Artificial Electron Beams Research Articles

Slice emittance measurements using a slit-grid system and a fast wall-current monitor.

The time evolution of beam properties in an electron bunch with the duration of a nanosecond was measured with a time resolution of several tens of picoseconds. A combination of horizontal and vertical slits cuts the beamlet from the original beam, with the current waveform of the beamlet measured using a fast wall-current monitor. The reconstruction of the waveform data obtained by scanning these two slits over the entire beam area provided the time evolution of the spatial profile. A similar measurement using two horizontal (vertical) slits separated by a certain distance also provides the time evolution of the phase-space profile. Using this method, the initial beam extracted from the CeB6 thermionic electron gun of the x-ray free-electron laser (XFEL) SACLA was evaluated. Although the slice emittance in the bunch was measured to be constant, the centroid of the spatial profile moved in the transverse direction by a few hundred micrometers in the 0.6ns flat-top region. This movement arises from the temporal variation in the rectangular high-voltage pulse of the beam chopper and can cause an increase in the projected emittance. These measurements are important for evaluating the conditions of the initial beam emitted from the cathode and processed downstream of the gun. Hence, the proposed diagnostic system will play an important role in developing an extremely low-emittance electron beam or an artificial electron beam with a multi-bunch or micro-bunch structure that enhances the brightness of the XFEL light.

Fast Spacecraft Charging Mitigation With Plasma Contactors During High‐Power Electron Beam Emission in the GEO Environment

AbstractThe emission of artificial electron beams causes an active spacecraft charging effect that poses challenges for electron beam sounding experiments. Plasma contactors have been suggested as a potential solution to this issue based on theoretical and simulation studies, but the impact of these contactors on spacecraft potentials in complex space environments remains unexplored. This paper investigates the fast active charging effect of spacecraft under the action of plasma contactors in the geostationary Earth orbit environment by using a two‐dimensional Particle‐in‐Cell model. We analyze and compare the effects of the “ion emission” process of the plasma contactor and the “electron collection” process of the background plasma on the spacecraft potential at different beam and environment parameters, as well as the coupling effects when both processes occur simultaneously. Our findings indicate that the “ion emission” and “electron collection” processes are the primary factors in determining the spacecraft potential in relatively thin and dense plasma environments, respectively. When the two processes occur together, the contactor plasma cloud increases the “electron collection” return current by expanding the range of the positive potential area around the spacecraft, while the dense background plasma mediates and provides the return current to ensure that the plasma contactor can effectively suppress the active charging effect even after the ion sheath is present. These results suggest that a reasonable adjustment of the contactor parameters based on the relationship between the electron beam and the background environment can effectively extend the normal emission time of the electron beam.

Physical properties of relativistic electron beam during long-range propagation in space plasma environment* *Project supported by the Joint Funds of the National Natural Science Foundation of China (Grant Nos. 61372050 and U1730247).

It is known that ion channel can effectively limit the radial expansion of an artificial electron beam during its long-range propagation in the space plasma environment. Most prior studies discussed the focusing characteristics of the beam in the ion channel, but the establishment process and transient properties of the ion channel itself, which also plays a crucial role during the propagation of the relativistic electron beam in the plasma environment, were commonly neglected. In this study, a series of two-dimensional (2D) particle-in-cell simulations is performed and an analytical model of ion channel oscillation is constructed according to the single-particle motion. The results showed that when the beam density is higher than the density of plasma environment, ion channel can be established and always continues to oscillate periodically over the entire propagation. Multiple factors, including the beam electron density, initial beam radius, and the plasma density can affect the oscillation properties of ion channel. Axial velocity of the beam oscillates synchronously with the ion channel and this phenomenon will finally develop into a two-stream instability which can seriously affect the effective transport for relativistic electron beam. Choosing appropriate beam parameters based on various plasma environments may contribute to the improvement of the stability of ion channel. Additionally, radial expansion of the beam can be limited by ion channel and a stable long-range propagation in terrestrial atmosphere may be achieved.

