High Current Density Electron Beam Research Articles

Characterization of Pseudospark Discharge-Based Multigap Plasma Cathode Electron Source for the Generation of Short Pulsed Energetic Electron Beam

Pseudospark (PS) discharge-based devices are known as excellent source for the generation of high current density and energetic self-focused electron beam in the hollow cathode (HC) phase. In this article, short pulsed ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\!\!\!&lt; 100$ </tex-math></inline-formula> ns) and high energetic (~20 keV) electron beam has been generated and propagated (up to ~60 mm) in the drift region without using any external guiding magnetic field from the four-gap configuration of PS discharge-based plasma cathode electron (PD-PCE) source. The particle-in-cell (PIC) simulation code OOPIC Pro has been employed, whose results have shown the strong dependence of beam propagation into the drift region on the penetration and distribution of potential lines. The combined experimental and simulation investigations have also been carried for the multigap (four-gap) with wide range of external storage capacitor (40 pF–18 nF) and operating voltages (5–35 kV). The circuit parameter controls the appearance of HC phase for the energetic (50%–70% applied voltage) electron beam and conductive phase for the high current (~10 A to ~0.5 kA) electron beam. The potential distribution has clearly indicated that the electron beam with higher applied voltages can propagate more focused in the drift region. The investigations have evidently shown the generation of low energy and high current to high energy and low current electron beams suitable to cover the potential applications in the field of extreme ultraviolet (EUV)/soft X-ray radiation generation, surface modification, and microwave-terahertz radiation generation.

PIC Simulation of Pseudospark Discharge-Based Plasma Cathode Electron Source for the Generation of High Current Density and Energetic Electron Beam

A plasma cathode electron (PCE) source using single-gap to multigap pseudospark discharge (PD) for the generation of high density and energetic electron beams has been proposed based on our study with the help of OOPIC PRO simulation code. The simulation model comprises the successive high voltage breakdown and plasma discharge processes for single-gap to multigap PD-PCE sources. A combined analysis of a single gap to multigap PD-PCE source with the operating gas pressures 20–60 Pa, the applied voltages 20–35 kV, and the electrode aperture sizes 2–6 mm for the generation of pulsed electron beams suitable for potential and growing applications in extreme ultraviolet (EUV)/Soft X-ray, microwave, and surface modification is presented. Trigger electrons are used to form the controlled and efficient plasma inside the hollow cathode cavity. The generated high density ( $\sim \!\!{2.8} \times {10}^{{{5}}}$ A/cm2) and up to 32-kV electron beams are governed by the penetrated dynamic equipotential lines. The electron beam current increases with the increase of gas pressure and the number of gaps and decreases for the increase in the diameter of the electrode apertures of the PD-PCE source. The investigation has helped to generate the required electron beam by optimizing the operating and geometrical parameters of the PD-PCE source.

A tapered multi-gap multi-aperture pseudospark-sourced electron gun based X-band slow wave oscillator

The experimental study of a tapered, multi-gap, multi-aperture pseudospark-sourced electron gun based X-band plasma assisted slow wave oscillator is presented. The designed electron gun is based on the pseudospark discharge concept and has been used to generate a high current density and high energy electron beam simultaneously. The distribution of apertures has been arranged such that the field penetration potency inside the backspace of the hollow-cathode is different while passing through the tapered gap region. This leads to non-concurrent ignition of the discharge through all the channels which is, in general, quite challenging in the case of multi-aperture plasma cathode electron gun geometries. Multiple and successive hollow cathode phases are reported from this electron gun geometry, which have been confirmed using simulations. This geometry also has led to the achievement of ∼71% fill factor inside the slow wave oscillator for an electron beam of energy of 20 keV and a beam current density in the range of 115–190 A/cm2 at a working argon gas pressure of 18 Pa. The oscillator has generated broadband microwave output in the frequency range of 10–11.7 GHz with a peak power of ∼10 kW for ∼50 ns.

