Explosive Emission Cathode Research Articles

Experimental study of flake graphite large-area explosion electron emission cathode performance based on particle size discrepancy

Graphite is the most commonly used large-area cathode material in the high-power vacuum electron device for its long life and good repetitive-frequency performance, but its particle size selection scheme has not been elucidated. Therefore, in the present work, the explosive electron emission properties of flake graphite large area explosive electron emission cathodes with varied particle sizes are studied. The 320 mesh flake graphite has a current delay reduction of 0.2–0.9 ns compared to other samples at the same peak voltage. According to the Murphy–Good equation, the emission performance of each sample during the priming stage was calculated, and the threshold characteristics of 320 mesh flake graphite were significantly better than the other samples. After the space charge limited current is formed, the plasma expansion rate of all samples is 1.2±0.2cm/μs with a difference not exceeding 4% between samples. Our research provides a strong basis for the selection of raw materials for graphite based large area explosive electron emission cathodes and contributes to the development of cross-field high-power vacuum devices.

Influence of Anode and Cathode Plasmas on the Operation of an Electronic Diode with an Explosive-Emission Cathode

Influence of Anode and Cathode Plasmas on the Operation of an Electronic Diode with an Explosive-Emission Cathode

Characterizing the high-power-microwaves radiated by an axial output compact S-band A6 segmented magnetron fed by a split cathode and powered by a linear induction accelerator

In a recent publication [J. Appl. Phys. 131, 023301 (2022)], microwave generation by a segmented A6 S-band axial output relativistic magnetron (RM) fed by a split cathode and powered by a small pulsed power generator was demonstrated for the first time. In the present article, we add to the same RM a mode converter and an antenna and drive it by using a modified linear induction accelerator (LIA) (≤450 kV, ≤4 kA, ∼150 ns). It was found that the operation of the RM for a split cathode or a common explosive emission cathode differs significantly. For a split cathode, we optimize for the best microwave output, its geometrical parameters, the external magnetic field, and the LIA's charging voltages. For the optimal choice of these parameters, the RM generates ∼160 ns long microwave pulses of ∼130 MW, 1.78 GHz frequency with an electronic efficiency of ∼40%, without pulse shortening. On the other hand, a common solid cathode fed RM showed microwave pulse shortening. We demonstrated that the segmented anode allows using a μs-timescale magnetic field, making it possible for the RM to operate repetitively.

Formation of Anode Plasma in an Electron Diode with an Explosive-Emission Cathode

Formation of Anode Plasma in an Electron Diode with an Explosive-Emission Cathode

Study on explosive emission characteristics of the silicon carbide modified graphite cathodes with varied morphologies

Explosive emission cathodes are extensively used in high power microwave sources. Growing requirements are highlighted in the performance of the explosive emission cathode with the development of the high power microwave technology. In order to improve the emission properties of the graphite cathode, modifications using SiC particles or whiskers are carried out by the in situ chemical vapor reaction method. The experiments demonstrate that SiC whiskers accelerate the explosive emission turn-on speed of the graphite cathode for large field enhancement factors and improve the emission uniformity due to the surface flashover plasma generation mechanism. SiC particles increase the emission capability of the graphite cathode, possibly corresponding to the dielectric properties of SiC particles. These results suggest that the SiC whiskers modified graphite cathode is suitable for applications under low magnetic field and large emission area conditions, while the SiC particles modified graphite cathode has a brighter application prospect in the long-time stable operation.

CNT/Cu composite cathode: A new approach to long lifetime for explosive emission cathode

Carbon nanotube (CNT) cathodes have attracted much attention in recent years due to the advantages of large field enhancement factor and low emission threshold. However, the severe ablation under intense emission makes the lifetime short and therefore limits the application in the field such as high power microwave generation. To resolve this problem, this paper proposes to mix CNTs with metals, and a novel CNT/Cu composite cathode is manufactured. The lifetime experiments under voltage of 940 kV and repetition frequency of 20 Hz demonstrate that the lifetime of the CNT/Cu composite cathode is over 3 × 105 pulses, which is much longer than that of the normal copper cathode by at least one order of magnitude. The microscopic morphology analysis reveals that the CNT micro-protrusions and whiskers should be vital for the good emission property of the new cathode.

Quantitative evaluation of emission uniformity of the annular explosive emission cathode

Explosive emission cathodes (EECs) are widely used in the field requiring intense electron beams. The uniform emission of EECs is vital for applications. To effectively evaluate the emission uniformity of annular EECs, this paper proposes a quantitative evaluation model based on the analysis of cathode plasma images and constructs a parameter which is named as the non-uniformity value to describe the extent of emission uniformity. The experiment designed to investigate the cathode emission uniformity under different circumstances demonstrates that the model is valid. The experimental results give quantitative information of uniformity change in explosive emission and show that the non-uniformity values of graphite and aluminum cathodes can be significantly decreased by increasing the guiding magnetic field and the diode voltage.