Influence of Geomagnetic Field on the Long-Range Propagation of Relativistic Electron Beam in the Atmosphere

It is known that Earth’s atmosphere and geomagnetic field affect the propagation of an artificial electron beam; many previous studies have neglected the effect of their variation on the propagation dynamics. In this article, parameters in envelope equations were modified according to the International Geomagnetic Reference Field (IGRF) model and MSIS-90 atmosphere model; then, the model was constructed to study the propagation of artificial electron beam in the atmosphere. Calculation of the beam’s energy loss and radial expansion in propagation showed that the variation of geomagnetic field intensity can be the main reason for the radial pinch in the altitudes above 150 km, and the ambient particle density plays a dominant role in the beam’s radial expansion in altitudes below 150 km. The Monte Carlo model was used to simulate the interactions of a relativistic (1 MeV) electron beam with the atmosphere in the altitude range 100–150 km. Energy deposition density demonstrates that the geomagnetic field can limit the radial expansion of the beam. The relativistic electron beam was emitted horizontally in the atmosphere. The geomagnetic field can limit the beam’s radial expansion and cause the beam to propagate along the field line, irrespective of the direction in which the beam is emitted.

Diagnostics of an artificial relativistic electron beam interacting with the atmosphere

AbstractWe use a Monte Carlo model to simulate the interaction of a beam of relativistic (0.5–10 MeV) electrons with the upper atmosphere as they are injected downward from a notional high‐altitude (thermospheric/ionospheric) injection platform. The beam parameters, defined by realistic parameters of a compact linear accelerator, are used to create a distribution of thousands of electrons. Each electron is injected downward from 300 km altitude toward the dense atmosphere, where it undergoes elastic and inelastic collisions, leading to secondary ionization, optical emissions, and X‐rays via bremsstrahlung. In this report we describe the model initialization (i.e., development of the electron distribution), essential features of the Monte Carlo model, and secondary outputs, including optical emissions, X‐ray fluxes, secondary ionization, and backscattered energetic electron fluxes. Optical emissions are propagated to the ground through the lower atmosphere, including the effects of atmospheric absorption and scattering, to estimate the brightness of the emission column for a given beam current and energy. Similarly, X‐ray fluxes are propagated to hypothetical detectors on balloons and satellites. Secondary ionization is used to estimate the radar signal returns from various ground‐based radar facilities. Finally, simulated backscattered electron fluxes are measured at the injection location. The simulation results show that each of these diagnostics should be readily detectable by appropriate instruments.

Electric Fields and Magnetic Fields in the Plasmasphere: A Perspective From CLUSTER and IMAGE

The electric field and magnetic field are basic quantities in the plasmasphere measured since the 1960s. In this review, we first recall conventional wisdom and remaining problems from ground-based whistler measurements. Then we show scientific results from Cluster and Image, which are specifically made possible by newly introduced features on these spacecraft, as follows. 1. In situ electric field measurements using artificial electron beams are successfully used to identify electric fields originating from various sources. 2. Global electric fields are derived from sequences of plasmaspheric images, revealing how the inner magnetospheric electric field responds to the southward interplanetary magnetic fields and storms/substorms. 3. Understanding of sub-auroral polarization stream (SAPS) or sub-auroral ion drifts (SAID) are advanced through analysis of a combination of magnetospheric and ionospheric measurements from Cluster, Image, and DMSP. 4. Data from multiple spacecraft have been used to estimate magnetic gradients for the first time.

Electron temperature variation associated with the auroral energy input during the DELTA campaign

Abstract Japanese sounding rocket “S-310-35” was launched from Andøya Rocket Range in Norway on December 13, 2004 during Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign, in which the rocket-borne in-situ measurements and ground-based measurements were coordinated to conduct a comprehensive observation of the upper atmospheric response against the auroral energy input. The Fast Langmuir Probe (FLP) was installed on the sounding rocket to study thermal structure and energy balance of the plasma by measuring the electron temperature in the polar lower ionosphere. The FLP observations indicate that the electron temperatures were found to be remarkably high in an altitude range from 106 km to 114 km during the ascending phase of the rocket. The lowest part of this high temperature region might be affected by artificial electron beam which was generated by the N2 temperature instrument on the same rocket. On the other hand, a small increase of the electron temperature was observed at the altitude from 114 to 119 km in the descending phase. This is possibly the first time that both the temperature increase and density fluctuation that may be caused by the Farley-Buneman instability were detected by in-situ observation.