Electron Beam Transport System for 263-GHz Sheet Beam TWT

The efficient transport of high current density electron beams is essential for the success of millimeter wave and terahertz vacuum devices. To achieve high current density sheet electron beam transmission, an advanced periodic cusped magnet-tunable quadrupolar magnet (PCM-TQM) is proposed to improve the transmission factor of the sheet beam. Through numerical a nalysis, the TQM is shown to provide the ability to be compatible with a larger initial transverse velocity spread and match the current density increasing caused by beam bunching. This new PCM-TQM focusing system is employed to transport a 19 kV, 0.15 A, $\sim 8.5$ :1 aspect ratio sheet beam with an elliptical cross section of 0.60 mm (width) $\times0.07$ mm (height), whose current density exceeds 400 A/cm2, through a 45-mm long circuit, whose beam tunnel is 0.7 mm $\times0.12$ mm. CST PARTICLE STUDIO is used to model the beam transport system and verify the analytical results. With the tunable feature of the QM poles, the “no RF” and “with RF” beam transmission factors under the focusing magnetic field of the advanced PCM-TQM are predicted to achieve 99.3% and 97.6%, respectively.

Influence of the electrode gap separation on the pseudospark-sourced electron beam generation

Pseudospark-sourced electron beam is a self-focused intense electron beam which can propagate without any external focusing magnetic field. This electron beam can drive a beam-wave interaction directly or after being post-accelerated. It is especially suitable for terahertz radiation generation due to the ability of a pseudospark discharge to produce small size in the micron range and very high current density and bright electron beams. In this paper, a single-gap pseudospark discharge chamber has been built and tested with several electrode gap separations to explore the dependence of the pseudospark-sourced electron beam current on the discharge voltage and the electrode gap separation. Experimental results show that the beam pulses have similar pulse width and delay time from the distinct drop of the applied voltage for smaller electrode gap separations but longer delay time for the largest gap separation used in the experiment. It has been found that the electron beam only starts to occur when the charging voltage is above a certain value, which is defined as the starting voltage of the electron beam. The starting voltage is different for different electrode gap separations and decreases with increasing electrode gap separation in our pseudospark discharge configuration. The electron beam current increases with the increasing discharge voltage following two tendencies. Under the same discharge voltage, the configuration with the larger electrode gap separation will generate higher electron beam current. When the discharge voltage is higher than 10 kV, the beam current generated at the electrode gap separation of 17.0 mm, is much higher than that generated at smaller gap separations. The ionization of the neutral gas in the main gap is inferred to contribute more to the current increase with increasing electrode gap separation.

A multiple gap plasma cathode electron gun and its electron beam analysis in self and trigger breakdown modes.

In the present paper, a pseudospark discharge based multiple gap plasma cathode electron gun is reported which has been operated separately in self and trigger breakdown modes using two different gases, namely, argon and hydrogen. The beam current and beam energy have been analyzed using a concentric ring diagnostic arrangement. Two distinct electron beams are clearly seen with hollow cathode and conductive phases. The hollow cathode phase has been observed for ∼50 ns where the obtained electron beam is having low beam current density and high energy. While in conductive phase it is high current density and low energy electron beam. It is inferred that in the hollow cathode phase the beam energy is more for the self breakdown case whereas the current density is more for the trigger breakdown case. The tailor made operation of the hollow cathode phase electron beam can play an important role in microwave generation. Up to 30% variation in the electron beam energy has been achieved keeping the same gas and by varying the breakdown mode operations. Also, up to 32% variation in the beam current density has been achieved for the trigger breakdown mode at optimized trigger position by varying the gas type.

Study of mini-themionic electron sources for vacuum electron THz devices

THz technology has attracted great attention for decades of years. Among the wide research areas of THz technologies, vacuum electron terahertz radiation sources have obvious advantages in high power region. For the THz vacuum electro devices (VED), high current density electron beams with small dimensions are required. Nanosized scandia doped dispenser (SDD) cathodes have the capability to operate stably at pulsed current densities of over 100 A/ cm2 at 950 ℃ so it becomes the most promising cathodes to meet the requirements for THz VEDs. In this paper, we report a new approach for developing miniaturized electron beam sources on normal SDD cathodes. An electron beam of 400 μm in diameter has been directly generated on an SDD cathode by deposition of a Zr/W double-layer anti-emission film and followed by a focused ion beam (FIB) milling. Results indicate that the electron beam is able to provide a space charge limited (SCL) current density of over 50 A/cm2 at the operating temperature of 950 ℃ with proper laminarity and works stably for more than 1000 hours. The beam emission characteristics and the function of the anti-emission film have been discussed and related to the surface analysis results. The approach opens a new way for producing high emission mini-electron sources to satisfy the requirment of THz VEDs.

