Pseudospark-sourced Electron Beam Research Articles

Design and Non-Uniform Current Analysis of a 0.35-THz Extended Interaction Oscillator Based on Pseudospark-Sourced Multiple Sheet Electron Beams

A novel method, which combines a multiple-beam extended interaction oscillator (EIO) with pseudospark-sourced (PS) sheet electron beams, is applied to generate high-power terahertz sources. For a multiple-beam EIO, the beam cross-section is significantly improved by replacing the commonly used pencil electron beams with sheet electron beams. The PS electron beams have the advantage of high current density and operate without a focus magnetic field. The volume of the cavity is larger when the EIO operates in the TM31-3π mode than in the conventional TM01-2π mode at the same operating frequency. The EIO operating at the terahertz frequency has a larger cavity volume, which means greater power capacity and lower manufacturing difficulty. For a PS multiple-beam EIO, the non-uniformity of electron beam currents is a common problem. In order to study this problem, an original high-order mode EIO driven by PS multiple sheet electron beams is presented with enhanced output power at 0.35 THz. The authors analyze electron beams with different currents through particle-in-cell (PIC) simulations. Simulation results show that the EIO can operate stably even in the case of non-uniform PS electron beam currents. When each current is 1.4 A, simulation results show the EIO’s output power of 4.9 kW at 0.35 THz. Considering the low conductivity of 1.1 × 107 S/m, the efficiency is still 1.42%.

Preliminary Study of a G-Band Extended Interaction Oscillator Operating in the TM31-3π Mode Driven by Pseudospark-Sourced Multiple Electron Beams

This paper presents the first design that combines pseudospark-sourced (PS) electron beams with a multiple-beam extended interaction oscillator (EIO). The PS electron beam is an excellent choice for driving EIOs because it has high current density and does not require a focusing magnetic field. The EIO with coaxial structure adopts the method of multiple electron beams, which plays a crucial role in improving the average output power. At the same frequency, the EIO operating in the high-order TM31-3π mode has a larger cavity size than the EIO operating in the traditional TM01-2π mode. The high-order TM31-3π mode solves the problem of the EIO’s manufacture at high frequency. In order to verify the above points, a G-band PS multiple-beam EIO operating in TM31-3π mode has been designed. The beam–wave interaction particle-in-cell simulation results show that the EIO’s peak output power is 39.2 kW at 217 GHz, and that its efficiency is around 6.1%. The EIO with six pencil beams operates at a voltage of 43 kV. The total current of the six electron beams is 15 A (equally distributed among the six beams), and the corresponding current density is about 5000 A/cm2. Considering the ohmic loss and the effect of skin depth, the conductivity used in these simulations is 2 × 107 S/m. The design is an excellent way to improve the output power of EIO operating at high frequency.

Design, Simulation, and Cold Test of a W-Band Double Nonparallel Staggered Grating Backward Wave Oscillator

A novel double nonparallel staggered grating (DNPSG) slow wave structure (SWS) is proposed to enhance the coupling impedance in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band backward wave oscillator (BWO) driven by a pseudospark-sourced sheet electron beam. The DNPSG SWS has been shown a broadband of 72–125 GHz and a higher coupling impedance compared with the traditional double staggered grating (DSG) SWS in simulation. The DNPSG BWO structure consisting of ten SWS units and a broadband output structure is designed and fabricated. In the cold test of the DNPSG BWO, the measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{11}$ </tex-math></inline-formula> (double of the losses in the DNPSG BWO) is above −10 dB in most of the band, which is satisfactory in the pseudospark-driven high-power device. The hot-test performance of the DNPSG BWO is analyzed by particle-in-cell (PIC) simulation in a beam voltage range of 14–90 kV and a current density range of 1.5– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times10$ </tex-math></inline-formula> 7 A/m2, obtaining a high output power (max. 190 kW) over an ultrawide tuning band of 38 GHz (75–113 GHz) due to enhanced coupling impedance.

Research of the Oscillation Start-Up Time in an Extended Interaction Oscillator Driven by a Pseudospark-Sourced Sheet Electron Beam

High current density and high brightness are critical factors for high-power and compact extended interaction oscillators (EIOs) which are operated in the terahertz (THz) waveband. The pseudospark-sourced (PS) sheet electron beam, which combines merits including high current density, a relatively big beam cross-section and no requirement for the external focusing magnetic field, is a good choice for application to high-frequency EIO. The pulse generated by the PS electron beam can last around tens of nanoseconds or even less, thus the EIO’s oscillation start-up time (OST) should be short enough. This paper researched how to reduce OST in an EIO driven by the PS sheet electron beam. The authors realized that the OST of EIO was very sensitive to the gap length under the equal period. The distribution of the electric field is optimized by adjusting the length of the gap. The strong electric field strength is conducive to the beam-wave interaction, and the OST is affected by the beam-wave interaction. When the gap length reaches a suitable value, the OST becomes the shortest. The simulation results showed the EIO’s shortest OST was 8 ns and the corresponding peak output power was 2 kW at 0.19 THz, while the current density was 500 A/cm2. When current density reached 10,000 A/cm2, the shortest OST could even be 1.9 ns.