On the performance of a cylindrical dipole antenna in diagnostics of wave phenomena in space plasma

There is a pronounced, permanent, variable frequency banded enhancement in high frequency spectra recorded in the ionospheric electron plasma on APEX and CORONAS-I. Spectral characteristics of this banded structure in diverse geophysical regions are discussed. The local plasma density derived with independent techniques is crucial for the interpretation of the banded enhancement. The quasi-simultaneous local plasma density is derived from the antenna impedance measurement, artificial electron beam induced spectra and topside sounder data. Equivalent circuit simulations of wave and impedance measurements reveal spectral signatures of cold plasma density, which match both types of data recorded under stationary conditions. The major spectral characteristics of the banded enhancement are also consistent with equivalent circuit simulations. The data and simulations suggest that the upper hybrid band may be misinterpreted as a circuit resonance. Circuit resonance can be neatly controlled to provide very low interference technique to monitor the density of cold plasma. Circuit resonance monitoring, if properly coupled with impedance measurements and electron beam injection may be a very useful low interference tool for diagnostics of highly structured plasmas like those in the auroral region.

Dynamics and relaxation of an artificial electron beam

We present results of analytical and numerical investigation of the motion of electron beam injected into the ionosphere from spacecraft. We show that the development of beam driven instability results the formation of a virtual cathode, where the ambient plasma is turbulently heated and the beam is relaxed on longitudinal velocities.

The stratosphere active experiments program with artificial electron beam injection

Active experiments with artificial electron beam injection from balloons within the stratosphere altitude range of 30–45 km are proposed to explore this atmospheric region. For a wide spectrum of science interests a pulse injector was developed and tested with main parameters: energy of beam electron ≥50 keV; pulse beam power ≤1 MW; pulse duration −10÷15 μs; average power −1 kW; frequency of pulse beam repetition — up to 200 Hz.

Observations of the beam-plasma interaction during the APEX artificial electron beam emission

During the APEX project the pulsed electron generator injected an intensive 8-kV electron beam at different angles to the Earth's magnetic field. Multiangular electrostatic analyzers (PEAS spectrometer) registered the flux of electrons and ions in the energy range from 50 eV to 30 keV in two perpendicular planes. The satellite potential was balanced by a low energy Xenon plasma generator during the electron beam injection. The observation of the charged particle distribution was accompanied by the measurement of the spacecraft potential by the floating probe. The energy spectrum of returning electrons exhibits a pronounced maximum near 1 keV. The electron distribution is not gyrotropic and the energy and number flux of the main electron population decreases with pitch angle. These two observational facts suggest a space-limited perpendicular acceleration process. The accelerated electrons are observed from both sides of the satellite with respect to the magnetic field, so the process is not caused directly by interaction of the primary beam and the ambient plasma.

Artificial electron and ion beam effects: Active Plasma Experiment

The Active Plasma Experiment uses intensive electron beam emission for the study of dynamic processes in the magnetosphere and upper ionosphere. The beam energy and current are as high as 8 keV and 100 mA and the pitch angle of the emission varies in the range from 50° to 80°. The basic cycle of electron injection is formed by current pulses of different duration, intensity, and frequency. The spacecraft potential is balanced by a low‐energy xenon plasma generator during the electron beam injection. The spacecraft potential is measured by the floating probe, and the response of the environment is studied by the charged particle spectrometer working in the energy range from 0.05 to 25 keV. During the neutral or ionized Xe release without the electron gun operation the spacecraft potential remains nearly unchanged and the observed energy spectra of charged particles do not exhibit the presence of any acceleration process. The spacecraft potential during electron beam emission does not exceed 50 V if the Xe plasma or the neutral gas was released together with the electrons. The electron gun firing creates a disturbance which produces a broad spectrum of energetic electrons extending up to 1.5 keV. The acceleration process can be explained by the introduction of the electric field with intensity of about 100 V/m. This intensity is in agreement with the observed E × B drift velocity. The spatial extent of the disturbance is established to be tens of meters.

