASA NTRS1984

Chaotic ion motion in magnetosonic plasma waves

The motion of test ions in a magnetosonic plasma wave is considered, and the 'stochasticity threshold' of the wave's amplitude for the onset of chaotic motion is estimated. It is shown that for wave amplitudes above the stochasticity threshold, the evolution of an ion distribution can be described by a diffusion equation with a diffusion coefficient D approximately equal to 1/v. Possible applications of this process to ion acceleration in flares and ion beam thermalization are discussed.

Varvoglis, H.↗

NASA NTRS1986

Plasma processes driven by current sheets and their relevance to the auroral plasma

Plasma processes dealing with ac and dc electric fields, the formation of ion beams and conics, and electron acceleration are considered, and similarities between simulation results and satellite-based observations are discussed. Electrostatic shock-type electric fields are found to occur near the current sheet edges, and double layers having upward electric fields form inside the sheet and are distinguishable from the large perpendicular electric fields only in wide sheets with thicknesses much greater than the ion Larmor radius. It is found that the most energetic ions have pitch angles near 90 deg, indicating a large perpendicular acceleration of the ions, and that the downward accelerating electrons inside the sheet are neither monoenergetic nor perfectly field aligned.

Singh, Nagendra↗

NASA NTRS1987

Advanced electric propulsion and space plasma contactor research

A theory of the plasma contacting process is described and experimental results obtained using three different hollow cathode-based plasma contactors are presented. The existence of a sheath across which the bulk of the voltage drop associated with the contacting process occurs is demonstrated. Test results are shown to agree with a model of a spherical, space-charge-limited double sheath. The concept of ignited mode contactor operation is discussed, which is shown to enhance contactor efficiency when it is collecting electrons. An investigation of the potentials in the plasma plumes downstream of contactors operating at typical conditions is presented. Results of tests performed on hollow cathodes operating at high interelectrode pressures (up to about 1000 Torr) on ammonia are presented and criteria that are necessary to ensure that the cathode will operate properly in this regime are presented. These results suggest that high pressure hollow cathode operation is difficult to achieve and that special care must be taken to assure that the electron emission region remains diffuse and attached to the low work function insert. Experiments conducted to verify results obtained previously using a ring cusp ion source equipped with a moveable anode are described and test results are reported. A theoretical study of hollow cathode operation at high electron emission currents is presented. Preliminary experiments using the constrained sheath optics concept to achieve ion extraction under conditions of high beam current density, low net accelerating voltage and well columniated beamlet formation are discussed.

Wilbur, Paul J.↗

NASA NTRS1988

A predicted energetic ion signature of a neutral line in the geomagnetic tail

Under certain assumptions one can predict the formation of a magnetic neutral line on theoretical grounds, generally as a result of an unstable current sheet. Neutral points also seen to form spontaneously in a wide variety of plasma simulatioons, from MHD to fully kinetic. Hence, such magnetic null regions are central to some models of magnetotail energization and magnetic substorms. Yet the observational evidence for such regions remains controversial. This paper presents a new signature of a neutral line: the field-aligned ion beam characteristic of current sheet acceleration becomes dispersed into a 'ridge' in the velocity space distribution function as a neutral line is approached. The results were obtained using single-particle simulations in a fixed magnetic field model. The origin of the ridge is shown to be due to initially earthward going ions which pass through the neutral line region. These ions come from a smaller part of the initially antiearthward flowing distribution, generating the depletion of the distribution function below the ridge.

Martin, R. F., Jr.↗