Investigation of the characteristics of the explosive-emission cathode based on carbon fiber in pulsed-periodic electron beam generation

Explosive electron emission cathodes provide a high current density without auxiliary systems and, therefore, have long been the subject of research for high-power pulsed accelerators. The paper summarizes the results of studies on the operation of an explosive-emission cathode based on carbon fiber in operating modes with a pulse repetition rate of 5–15 pps, at a voltage rise rate of ≈ 2.5 × 1012 V/s. The change in current and voltage as a function of the number of pulses and the diode impedance in the pulsed-periodic mode with a voltage pulse duration of 200 ns at full width half maximum (FWHM) are studied. During testing, the total diode current reduced by 25%. The change in the characteristics of the cathode at different pulse repetition rates is given up to 3.2 × 105 pulses. Changes in characteristics are correlated with changes in the microstructure of the cathode surface, presented in the enlarged images of the cathode surface.

Direct comparison of analytically derived “Fedosov” solution with experimental measurements and numerical simulations of relativistic thin annular electron beam generation and propagation in magnetically insulated coaxial diode of SINUS-6 high-current electron-beam accelerator

The analytically derived solution for a thin annular electron-beam generation in a magnetically insulated coaxial diode (MICD), also known as “Fedosov solution,” is validated by direct experimental measurements of current–voltage (I–V) characteristics of MICD driven by the high-current electron-beam accelerator SINUS-6 and numerical particle-in-cell (PIC) simulations of a thin annular electron-beam generation and propagation in computer model of MICD of corresponding geometry and initial/boundary conditions. The experimental measurements of I–V characteristics of MICD are performed by monitoring and analyzing a voltage applied to MICD and an electron-beam current generated by an explosive-emission cathode of MICD. Numerical simulations of MICD operation are performed using ICEPIC code. The comparisons of analytically derived “Fedosov” solution with experimentally measured I–V characteristics of MICD as well as with results of ICEPIC simulations of a three-dimensional computer model of MICD operation are analyzed, and a conclusion is made that all three measures are pretty much consistent with each other. The appliance of the analysis is purposed now in using SINUS-6 accelerator for designing, prototyping, and studying different variants of O-type slow-wave high-power microwave/millimeter-wavelength vacuum electronic devices.

Influence of anode and cathode plasmas on the operation of an electronic diode with an explosive-emission cathode

The results of modeling and experimental investigation of the formation of anode and cathode plasmas in a vacuum diode with an explosive-emission cathode during the generation of a pulsed electron beam with a current density of 0.3-0.4 kA/cm2 and an accelerating voltage of 300-500 kV are presented. It is shown that the concentration of the anode plasma does not exceed 1010 cm-3 and it does not significantly contribute to the operation of the diode. However, the complete desorption of molecules from the working surface of the explosive-emission cathode and the high efficiency of shock ionization of atoms ensure the formation of a cathode gas plasma with a concentration of ~1016 cm-3. It is found that the charge of the explosive-emission plasma layer is significantly less than the charge of the electron beam and the main source of electrons is not an explosive-emission plasma, but a cathode gas plasma. In this case, the electron current is limited by the concentration of the cathode plasma. The use of a cathode with a developed surface (a cathode with a carbon fabric coating) allows increasing the total charge of the electron beam by more than 1.5 times without changing the cathode diameter and the anode-cathode gap. Keywords: high-current electron beam, explosive emission, plasma concentration, electronically stimulated desorption.

Влияние анодной и катодной плазмы на работу электронного диода со взрывоэмиссионным катодом

The results of modeling and experimental investigation of the formation of anode and cathode plasmas in a vacuum diode with an explosive-emission cathode during the generation of a pulsed electron beam with a current density of 0.3-0.4 kA/cm^2 and an accelerating voltage of 300-500 kV are presented. It is shown that the concentration of the anode plasma does not exceed 10^10 cm^-3 and it does not significantly contribute to the operation of the diode. However, the complete desorption of molecules from the working surface of the explosive-emission cathode and the high efficiency of shock ionization of atoms ensure the formation of a cathode gas plasma with a concentration of 10^16 cm^-3. It is found that the charge of the explosive-emission plasma layer is significantly less than the charge of the electron beam and the main source of electrons is not an explosive-emission plasma, but a cathode gas plasma. In this case, the electron current is limited by the concentration of the cathode plasma. The use of a cathode with a developed surface (a cathode with a carbon fabric coating) allows increasing the total charge of the electron beam by more than 1.5 times without changing the cathode diameter and the anode-cathode gap.