Comparative simulation studies of plasma cathode electron (PCE) gun

Pseudospark discharge based plasma cathode has capability to provide high current density electron beam during discharge process. In this paper an effort has been made to simulate the breakdown processes in the pseudospark discharge based plasma cathode electron gun. The two-dimensional plasma simulation codes VORPAL and OOPIC-Pro have been used and results are compared. The peak discharge current in the plasma cathode electron gun is found to be dependent on aperture size, hollow cathode dimensions, anode voltage and seed electrons energy. The effect of these design parameters on the peak anode current has been analysed by both the codes and results matches well within 10% variation. For the seed electron generation an electron beam trigger source is used to control the discharge process in the hollow cathode cavity. The time span of trigger source has been varied from 1-100 ns to analyze the effect on the peak anode current.

Ion beams in SEM: An experiment towards a high brightness low energy spread electron impact gas ion source

A next generation ion source suitable for both high resolution focused ion beam milling and imaging applications is currently being developed. The new ion source relies on a method of which positively charged ions are extracted from a miniaturized gas chamber where neutral gas atoms become ionized by direct electron impact. The use of a very small gas chamber and a very narrow electron beam (&amp;lt;100 nm) allows for a very small ionization volume, which, in turn, yields a small virtual source size and low energy spread. The authors estimate that using a high current density electron beam from a Schottky electron gun the reduced brightness of this source can exceed that of the Gallium Liquid Metal Ion Sources and the energy spread can be well below 1 eV at an optimal gas pressure and gas chamber spacing while producing more than 1 nA of usable ion beam current. In a proof-of-concept study, the authors have produced ions of helium, argon, xenon, and air from a prototype gas chamber using an electron probe inside a scanning electron microscope. Using micro-channel plates and a phosphor screen, ion beam patterns have been acquired demonstrating that a beam of ions can be produced from a miniaturized gas chamber. The authors have measured up to several hundreds of pico-amperes of ion current in a Faraday cup using an input electron probe current of ∼14 nA with 1 keV incident energy. The authors have also verified that the ion beam current is dependent on the incident electron beam energy, gas chamber bias voltage, and the gas pressure inside the ionization chamber.

Investigation of terahertz sheet beam traveling wave tube amplifier with nanocomposite cathode

Particle-in-cell simulations of a staggered double grating array traveling wave tube intended as a wideband amplifier for terahertz communications, sensing, and imaging applications showed that, for an electron beam power of 5 kW, it produces 150–275 W, corresponding to 3%–5.5% electronic efficiency, at 0.22 THz with over ∼30% bandwidth and with greater than 12 dB/cm growth rate. The circuit has been fabricated by both UV lithography and high precision computer-numerical-control machining with ∼2–3 μm dimensional tolerance and ∼50 nm surface roughness. A scandate nanocomposite (Sc2O3–W) cathode for the electron beam source has successfully emitted 120 A/cm2 (space charge limited) at 1150 °C and 50 A/cm2 at 1050 °C for 8000 h as required to produce the requisite high current density electron beam.

High-current fast electron beam propagation in a dielectric target

Recent experiments demonstrate an efficient transformation of high intensity laser pulse into a relativistic electron beam with a very high current density exceeding 10(12) A cm(-2). The propagation of such a beam inside the target is possible if its current is neutralized. This phenomenon is not well understood, especially in dielectric targets. In this paper, we study the propagation of high current density electron beam in a plastic target using a particle-in-cell simulation code. The code includes both ionization of the plastic and collisions of newborn electrons. The numerical results are compared with a relatively simple analytical model and a reasonable agreement is found. The temporal evolution of the beam velocity distribution, the spatial density profile, and the propagation velocity of the ionization front are analyzed and their dependencies on the beam density and energy are discussed. The beam energy losses are mainly due to the target ionization induced by the self-generated electric field and the return current. For the highest beam density, a two-stream instability is observed to develop in the plasma behind the ionization front and it contributes to the beam energy losses.

