Plasma Cathode Electron Gun Research Articles

Electron-Beam-Pumped UVC Emitters Based on an (Al,Ga)N Material System.

Powerful emitters of ultraviolet C (UVC) light in the wavelength range of 230-280 nm are necessary for the development of effective and safe optical disinfection technologies, highly sensitive optical spectroscopy and non-line-of-sight optical communication. This review considers UVC emitters with electron-beam pumping of heterostructures with quantum wells in an (Al,Ga)N material system. The important advantages of these emitters are the absence of the critical problem of p-type doping and the possibility of achieving record (up to several tens of watts for peak values) output optical power values in the UVC range. The review consistently considers about a decade of world experience in the implementation of various UV emitters with various types of thermionic, field-emission, and plasma-cathode electron guns (sources) used to excite various designs of active (light-emitting) regions in heterostructures with quantum wells of AlxGa1-xN/AlyGa1-yN (x = 0-0.5, y = 0.6-1), fabricated either by metal-organic chemical vapor deposition or by plasma-activated molecular beam epitaxy. Special attention is paid to the production of heterostructures with multiple quantum wells/two-dimensional (2D) quantum disks of GaN/AlN with a monolayer's (1 ML~0.25 nm) thickness, which ensures a high internal quantum efficiency of radiative recombination in the UVC range, low elastic stresses in heterostructures, and high-output UVC-optical powers.

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.

Analysis of Geometrical Design Parameters for High-Energy and High-Current-Density Pseudospark-Sourced Electron Beam Emission

Four different configurations of plasma cathode electron (PCE) guns have been designed and developed in which pseudospark discharge concept has been used. Experimental studies have been performed to identify the dependence of geometrical design parameters like shape of aperture, distribution of apertures, and the number of gaps between hollow cathode and anode on high-energy and high-current-density electron beam emissions. The beam current density has been increased around five times by changing the shape of aperture from circular to sheet keeping the aperture area and other geometrical parameters identical. The perveance of the electron beam has also been increased by increasing the number of apertures on hollow cathode and anode surface. The beam current density from single aperture to multiple apertures has increased from 225 to 900 A/cm2 with low beam energy for the multiple-aperture case. The multiple gaps between hollow cathode and anode increase the electron beam energy. The maximum beam energy is found to be 16–14.5 keV at an applied gap voltage of 16 kV from the developed multigap PCE gun, which is much higher than that from the single gap configuration.

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.

The Use of an Electron Microchannel as a Self-Extracting and Focusing Plasma Cathode Electron Gun

A new and simple type of electron gun is presented. Unlike conventional electron guns, which require a heated filament or extractor, accelerator and focusing electrodes, this gun uses the collimated electron microchannels of an inertial electrostatic confinement (IEC) discharge to achieve the same outcome. A cylindrical cathode is placed coaxially within a cylindrical anode to create the discharge. Collimated beams of electrons and fast neutrals emerge along the axis of the cylindrical cathode. This geometry isolates one of the microchannels that emerge in a negatively biased IEC grid. The internal operating pressure range of the gun is 35-190 mTorr. A small aperture separates the gun from the main vacuum chamber in order to achieve a pressure differential. The chamber was operated at pressures of 4-12 mTorr. The measured current produced by the gun was 0.1-3 mA (0.2-14 mA corrected measurement) for discharge currents of 1-45 mA and discharge voltages of 0.5-12 kV. The collimated electron beam emerges from the aperture into the vacuum chamber. The performance of the gun is unaffected by the pressure differential between the vacuum chamber and the gun. This allows the aperture to be removed and the chamber pressure to be equal to the gun pressure if required.

Simulation and experimental studies of 20 kV/200 A PCE-Gun for discharge current analysis

In this paper, an effort has been made to study the recently developed pseudospark discharge-based 20 kV/200 A plasma cathode electron gun by using 3-D particle-in-cell simulation code VORPAL for discharge current analysis to validate the experiments. The experimental and simulation studies have been performed at different working pressures between 50 and 100 mTorr and at different applied voltages between 10 and 20 kV. Argon has been used as the working gas. The temporal behavior of the discharge current, discharge voltage, potential distribution within the hollow cathode region and the electron plasma density buildup has been analyzed from the simulations. The experiments have been performed to measure the dynamic nature of the discharge voltage and discharge current at different operating conditions. An agreement between the simulated and experimental results has been obtained well within 10 % for the discharge currents at different applied voltages, qualifying the design.

Experimental investigation of a 1 kA/cm² sheet beam plasma cathode electron gun.

In this paper, a cold cathode based sheet-beam plasma cathode electron gun is reported with achieved sheet-beam current density ∼1 kA/cm(2) from pseudospark based argon plasma for pulse length of ∼200 ns in a single shot experiment. For the qualitative assessment of the sheet-beam, an arrangement of three isolated metallic-sheets is proposed. The actual shape and size of the sheet-electron-beam are obtained through a non-conventional method by proposing a dielectric charging technique and scanning electron microscope based imaging. As distinct from the earlier developed sheet beam sources, the generated sheet-beam has been propagated more than 190 mm distance in a drift space region maintaining sheet structure without assistance of any external magnetic field.