Dual-mode terahertz extended interaction oscillator driven by a pseudospark-sourced sheet electron beam

A terahertz dual-mode extended interaction oscillator (EIO) driven by a pseudospark-sourced sheet electron beam (SEB) was presented. The major advantages of the newly developed circuit include 1) high-density SEB interacting with the TM11 and TM31 modes, respectively, and 2) high output power of over 1 ​kW ​at the sub-terahertz frequency range. Two different types of 2π modes and their output characteristics were studied, and the circuit was optimized to ensure efficient outputs of two standing-wave modes. The three-dimensional (3D) particle-in-cell (PIC) simulation predicts the maximum output power of 1.3 ​kW with a 3-dB bandwidth of ~0.5 ​GHz ​at 303 ​GHz when operating at the TM11 mode, and 3.18 ​kW, 3-dB bandwidth of ~0.85 ​GHz ​at 364 ​GHz when operating at the TM31 mode.

Simulations of the Self-Focused Pseudospark-Sourced Electron Beam in a Background Ion Channel

Using pseudospark discharge sourced electron beams for the generation of high-peak-power millimeter and terahertz radiation has attracted increasing research interest in recent years. However, one of the crucially important and hitherto unanswered questions is what is the upper-frequency limit at which millimeter-wave devices can be driven by pseudospark discharge sourced electron beams?. In this paper, we studied this question from the perspective of beam transportation in a plasma background, more specifically an ion channel using particle-in-cell simulations to find the limitations. The parameter ranges of the beam transportation with small oscillations in the beam diameter were investigated and summarized, through simulations of beam propagation in a large diameter drift tube with different ion densities, plasma electron densities, beam density distributions, and beam energies. The beam transportation in a small diameter beam tunnel was also simulated. It showed the maximum beam current with a small velocity spread that can be transported in the beam tunnel was determined by the diameter of the beam tunnel and the ion density. High injected current will cause significant beam loss and reduce the overall efficiency. The simulation results indicate a minimum diameter of the beam tunnel in a millimeter-wave circuit that can be effectively driven by a pseudospark-sourced electron beam. The equivalent upper limit in the operating frequency is about 400 GHz.

Plasma simulation to analyze velocity distribution characteristics of pseudospark-sourced electron beam**Project supported by the Sichuan Science and Technology Program, China (Grant No. 2019YJ0188) and the National Natural Science Foundation of China (Grant Nos. 61671116 and 11905026).

Pseudospark-sourced electron beam is a promising candidate for driving vacuum electronic devices to generate millimeter wave and terahertz wave radiation as it has a very high combined beam current density. However, the inherent velocity spread of the beam, which is difficult to measure in experiment, has a great influence on the operating frequency and efficiency of the vacuum electronic device. In this paper, the velocity distribution characteristics of the electron beam produced by a single-gap hollow cathode electron gun are numerically studied and a three-dimensional kinetic plasma simulation model of a single-gap hollow cathode electron gun is built by using particle in cell and Monte Carlo collision methods in Vorpal. Based on the simulation model, the time-dependent evolution of the plasma formation inside the hollow cathode and electron beam generation process are observed. It is demonstrated that the pseudospark-sourced electron beam has a relatively large velocity spread. The time-dependent velocity distribution of the beam is analyzed, and the dependence of the beam velocity distribution under various operating conditions such as anode–cathode potential difference, gas pressure, and cathode aperture size are also studied.

Effect of Tapered Interelectrode Gap Region on Pseudospark-Sourced Electron Beam Emission

In this article, a comparative experimental and simulations analysis has been carried out to understand the role of tapered gap region on current density and energy of pseudospark (PS)-based electron beam emission. These studies have been performed for 5-gap and 8-gap regions in a tapered multigap multiaperture-based PS electron beam source, keeping the same electron beam fill factor, and other geometrical parameters identical. The source has been operated in a self-breakdown mode. Around four-times increase in the beam current has been observed in the 8-gap case at the fixed applied gap potential ranging between 20 and 25 kV, whereas the tapered interelectrode gap angle has been reduced from 7.5° to 4.5°. The field penetration inside the hollow cathode cavity and tapered region have also been analyzed using simulations, and the obtained results are explained based on built-up space charge in these regions.