Microwave Excitation by a Constrained Large-Orbit Electron Beam—A Unified Dispersion Relation for Slow- and Fast-Wave Devices

A unified linear dispersion relation that describes both slow- and fast-wave devices excited by a constrained large gyration orbit (LO) monoenergetic electron beam of infinitely small thickness has been derived and studied numerically. Beam electrons in a sufficiently long sinusoidally corrugated metal slow-wave structure are assumed completely neutralized by the background ions in equilibrium state. An exact dispersion relation of an LO backward-wave oscillator that can reasonably describe instabilities in the slow-wave device region has been obtained. A parameter a, defined as a ratio of the transverse to the longitudinal component of the electron velocity, is found to have a critical value above which the excitation of a nonaxisymmetric quasi-TE/sub 11/ mode caused by the fast cyclotron instability dominates the conventional Cherenkov instability. However, for an SWS having infinitely small amplitude of corrugation, radiation with w W is altogether suppressed; instead, an alternate mechanism, namely, Cherenkov instability in the azimuthal direction with w<W found first in the current paper leads to the excitation of microwaves. This suggests that the radiation from some previous CRM experiments with a high current density electron beam neutralized by ions might have been caused not by CRM instability but by the present Cherenkov instability in the azimuthal direction.

XPS Evaluation of Samples Surface Cleaned by the XEI Evactron®

With the advent of field emission gun equipped SEMs and TEMs with clean vacuum systems, it has been shown repeatedly that the surface cleanliness of samples exposed to the small, high current density electron beam available in these instruments is critical with respect to controlling hydrocarbon contamination under the electron beam. in previous studies of dedicated plasma cleaners for TEM samples by one of the authors (SDW), it was shown that surface analytical techniques are the best methods for determining the effectiveness of such cleaners in removing hydrocarbons from the samples’ surfaces. in the current study, some of the experiments that were used in these previous studies were repeated using the XEI Evactron® system in order to compare the relative effectiveness of this system for surface cleaning of EM samples.Figure 1 shows a picture of the sample introduction chamber of a VG Scientific ESCALAB Mkll system (XPS) with the Evactron® in place. The Evactron® system consists of an RF head unit that attaches to the vacuum chamber with an adaptable flange, an RF impedance matching unit, and an RF generator/vacuum controller. Although a 100% O2 system can be used and is recommended, for safety reasons, a 20%O2/80%Ar by volume gas mixture was selected because of the hydrocarbon oil used in the mechanical pump. Cleaning was also performed using laboratory air. The major difference between the dedicated plasma cleaners and the Evactron® configuration is that the sample is not immersed in the plasma but is exposed to the activated oxygen species in the downstream flow.

Arcing and voltage breakdown in vacuum microelectronics microwave devices using field emitter arrays: Causes, possible solutions, and recent progress

There is growing interest in high current field emitter arrays (FEAs) capable of delivering high current density and high conductance electron beams, particularly for microwave applications. Large, high packing density molybdenum and silicon FEAs have been placed in ultrahigh vacuum chambers, carefully conditioned, and tested for maximum performance and have yielded total FEA currents of 20–200 mA and beam current densities of 5–2000 A/cm2 at gate voltages of 80–150 V. However similar Mo and Si FEAs, and GaAs edge arrays, when placed in a prototype 10 GHz klystrode amplifier, have failed at 1–4 mA, even for pulsed operation at a low duty factor. Hence, the current must be increased by 25–50 in order to meet klystrode design objectives. We compare the intrinsic FE stable current limits of various materials: Mo, Si, GaAs, ZrC, and ZrC films on Mo emitters. We conclude that Mo FEAs have a high FE current limit but demand an extremely clean environment, Si or GaAs FEAs are more tolerant of a poorer environment but have a relatively low FE current limit, while ZrC/Mo FEAs have at least as equally high a FE current limit as Mo FEAs but are much more robust and tolerate a much poorer environment. We hypothesize that failures of high density Mo FEAs at relatively low current levels (below 1 μA per tip) in a microwave tube are due to ion bombardment of the FEA creating sharp nanoprotrusions on the sides of the emitters, which emit intense, focused electron beams directed at the gate, leading to a vacuum arc. Binh’s recent study presents strong evidence for the ongoing creation and destruction of nanoprotrusions on Mo FEAs. A ZrC thin film coating protects and passivates the Mo FEA surface, thus minimizing nanoprotrusion formation and allowing more stable operation at higher current levels and/or in more degraded environments. Studies of carbide (ZrC, HfC) film coatings on W, Mo, and Si single tips and on Mo and Si FEAs show that indeed the carbide film reduces the work function and operating voltage (by about 25% and 40%, respectively) and increases stability. Other studies still in progress show that blunt ZrC or ZrC/Mo single tips can operate for reasonable periods (∼1 h) at currents averaging 400 μA in ultrahigh vacuum or 200 μA in 10−5 Torr air. More extensive studies are planned to verify and to quantify the advantages of carbide film coatings for producing lower voltage, higher current, environment tolerant, and long life FEAs for various applications.

