Structure of electron-beam generated plasma
Electron beam generated plasma behavior, showing electron pressure local nonisotropy capability for generating supersonic-subsonic transitions
Bienkowski, G. K.↗
SEARCH · Engineering Papers
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Particle-in-cell modeling of electron beam generated plasma
Plasmas generated using energetic electron beams are well known for their low electron temperature (T e ) and plasma potential, which makes them attractive for atomic-precision plasma processing applications such as atomic layer etch and deposition. A 2-dimensional particle-in-cell model for an electron beam-generated plasma in argon confined by a constant applied magnetic field is described here in this article. Plasma production primarily occurs in the path of the beam electrons in the center of the chamber. The resulting plasma spreads out in the chamber through non-ambipolar diffusion with a short-circuiting effect allowing unequal electron and ion fluxes to different regions of the bounding conductive chamber walls. The cross-field transport of the electrons (and thus the steady-state characteristics of the plasma) are strongly impacted by the magnetic field. T e is anisotropic in the electron beam region, but low and isotropic away from the plasma production zone. The plasma density increases and the plasma becomes more confined near the region of production when the magnetic field strengthens. The magnetic field reduces both electron physical and energy transport perpendicular to the magnetic field. T e is uniform along the magnetic field lines and slowly decreases perpendicular to it. Electrons are less energetic in the sheath regions where the sheath electric field repels and confines the low-energy electrons from the bulk plasma. Even though electron and ion densities are similar in the bulk plasma due to quasi-neutrality, electron and ion fluxes on the grounded chamber walls are unequal at most locations. Electron confinement by the magnetic field weakens with increasing pressure, and the plasma spread out farther from the electron beam region.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Beam Current Effect on Microstructure and Properties of Electron-Beam-Melted Ti-6Al-4V Alloy
In this study, a noncommercial 3D printing machine was used to fabricate Ti-6Al-4V alloy by electron-beam melting (EBM). The influence of beam current on the microstructure, phase composition and mechanical properties of electron-beam-melted Ti-6Al-4V alloy was investigated. Numerical simulation is implemented to evaluate thermal fields during electron-beam melting of Ti-6Al-4V powder. The decrease in beam current from 3.5 to 2.5 mA leads to refinement of microstructure: The average width of α plates decreases from 10 down to 6 μm. The formation of finer microstructure is attributed to higher cooling rate at lower beam current confirmed by simulation. The phase composition of EBM Ti-6Al-4V indirectly depends on the beam current. High content of β phase (7%) was achieved at the beam current of 3 mA. The produced Ti-6Al-4V samples are characterized by high microhardness (470-520 HV).
36 MATERIALS SCIENCE↗
Metastable atom probe for measurement of electron beam density profiles
Electron beam current density measurement using neutral Ar atoms electron impact excitation to metastable states
Lockhart, J. M.↗
Electrostatic ion cyclotron, beam-plasma, and lower hybrid waves excited by an electron beam
It is pointed out that electrostatic ion cyclotron (EIC) waves have been extensively investigated in connection with both space and laboratory plasmas. The present investigation has the objective to study the excitation of low-frequency waves in a multiion plasma by electron beams. The frequencies considered range from below the lowest gyrofrequency of the heaviest ion to about the lower hybrid frequency. It is shown that electron-beam instabilities can produce peaks in the growth rate below the cyclotron frequency of each ion species if nonzero perpendicular wave number effects are included in the ion dynamics. The dispersion relations for neutralized ion Bernstein (NIB) and pure ion Bernstein (PIB) waves are considered along with an instability analysis for a cold plasma and warm electron beam, the electron beam-plasma mode, banded ion cyclotron (EIC) waves with small perpendicular wavelengths, and the growth lengths of the waves.
Singh, N.↗
A kinetic description of electron beam ejection from spacecraft
A model based on the Boltzmann kinetic equation describing electron beam ejection from spacecraft is presented. Results are shown for steady-state, beam-atmosphere interaction (BAI) and for beam-plasma interaction (BPI). The BPI considered is that of elastic scattering of beam electrons with strong, short-scale Langmuir turbulence. This mechanism has been suggested for prompt echoes of beam electrons observed in sounding rocket experiments. It is shown that the interaction is sufficiently strong to account for observations of back-scattered electrons at the beam accelerator energy. However, it is clear from observations that particles undergo diffusion in energy at a rate exceeding levels expected for BAI. Inelastic scattering in BPI must therefore also be important.