The SEPAC artificial aurora

The Space Experiments with Particle Accelerators (SEPAC) and the Atmospheric Emission Photometric Imager (AEPI) were flown on the ATLAS‐1 shuttle mission in the spring of 1992. In a series of artificial aurora experiments, SEPAC injected electron beam pulses (6.25 keV, 1.2 A) into the Earth's upper atmosphere while the AEPI camera recorded optical emissions in white light continuum and 4278 Å. The intensity, overall size, and structure of the artificial auroras have been compared to results from a two‐stream code which models collisional and transport properties of natural and artificial electron beams. Using the curvature of the magnetic field to resolve the altitude dependence of observed optical emissions, it is found that the beam reaches 110–130 km altitude and excites emissions at intensities that are consistent with model calculations (1 kR). However, it is also concluded that the near‐field intensity (5 kR) exceeds model estimates by one order of magnitude. It is argued that this unexpected enhancement is caused by emissions generated within the first 10's of kilometers of the Orbiter by suprathermal electron fluxes at levels that are orders of magnitude larger than predicted by the purely collisional model. It is suggested that the enhanced flux is due to wave‐particle interactions heating the ambient plasma.

Semi-relativistic maser cyclotron instabilities: can active experiments help to understand AKR?

Injection of an artificial energetic electron beam into the ionosphere during the PICPAB experiment onboard Spacelab-1 in 1983 has been found to trigger electromagnetic waves near the electron gyrofrequency. While the role of the primary beam-cold plasma interaction is to initiate the whole process by giving the neutralization return beam a loss-cone-like distribution, dispersion relation calculations based on a numerical semi-relativistic model developed from Rönnmark's WHAMP (Kiruna Geophysical Institute Report No. 179, Kiruna, Sweden, 1982), show that the observed waves can be adequately described as maser emissions generated in the relativistic mode through a return beam-cold plasma interaction, which almost solely determines the growth rate level. Similarities with AKR observations are stressed, and offer a good opportunity to gain some knowledge about natural phenomena by means of active experiments.

Artificial auroras in the upper atmosphere 1. Electron beam injections

Artificial electron beams from the Space Experiments with Particle Accelerators (SEPAC) on the ATLAS 1 Spacelab payload were used to stimulate auroral emissions at southern auroral latitudes. The emitted electron beams were monoenergetic at 6.25 keV and were fired in one‐second pulses every fifteen seconds with currents of 1.21 A. Optical measurements of the beam were made in the vicinity of the Shuttle Orbiter by its on‐board television camera and in the upper atmosphere by the Atmospheric Emissions Photometric Imager (AEPI). AEPI imaged auroral emissions in both white light and at the 427.8 nm N2+ emission line. Energy deposition calculations and the results of previous sounding‐rocket experiments had suggested that emissions with scale sizes of about 130 meters would result from the artificial electron beams with the visible emissions extending from about 110 to 130 km altitudes. In the ATLAS 1 experiments the auroral imaging was performed from the Shuttle, providing a new perspective on the artificial auroras and allowing the emissions to be traced from altitudes near the 295 km Shuttle altitude down to the 110 km level along the curved magnetic field lines.

Controlled experiments in the earth's magnetosphere with artifical electron beams

This article describes the Electron Echo project, which used artificial electron beams to determine how natural particles are accelerated and lost from the trapping region in the earth's magnetosphere. The problems of injecting electron beams into the ionosphere, including vehicle charging and beam-plasma instabilities, are discussed, and images of the Echo beams both in space and in the laboratory are shown. The beam pulses were detected and analyzed after reflection from the conjugate hemisphere mirror points. Although the injected beams contained a wide spread of energies, the echoes displayed a monoenergetic peak, which together with the exact drift displacement during one bounce permitted a separate evaluation of electric and magnetic components of the transverse drifts in the distant magnetosphere. Thus distant electric fields were measured and found to have large turbulent fluctuations compared to the local ionospheric fields, and to differ in average value from the local fields by about 50 mV/m, indicating the existence of parallel potential drops. The measured bounce times and field-line lengths were compared with model magnetic geometries. The best fit was with the Tsyganenko-Usmanov 1982 version. Pitch-angle diffusion out of the loss cone resulted in a large loss in beam intensity during one bounce, showing the presence of important beam perturbations near the electron gyrofrequency range. Thus the Echo experiments evaluated many effects on the natural trapped particles, but were limited to a moderately disturbed condition of the magnetosphere when the magnetic morphology was stable.