Cathodes for electron guns

This paper reviews cathodes for high current density electron beams. It describes the following types of emitters: oxide, dispenser, LaB6, IrCe. Photo and ferroelectric cathodes are presented briefly. Current density from the cathodes, their lifetimes, some features due to the cathode surface properties and influence on the beam of filament magnetic field are discussed.

A new beam source for free electron lasers

Abstract A high power, high current density and high voltage electron beam was generated with the pseudospark discharge (PS). This is a new approach to high brightness beam source for free electron lasers. The design and construction of the pseudospark discharge is described. The device has low cost and is easy to fabricate. The experimental results are presented, the configuration of the modified pulse line accelerator (PLA) is described. The intense electron beams with a voltage of 200 keV, a current of 2kA, a beam diameter of less than 1 mm, a beam pulse duration of 400 ns and beam emittance of 48 mm mrad were measured. The compact free electron laser with a table size is discussed.

Characterization of an insulated SCL non-relativistic electron-beam diode operating at 300 kV/cm

High current density (>100 A/cm/sup 2/) electron beam diodes operating beyond 250 kV/cm in a 1 cm gap configuration are described. The principal features of the electron source are the high current density and high field without the onset of vacuum arcs or arc collapse prior to gap closure. The electron beam diode was controlled by a variable pulse-width output Marx generator. Fields to 600 kV/cm were applied for 25 to 30 ns, and to 300 kV/cm during space-charge limited current conduction at pulse lengths of 100 to 120 ns. Evidence of the transition to space-charge limited unipolar flow and transition to bipolar space-charge limited flow was obtained. Traces of the typical behavior and the different transitions are shown. Beam uniformity was measured by using a set of Faraday cups. The Faraday cup setup was then used to demonstrate suppression of electron emission from surfaces coated with a dielectric film.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High current density electron beam generation from field emission tip cathodes

Electron beams with current up to 1.2 A and current density over 107 A/cm2 have been generated from a pulsed field emission tip cathode etched from commercial grade tungsten wire. Electron beams with current up to 0.8 A have also been generated from a molybdenum tip. Tip radius was 0.1–1 μm. Applied tip voltage was up to 50 kV with pulse duration of 0.3–1.2 μs. Depending on brightness, these types of electron beams could be suitable for channeling radiation x-ray lasers and might considerably decrease the operating wavelength of free-electron lasers at moderate beam energies.

Electron-beam-assisted dry etching for GaAs using electron cyclotron resonance plasma electron source

Electron-beam (EB)-assisted dry etching of GaAs using Ar electron cyclotron resonance (ECR) plasma as an electron shower source is developed to achieve a low energy and high current density electron beam (EB). The rate of EB-assisted dry etching is more than ten times larger than for Cl2 gas etching.It is confirmed, through photoluminescence measurement, that this etching method causes less damage than ion beam techniques and is very effective for damaged layer removal. Using this technique, a 0.4 μm linewidth low-damage fine structure of GaAs was fabricated.

HREM of electron-beam-induced damage in l-Ta 2O 5

Samples of l-Ta 2O 5 exposed to a high current density electron beam were observed by high resolution electron microscopy (HREM) to reduce to suboxide and metallic Ta phases. Data are presented for crystals aligned along [001], [020], [200], and [130], in which surface areas are seen to reduce to Ta 2O, TaO 2, bcc Ta, or a β-Ta phase via an amorphous intermediate phase. The driving force appears to involve desorption of oxygen stimulated by electronic transitions (DIET), and the possible involvement of the “infinitely adaptive nature” of the initial l-Ta 2O 5 structure in the damage process is discussed. Comparison of results obtained from an ultrahigh vacuum HREM to those from a conventional HREM suggests that contamination effects in lower vacuum systems influences the reliability of the results, especially in complex systems like Ta&1bO. Finally, the results follow a symmetry selection rule, which explains why l-Ta 2O 5 does not reduce to the monoxide phase as observed in similar experiments with TiO 2, V 2O 5, and Nb 2O 5.