Khazanov, George V.↗
Injection of electron beams into two laser wakefields and generation of electron rings
Throughout this work, mutual injection of electron beams into two laser plasma wakefields was observed experimentally when driving laser pulses interfered in plasma at a small crossing angle and were slightly relatively delayed, approximately by one pulse duration. Particle-in-cell simulations revealed that the mutual injection was sensitive to the spatial overlap of the laser pulses, which therefore could be used to control the mutual injection. The dual synchronized, femtosecond electron beams are potentially useful for pump-probe experiments in ultrafast science. In addition, out-of-axis ring-shaped electron beams were detected in both experiments and simulations.
47 OTHER INSTRUMENTATION↗
Disabling CNT Electronic Devices by Use of Electron Beams
Bombardment with tightly focused electron beams has been suggested as a means of electrically disabling selected individual carbon-nanotubes (CNTs) in electronic devices. Evidence in support of the suggestion was obtained in an experiment in which a CNT field-effect transistor was disabled (see figure) by focusing a 1-keV electron beam on a CNT that served as the active channel of a field-effect transistor (FET). Such bombardment could be useful in the manufacture of nonvolatile-memory circuits containing CNT FETs. Ultimately, in order to obtain the best electronic performances in CNT FETs and other electronic devices, it will be necessary to fabricate the devices such that each one contains only a single CNT as an active element. At present, this is difficult because there is no way to grow a single CNT at a specific location and with a specific orientation. Instead, the common practice is to build CNTs into electronic devices by relying on spatial distribution to bridge contacts. This practice results in some devices containing no CNTs and some devices containing more than one CNT. Thus, CNT FETs have statistically distributed electronic characteristics (including switching voltages, gains, and mixtures of metallic and semiconducting CNTs). According to the suggestion, by using a 1-keV electron beam (e.g., a beam from a scanning electron microscope), a particular nanotube could be rendered electrically dysfunctional. This procedure could be repeated as many times as necessary on different CNTs in a device until all of the excess CNTs in the device had been disabled, leaving only one CNT as an active element (e.g., as FET channel). The physical mechanism through which a CNT becomes electrically disabled is not yet understood. On one hand, data in the literature show that electron kinetic energy >86 keV is needed to cause displacement damage in a CNT. On the other hand, inasmuch as a 1-keV beam focused on a small spot (typically a few tens of nanometers wide) deposits a significant amount of energy in a small volume, the energy density may suffice to thermally induce structural and/or electronic changes that disable the CNT. Research may be warranted to investigate this effect in detail.
Petkov, Mihail↗
DEVELOPMENT OF A QUANTUM ELECTRON BEAM DIAGNOSTIC APPARATUS
Spatial properties of electron beams are the essential parameters needed in particle physics and accelerator research. Beam characterization using optical detection methods through the interaction between photons and electrons such as Compton scattering, or electro-optic sampling of the electric field of electrons have been extensively explored and successfully implemented as non-invasive diagnostics at various accelerator facilities. However, such methods often suffer from inherently low sensitivity. Here we present the study of a new type of electron beam diagnostic device for direct optical imaging of electron beams at various energy levels. The concept is based on the extremely high sensitivity of atoms, prepared in a specific "dark superposition" quantum state, to the external electric or magnetic perturbations induced by the passing charged particle, which allows atoms to change polarization of the probe light or absorb probe light and to fluoresce, enabling direct 3D imaging of the charged particles with high resolution. We report our recent experiment results and the design effort on a compact apparatus intended to be tested with the relativistic electron beams at Jefferson Laboratory.
Zhang, S.↗
Study of Microphonic Effects on the C100 Cryomodule for High Energy Electron Beam Accelerators
The Continuous Electron Beam Accelerator Facility (CEBAF) at Thomas Jefferson National Laboratory (JLab) is a particle accelerator which can accelerate an electron beam to relativistic speeds and apply the beam onto target samples. The C100 superconducting radio frequency (SRF) cavity is the primary accelerating structure of the C100 cryomodule, one of the many cryomodules which compose the CEBAF linear accelerator. SRF cavities are particularly sensitive to internal and external vibrations that can result in a phenomenon called microphonics which degrade the operational stability of a cryomodule. The purpose of this thesis is to investigate the significance of mechanical disturbances on the electromagnetic resonant frequency of a C100 SRF cavity. Knowledge of the mechanical resonance of the cavities and cryomodule sheds light into how these disturbances are most easily realized as deformation which causes radio frequency (RF) detuning. Three studies were conducted: the development and hammer test calibration of a Finite Element Analysis (FEA) model of a C100 cavity, the development and hammer test calibration of an FEA model of a C100 cavity string, and the hammer test of the C100-10R cryomodule at the Cryomodule Test Facility (CMTF). The cavity FEA model was found to accurately predict two modes found in two real cavities in a simply supported configuration. The cavity string FEA model leveraged the calibrated cavity FEA model but was not found to accurately predict the modal behavior of a real cavity string. Even so, the modal behavior of the cavity string inside the C100-10R cryomodule was captured during a hammer test while it was partially assembled. Finally, the C100-10R cryomodule was placed in the CMTF to study RF detuning. The RF detuning spectra during hammer hits and background noise was captured. The results of the hammer testing indicate two strong peaks at low frequencies (9-10 Hz and 22-23 Hz). These two frequencies were found to be nearly coincident to four instances of mechanical resonance found during the hammer testing done on the partially-assembled C100-10R. Because of this, these two modes are believed to contribute to RF detuning of the cryomodule. This test event also included the testing of the effectiveness of a configuration of BNNT canisters designed to act as dampers. While these tests show promising results, the lurking variables render these tests somewhat inconclusive.