Parallel electric fields detected via conjugate electron echoes during the Echo 7 sounding rocket flight

Electron detectors on the Echo 7 active sounding rocket experiment measured “conjugate echoes” resulting from artificial electron beam injections. Analysis of the electrons' drift motion after a complete bounce led to measurements of the magnetospheric convection electric field mapped to ionospheric altitudes. The magnetospheric field was highly variable, changing by tens of millivolts per meter on time scales of as little as hundreds of milliseconds. Sets of double probes on the payload measured the ionospheric electric field directly. While the smallest‐scale magnetospheric field irregularities were damped out by ionospheric conductivity, larger‐scale features were enhanced by up to 50 mV/m in the ionosphere. The mismatch between magnetospheric and ionospheric convection fields indicates a violation of the equipotential field line condition, i.e., the existence of parallel electric fields above the payload. The parallel fields occurred in regions ≈10 km across and probably supported a total potential drop of 10‐100 V. The flight took place in the premidnight diffuse auroral region and therefore was likely in a region of downward field‐aligned current. There is some evidence that the parallel fields also pointed downward and so could be responsible for creating the field‐aligned current by drawing electrons out of the ionosphere.

Packets of cyclotron waves induced by electron beam injection from the space shuttle 1. Linear theory

Measurements of high‐frequency wave emissions were made on board Spacelab 1 during artificial electron beam injection from the shuttle bay. The most unusual kind of spectrum is considered here, i.e., packets of electron cyclotron waves occurring at successive harmonics of the plasma frequency. Using a linear theory, it is shown that the emissions consist of the well‐known electronic Bernstein modes, which are thought to be generated by an interaction between the neutralization return current and the ambient ionospheric plasma, and not directly by the primary beam. A simplified model of the electron velocity distribution function of the return current is developed. Then, a numerical resolution of the dispersion equation is undertaken, with the experimental constraint that the waves have to fit the “rendezvous” condition with the instruments flying on board the orbiter. The observed electron cyclotron emissions, as well as their amplitude modulation around the fundamental of the plasma frequency, are found to be easily explained by such a linear theory. A companion paper [Mourenas and Béghin, this issue] will propose a nonlinear treatment of the same mechanism, which is thought to be responsible for the production of upper harmonic packets.

The interaction of an artificial electron beam with the Earth's upper atmosphere: Effects on spacecraft charging and the near‐plasma environment

The ionization created in the Earth's upper atmosphere by an electron beam emitted from a low altitude spacecraft is described by two coupled nonlinear differential electron transport equations for the up‐going (along a magnetic field line) and down‐going differential energy flux. The equations are solved numerically, using the mass spectrometer and incoherent scatter data (MSIS) atmospheric model and the international reference ionosphere (IRI) model, yelding estimates of the differential electron energy flux density at the spacecraft location. At altitudes below 200–250 km and for beam energies around 1 keV, it is shown that secondary electrons supply a significant contribution to the return current to the spacecraft and thereby reduce the spacecraft potential. Our numerical results are in good agreement with observations from the Charge 2 sounding rocket experiment.

Spacelab 2 electron beam wave stimulation: Studies of important parameters

The Spacelab 2 space shuttle mission included experiments on the production of waves in the ionospheric plasma by the injection of pulsed electron beams. The Spacelab 2 mission took place during July and August of 1985 and provided an extensive data set which continues to yield new results. The experimental results reported here were obtained with the combined use of the University of Iowa plasma diagnostics package (PDP) and the Stanford/Utah State vehicle charging and potential experiment (VCAP). To date most of the analysis has focused on the sequences performed during the release of the PDP as a free‐flying satellite. However, over 300 beam sequences were conducted with the PDP mounted in the orbiter's payload bay. The results of these experiments provide important new information which helps answer some of the outstanding questions which remain. The wave environments which exist in the orbiter payload bay and at several hundred meters away are compared both during ambient conditions and during electron beam injections. The dependence of beam‐generated wave characteristics on the duty cycle and the frequency of the pulsed beam are investigated. The duty cycle of the beam is found to be one of the most important parameters affecting the amplitude and the harmonic structure of the waves. Maximum amplitudes are produced by 50% duty cycles and amplitudes are proportional to instantaneous, rather than average, beam current. Harmonic structure also provides insight into the spatial integrity of the propagating beam. The dependence on pulsing frequency is found to show good agreement with theory for whistler mode waves produced through the Cherenkov resonance confirming previous results. Theoretical predictions for the dependence of wave amplitudes on parameters which could not be experimentally investigated are also provided. The results of both the experimental and theoretical studies have important practical consequences for experiments attempting to produce and detect propagating radiation using artificial electron beams.