Performance evaluation of self-breakdown-based single-gap plasma cathode electron gun

This paper presents the experimental studies on self-breakdown-based single-gap plasma cathode electron (PCE) gun (5–20 kV/50–160 A) in argon, gas atmosphere and its performance evaluation based on particle-in-cell (PIC) simulation code ‘OOPIC-Pro’. The PCE-Gun works in conducting phase (low energy, high current) of pseudospark discharge. It produces intense electron beam, which can propagate more than 200 mm in the drift space region without external magnetic field. The profile of this beam in the drift space region at different breakdown conditions (i.e., gas pressures and applied voltages) has been studied and the experimental results are compared with simulated values. It is demonstrated that ∼30% beam current is lost during the propagation possibly due to space charge neutralization and collisions with neutral particles and walls.

Experimental Investigation of Pseudospark generated electron beam

The pseudospark (PS) discharge is, however, more recently recognized as a different type of discharge which is capable of generating electron beams with the highest combined current density and brightness of any known type of electron source. PS discharge is a specific type of gas discharge, which operates on the left-hand side of the hollow cathode analogy to the Paschen curve with axially symmetric parallel electrodes and central holes on the electrodes. The PS discharge generated electron beam has tremendous applications in plasma filled microwave sources where normal material cathode cannot be used. Analysis of the electron beam profile has been carried out experimentally for different applied voltages. The investigation has been done at different axial and radial location inside the drift tube in argon atmosphere. This paper represents experimentally derived axial and radial variation of the beam current inside the plasma filled drift tube of PS discharge based plasma cathode electron (PCE) gun. With the help of current density estimation the focusing and defocusing point of electron beam in axial direction can be analyzed. It has been further confirmed the successful propagation of electron beam in confined manner without any assistance of external magnetic field.

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.

Investigation of electron beam parameters inside the drift region of plasma cathode electron gun

This paper presents experimental studies for the production and propagation of an electron beam from a single gap pseudospark discharge based plasma cathode electron (PCE) gun. The generated electron beam has been successfully propagated for more than 25 cm in a gaseous environment without application of external guiding magnetic field at different operating conditions. The electron beam losses due to recombination with ions and collision with walls of drift space have been estimated. The electron beam profile has also been analyzed in the drift region of the gun.

Development of low-pressure high-current plasma cathode electron gun and use of associated techniques

A plasma cathode electron (PCE) gun has capabilities for generating high-current, broad, and focused beams for plasma-assisted microwave sources. A pseudospark-based hollow cathode PCE gun has been designed and developed for microwave generation which is operated in argon atmosphere. An analysis of the electron beam profile inside the drift space at different operating conditions has been carried out. This has been performed at several axial and radial locations inside the drift space which shows coherent phases of beam profiles in radial direction. The focusing and defocusing points in axial direction are also obtained. The beam current at different axial location for different applied voltages has been estimated. The obtained beam current is in close agreement with the beam current estimated by the particle-in-cell simulation code for the same geometry.

Experimental Analysis of Pseudospark Sourced Electron Beam

The pseudospark (PS) discharge has been shown to be a promising source of high brightness, high intensity electron beam pulses. The PS discharge sourced electron beam has potential applications in plasma filled microwave sources where normal material cathode cannot be used. Analysis of the electron beam profile has been done experimentally for different applied voltages. The investigation has been carried out at different axial and radial location inside the drift space in argon atmosphere. This paper represents experimentally found axial and radial variation of the beam current inside the drift tube of PS discharge based plasma cathode electron (PCE) gun. With the help of current density estimation the focusing and defocusing point of electron beam in axial direction can be analyzed.

A high current density plasma cathode electron gun

The design, performance, and characteristics of a plasma cathode electron gun are presented. The plasma cathode is based on a hollow cathode direct current discharge, and the electron beam is accelerated by pulse voltage. By discharging at high gas pressure and operating at low gas pressure, both the maximum accelerating voltage and maximum emitting current could be increased. Utilizing argon, with the accelerating voltage up to 9 kV and gas pressure down to 52 mPa, the gun is able to generate an electron beam of about 4.7 A, and the corresponding emitting current density is about 600 A/cm2.

Generation of narrow focused beams in a plasma-cathode electron gun

The possibility of generating a narrow focused electron beam in a plasma-cathode electron gun has been studied. An operating mode in which the emitting plasma surface is deep in the emitting channel is optimal for obtaining the highest emission-current density. The beam diameter is reduced by a factor of 1.5–2.0 by choosing the operating mode of the plasma emitter. A two-lens focusing system is used to focus the electron beam. Studies have shown the possibility of generating beams with a power density five to seven times higher than that attained earlier in plasma-cathode guns. As a result of these studies, a plasma-cathode electron gun that allows generation of an electron beam with a diameter of ∼260 μm and a power of 3 kW at an accelerating voltage of 30 kV has been developed.