Hull, Caleb James↗
Erosion of an electron-beam front in a long beam-plasma system
The front of an electron beam injected into a uniform, magnetized plasma is observed to erode rapidly because of scattering of the beam electrons via collective beam-plasma interactions. The propagation of the beam front is measured with temporal and axial resolution via beam excitation light. A broadened light front can result that propagates much slower than the injected beam particle speed.
Chan, L. Y.↗
An R&D program for milliampere spin-polarized electron beams
The electron injector driving the positron conversion target for Ce+BAF requires a spin-polarized beam with at least 1 mA of average current. While the parameters are not unusual from a beam dynamics point of view, the high average current is challenging due to the finite charge lifetime of the GaAs-based high-polarization photocathodes, even when operated in state-of-the-art load-lock photo-guns. We discuss the dominant limitations and present a systematic experimental program at the Gun Test Stand aiming to overcome them. With a multi-pronged approach involving a redesign of the photo-gun electrodes and an optimized drive laser profile, we hope to demonstrate a charge lifetime in excess of 1 kC from a high-polarization photocathode, mitigating one of the main risks in the injector design.
Bruker, Max [Thomas Jefferson National Accelerator↗
Electron Beam Could Probe Recombination Centers
Electron beam probe technique estimate electron/hole capture cross sections in semiconductors with wide band gaps. Amplitude-modulated electron beam induces short-circuit current collected by ohmic contacts. Phase shift between this current and electron-beam current measured as function of frequency. Results of measurements used to ascertain recombination rates and energy levels.
Vonroos, O.↗
Rotational temperature measurements in electron-beam excited nitrogen.
Electron beam excitation-deexcitation process for N, discussing rotational temperature measurements for free jet
Ashkenas, H.↗
Concentric semi-circular split profiling for computed tomographic imaging of electronic beams
Apparatus and method for analyzing an electron beam including a circular sensor disk adapted to receive the electron beam, an inner semi-circular slit in the circular sensor disk; an outer semi-circular slit in the circular sensor disk wherein the outer semi-circular slit is spaced from the first semi-circular slit by a fixed distance; a system for sweeping the electron beam radially outward from the central axis to the inner semi-circular slit and outer second semi-circular slit; a sensor structure operatively connected to the circular sensor disk wherein the sensor structure receives the electron beam when it passes over the inner semi-circular slit and the outer semi-circular slit; and a device for measuring the electron beam that is intercepted by the inner semi-circular slit and the outer semi-circular slit.
42 ENGINEERING↗
Welding multiple plies with an electron beam
Method for electron-beam welding of multi-ply metal sheets eliminates ply separation and minimizes porosity. Method was developed for assembling bellows made of four plies of iron/nickel alloy sheets. Method consists of making successive stitch welds with electron beam until weld seam is completely filled in and all plies have been penetrated.
Kiluk, F. J.↗
Injection of an overdense electron beam in space
A three-dimensional particle simulation model is used to study the injection and propagation of an overdense electron beam in the vicinity of a conductor in space. Beam electrons with a density of more than 100 times the ambient electron density are modeled using large-scale plasma simulations; in these simulations the surface area of the conductor is several thousands times that of the beam cross section at the injection point. The parameters of the simulations are chosen to allow the realistic simulation of active space experiments such as the Space Shuttle's Spacelab 2 electron beam mission. These simulations confirm space observations that an overdense electron beam can at least partially escape the near vicinity of the spacecraft, even in a fully ionized plasma. Once they have escaped from the vicinity of the spacecraft, these beam electrons should be able to propagate away freely until their energy is dissipated. It is suggested that such large-scale simulations could be useful in interpreting data from space experiments.
Okuda, Hideo↗
High-resolution planar electron beam induced current in bulk diodes using high-energy electrons
Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here in this paper, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 10 16 electrons cm -2 led to an approximate 35% decrease in the minority carrier diffusion length.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