Study of a 0.35 THz Extended Interaction Oscillator Driven by a Pseudospark-Sourced Sheet Electron Beam

A compact high-power extended interaction oscillator (EIO) driven by a pseudospark-sourced (PS-sourced) sheet electron beam (SEB) is presented at 0.35 THz. It combines the advantages of a planar interaction circuit and a SEB generated from the PS discharge, including a large beam cross-section, high gain per unit length, and high current density with the additional benefit of not requiring an external focusing magnetic field. Staying within what is achievable with microfabrication techniques, the influence of tolerance on the Q value, resonance frequency, and characteristic impedance was investigated. The effect of surface roughness caused by the manufacturing method on Ohmic loss of the material surface was studied. The advanced microfabrication techniques of Ultra Violet Lithographie, Galvanik, and Abformung (UV-LIGA) and Nano-computer numerical control (Nano-CNC), which are capable of realizing high precision and a metal surface of sufficient smoothness, were proposed to manufacture the planar structures. The effect of plasma density in PS-sourced SEB on the resonance frequency of the EIO circuit was investigated. The simulation results showed that the output signal had a slight frequency upshift and a decrease of the output power as the plasma density increased at 0.35 THz, which is consistent with the theoretical analysis. Beam–wave interaction simulations for this planar EIO predicted a peak output power of 1.8 kW at ~0.35 THz using an effective value of conductivity of ${1.1} \times {10}^{{7}}$ S/m to take into account the skin depth and surface roughness.

Influence of Electrode Geometry and Gas on the Breakdown Voltage of Hollow Cathode Discharge With Spark Geometry Under Nanosecond Pulses

Pseudospark-sourced electron beam with high energy can be produced by the pseudospark device. In this paper, the breakdown voltages of the hollow cathode discharge with the pseudospark geometry influenced by electrode geometry and the gas have been studied and analyzed. In the wide range of pressure and gap, the breakdown voltage increases with the increase in the pressure and the gap distance. There is a pressure threshold and a gap distance threshold, below which the discharge mode will change. The product of the pressure threshold and the gap distance threshold is about 65 Pa · mm. The breakdown voltage is affected by the aperture diameter and the gap distance together. When the ratio of the gap distance to the aperture diameter is greater than 4, the electric field inside the small hole is almost zero and the discharge is converted to the plate electrode discharge. When the ratio is in the range of 0.67-4, the breakdown voltage decreases with the increase in the aperture diameter. When the ratio is less than 0.67, the breakdown voltage hardly changes with the aperture diameter.

Demonstration of a Planar ${{W}}$ -Band, kW-Level Extended Interaction Oscillator Based on a Pseudospark-Sourced Sheet Electron Beam

A ${{W}}$ -band extended interaction oscillator (EIO) driven by a pseudospark-sourced sheet electron beam (PS-SEB) is demonstrated in this letter. The idea of combining the advantages of a pseudospark-sourced electron beam (high beam current density), a sheet electron beam geometry (large beam cross-sectional area), and a ladder-like slow wave structure (high-gain per unit length) to generate powerful millimeter-wave radiation was experimentally verified. The PS-SEB based EIO produced ~1.2-kW peak output power, an increase of six times in the measured power from an EIO based on a pseudospark-sourced pencil electron beam. Such a methodology offers a promising solution for portable, low-cost, and powerful millimeter-wave and terahertz-wave radiation sources.

Experimental demonstration of a terahertz extended interaction oscillator driven by a pseudospark-sourced sheet electron beam

We have recently proposed to combine the advantages of a pseudospark-sourced sheet electron beam (PS-SEB) with a planar slow wave structure to generate high power terahertz radiation. To verify this idea, experimental investigation of an extended interaction oscillator based on the PS-SEB has been conducted and presented. A PS-SEB of approximately 1.0 mm × 0.17 mm in size with 21.5 A peak current (1.26 × 104 A/cm2 beam current density) and 34.5 kV peak voltage was measured after propagating a distance of 10-mm without the need of an external focusing magnetic field. A radiation pulse of ∼35 ns in duration and an output power of over 10 W at a frequency of ∼0.2 THz were measured.

Influence of electrode gap on breakdown voltages of a multi-gap pseudospark discharge device under nanosecond pulses

Pseudospark-sourced electron beams of high energy can be produced in multi-gap pseudospark devices under high breakdown voltages. The breakdown voltages and the gap separation of the discharge device have been studied. Collisional ionization in the gaps has been semi-quantitatively analyzed. Based on the results, the influence of the electrode gap on the breakdown voltages has been verified. Collisional ionization during device discharge begins initially in the first gap near the cathode. The electrons produced in the first gap move towards the second gap and contribute to the collisional ionization in the second gap. The process proceeds to successive gaps with collisional ionization occurring in all gaps. For wider gap separations, the number of collisional ionizations in the gap is large, and hence, more electrons move through the intermediate electrodes into the posterior gaps. This creates a cascading of collisional ionizations, leading to a decrease in breakdown voltage. The influence of the coefficient of collisional ionization on the whole process in the posterior gaps may be slight under different gap separations, as electrons moving into the posterior gaps are plentiful. The breakdown voltage mainly depends on the first gap separation near the cathode.