Experimental Study of the Plasma Cathode Electron Gun

The high-power microwave devices with plasma-filled have been unique properties. One of the major problems associated with plasma-filled microwave sources is that ions from the plasma drift toward the gun regions of the tube. This bombardment is particularly dangerous for the gun,where high-energy ion impacts can damage The cathode surface and degrade its electron emission capabilities. One of the techniques investigated to mitigate this issue is to replace the material cathode with plasma cathode. A unique plasma cathode electron gun are investigated in detail experimentally in this paper. The PCE-gun has been operated at pulser voltage up to 8KV,discharge current up to 166A,and pulse lengths of up to 60μs.

High current, low pressure plasma cathode electron gun

A plasma-cathode electron gun based on a moderate pressure (>5 mTorr) cold-cathode discharge and a high perveance, multiaperture accelerator was previously developed at Hughes Research Laboratories and produced electron beam currents of up to 1 kA at voltages of over 200 kV for pulse lengths of 100 μs. This gun was limited in pulse repetition frequency and duty by the gas-puff system that provided adequate gas pressure in the hollow cathode to operate the glow discharge while keeping the pressure in the beam transport region sufficiently low. We describe a new plasma cathode electron gun (PCE gun) that eliminates this problem by replacing the glow-discharge plasma generator in the electron gun by a low-pressure thermionic discharge in a magnetic multipole confinement chamber. Proper design of the plasma generator and electrical circuit provides high electron-current densities to the accelerator structure at very low gas pressure (<10−4 Torr). The static gas pressure permits the pulse repetition frequency to be very high (>1.5 kHz demonstrated) with electron beam currents up to 200 A at voltages up to 120 kV demonstrated. The design and performance of the PCE gun, along with several models used to predict and scale the performance, are discussed.

Hollow-cathode plasma electron gun for beam generation at forepump gas pressure

The characteristics, performance, and design feature of a filamentless plasma-cathode electron gun for beam generation in the forepump gas pressure range are presented. The plasma cathode is based on a hollow-cathode direct current (dc) discharge. Using the method of “grid stabilization” it was possible to generate an e beam at a background gas pressure as high as about 10−1 Torr. This pressure can be easily obtained by using mechanical pump only. The operation of the gun with a magnetic field up to 0.1 T was investigated. The presence of a magnetic field (B field) is often required, for instance in plasma chemistry and surface treatment processes. The effect of the B field both on discharge and emission parameters of the gun are observed. The results obtained can be explained based on the concept of electron confinement and motion across the B field. With the accelerating voltage up to 8 kV, the gun is able to generate an electron beam of about 0.7 A dc.

Efficiency enhancement in high power backward-wave oscillators

High power microwave (HPM) sources based on the backward-wave oscillator (BWO) have been investigated for the past two decades primarily because of their potential for very high efficiency (15 to 40%) operation. Several different effects have been proposed to explain this high efficiency compared to conventional BWOs. One of the major contributors to the high efficiency of the plasma-filled Pasotron HPM BWO source is the presence of optimally phased end reflections. The Pasotron uses a long-pulse (/spl ges/100 /spl mu/s) plasma-cathode electron-gun and plasma filled slow-wave structure to produce microwave pulses in the range of 1 to 10 MW without the use of externally produced magnetic fields. The efficiency of the Pasotron can be enhanced by up to a factor of two when the device is configured as a standing-wave oscillator in which properly phased reflections from the downstream collector end of the finite length SWS constructively interfere with the fundamental backward-wave modes and improve the coupling of the beam to the circuit. Operation in this configuration increases the efficiency up to 30% but causes the frequency to vary in discrete steps and the output power to change strongly with beam parameters and oscillation frequency.

Development of plasma cathode electron guns

The status of experimental research and ongoing development of plasma cathode electron guns in recent years is reviewed, including some novel upgrades and applications to various technological fields. The attractiveness of this kind of e-gun is due to its capability of creating high current, broad or focused beams, both in pulsed and steady-state modes of operation. An important characteristic of the plasma cathode electron gun is the absence of a thermionic cathode, a feature which leads to long lifetime and reliable operation even in the presence of aggressive background gas media and at fore-vacuum gas pressure ranges such as achieved by mechanical pumps. Depending on the required beam parameters, different kinds of plasma discharge systems can be used in plasma cathode electron guns, such as vacuum arcs, constricted gaseous arcs, hollow cathode glows, and two kinds of discharges in crossed E×B fields: Penning and magnetron. At the present time, plasma cathode electron guns provide beams with transverse dimension from fractional millimeter up to about one meter, beam current from microamperes to kiloamperes, beam current density up to about 100 A/cm2, pulse duration from nanoseconds to dc, and electron energy from several keV to hundreds of keV. Applications include electron beam melting and welding, surface treatment, plasma chemistry, radiation technologies, laser pumping, microwave generation, and more.