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.

The effect of velocity spread of pseudospark-sourced electron beam to Y-band extended interaction oscillator

Extended interaction oscillator (EIO) operation in the terahertz range puts greater demand on the current density and brightness of an electron beam. The pseudospark (PS)-sourced electron beam is a good candidate for driving such high frequency EIOs as it has a very high combined beam current density and brightness. However, the PS-sourced electron beam can have an inherent velocity spread unless some form of post acceleration is used. Before a new EIO device in the Y-band (220 GHz–325 GHz) based on a PS-sourced electron beam can be realized, it is first necessary to analyze the influence of the beam velocity spread on performance. This paper presents the numerical studies of the EIO performance with the inclusion of the beam velocity spread. It was found that the Y-band EIO circuit can operate in a relatively wide velocity spread range when a high beam current density is used. For an electron beam current density of 1 kA/cm2, the output power is not less than 0.9 times of the power obtained with an electron beam with no velocity spread when the velocity spread is within the range of 0%–15%. We also present our latest results that demonstrate the successful design and optimization of a Y-band EIO driven by a 36 kV, 255 mA (800 A/cm2) PS-sourced electron beam. A peak power over 1.67 kW at 283.7 GHz was achieved using an electron beam, which can be generated by PS discharge, with a velocity spread of 12.5%.

Study of the beam profile and position instability of a post-accelerated pseudospark-sourced electron beam

A pseudospark-sourced electron beam is a promising candidate for driving a THz millimeter wave radiation source. However, the physics governing the electron beam density profile and the beam center deviation from the axis of the structure, which may be caused by the randomness in the pseudospark discharge process, remains still unclear especially for the high energy component of the pseudospark-sourced electron beam, which is usually non-mono-energetic. It is essential to study the electron beam density profile and the beam center position distribution for optimizing the pseudospark discharge configuration. In this paper, images of some single-shot electron beam pulses have been captured using a 50 μm thickness stopping copper foil and a phosphor screen coated with a P47 scintillator to study the electron beam density profile and the beam center position distribution of the high energy component of the electron beam. The experiments have been carried out on two pseudospark discharge configurations with two different size hollow cathode cavities. The influence of the cathode aperture of each configuration has also been studied according to the beam images. Experimental results show that the beam profile of the high energy component has a Lorentzian distribution and is much smaller than the axial aperture size with the beam centers dispersing within a certain range around the axis of the discharge structure. The pseudospark-sourced electron beam with a larger hollow cathode cavity shows a smaller full width at half maximum radius and a more concentrated beam center distribution.

Advanced post-acceleration methodology for pseudospark-sourced electron beam

During its conductive phase, a pseudospark discharge is able to generate a low energy electron beam with a higher combined current density and brightness compared with electron beams formed from any other known type of electron source. In this paper, a configuration is proposed to post-accelerate an electron beam extracted from a single-gap pseudospark discharge cavity in order to achieve high quality high energy intense electron beams. The major advancement is that the triggering of the pseudospark discharge, the pseudospark discharge itself, and the post-accelerating of the electron beam are all driven by a single high voltage pulse. An electron beam with a beam current of ∼20 A, beam voltage of 40 kV, and duration of ∼180 ns has been generated using this structure. The beam energy can be adjusted through adjusting the amplitude of the voltage pulse and the operating voltage of the whole structure, which can be varied from 24 to 50 kV with an efficient triggering method under fixed gas pressure below ∼10 Pa.

Study of a 0.2-THz Extended Interaction Oscillator Driven by a Pseudospark-Sourced Sheet Electron Beam

The study of a planar G-band extended interaction oscillator (EIO) driven by a pseudospark-sourced (PS) sheet electron beam is presented. This enables the advantages of a planar interaction circuit combined with the merits of a PS sheet electron beam, including large beam cross section, high current density and the fact that a PS electron beam does not require the use of an external focusing magnetic field. Beam–wave interaction simulations for this planar EIO predicted a peak output power of 2.1 kW at ~0.2 THz. Investigations indicate that this planar EIO has a better tube performance with a higher radiation power compared with the PS pencil electron beam EIO.

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.

Simulation and Experiments of a &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$W$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Band Extended Interaction Oscillator Based on a Pseudospark-Sourced Electron Beam

This paper presents the first experimental results of an extended interaction oscillator (EIO) based on a pseudospark-sourced electron beam, which produced a peak output power over 38 W at $W$ -band. The advantages of the newly developed device are: 1) transport of the electron beam by the positive-ion focusing channel without the need of an external magnetic field and 2) high interaction impedance and high gain per unit length of the EIO circuit. The experimental results agree well with the 3-D particle-in-cell simulations.