Current Characteristics of a High-Current Electron Gun with Multi-Gap Initiation of Explosive Emission by Dielectric Surface Flashover

Current characteristics of a plasma-filled, high-current electron gun at different accelerating voltages and densities of plasma preliminary filling the space between the cahode and collector have been investigated. Multi-gap initiation of explosive emission is performed with the use of parallel operated, resistively decoupled arc plasma sources which electrodes and tube ceramic isolators are built-in to the explosive-emission cathode. Experiments have demonstrated operability of high-current electron gun with a new cathode assembly at accelerating voltage up to 30 kV and its good emissivity (average in area current density) which was approximately 1.4-2.4times higher than an emissivity of traditional electron gun with plasma anode and multi-wire copper cathode. Keywords: high-current electron beams, explosive emission, cathode assembly, multi-gap initiation, dielectric surface flashover.

Comprehensive Particle-In-Cell Simulations of Ten-Vane Microwave Oven Free-Running 2.45 GHz “Cooker” Magnetron With ICEPIC and CST-PS Codes

Particle-in-cell (PIC) simulations of a microwave oven free-running 2.45 GHz “cooker” ten-vane magnetron operating with a “cold” explosive-emission cathode in a uniform axial magnetic field are performed with two computer codes, improved concurrent electromagnetic PIC (ICEPIC), and CST Particle Studio (CST-PS). PIC simulations are done to compare results obtained by these two codes to each other and, in such a way, identify all possible advantages and disadvantages of each code performance as a reliable tool allowing to predict all important characteristics of the magnetron operation. Results of the PIC simulations show that the range of applied voltages within which the magnetron is able to operate in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> -mode at an external uniform axial magnetic field 0.19 T is much broader as it is predicted by the CST-PS code, from 4.00 to 4.70 kV, than as it predicted by the ICEPIC code, from 3.90 to 4.10 kV. It is also shown that while the magnetron startup time gradually decreases, as it is predicted by the ICEPIC code, from about 700 ns (3.90 kV) to about 100 ns (4.20 kV) with the applied voltage increase, it initially chaotically varies, as it is predicted by the CST-PS code, between about 1000 ns (4.03 kV) and about 300 ns (4.01 kV) and only then gradually decreases down to 10–20 ns (4.40–4.60 kV) with the applied voltage increase. Additional PIC simulation with MAGIC or VORPAL codes would be interesting to perform as well to compare the obtained with these codes simulation results with results described in this article.

Measurement of the electron beam energy in a source with a plasma anode and the beam extraction into the atmosphere through a foil window

Measurements of the energy of an electron beam in an electron beam source with an explosive-emission cathode, a plasma anode and a foil window, providing the extraction of an electron beam with a cross-section of (100–200) cm2 into the atmosphere, have been performed. The high voltage source was a Marx generator based long lines with matched loads. The energy values were calculated from the results of measurements of the temperature of a beam collector placed in vacuum using thermistors and an infrared imager. At an accelerating voltage of ~ 200 kV, a current of (1–2) kA, and an electron beam duration of 5 μs, the maximum values of the energy released in the collector with a cross section of 74 cm2 were (650–850) J/pulse. A decrease in the current and energy of the beam was recorded approximately by a factor of two after passing through an AMG-2n aluminum-magnesium foil with a thickness of 30 μm. The use of infrared imager for recording the beam structure in the plane of the output window in air has been tested.

Experimental Generation of 1.1-kA Gyrating Electron Beam Current From an Explosive Emission Cathode Magnetron Injection Gun

Most high-power gyrotrons are driven by the thermal cathodes, and the output power can only reach megawatt (MW) level. In the present study, a single anode explosive emission cathode (EEC) magnetron injection gun (MIG) with the high beam current, the pitch radio of 1.2 and the controlled beam velocity spread was designed. At the peak voltage of 300 kV, the peak beam current of 1.1 kA and pulsewidth of 170 ns were emitted by a carbon fiber array (CFA) emitter. Compared to the reported EEC MIG, large beam loss was avoided in this experiment. The guiding center radius of 14.1 mm was close to the simulation results. CFA has the stronger emission capability, the lower emission threshold electric field, and the better emission uniformity than sharpened edge graphite (SEG), so it is suggested as the emitter of the EEC MIG.

Production of low-energy, high-current electron beams in a gun with a controlled explosive emission cathode

The operating characteristics of an electron gun with a controlled explosive emission cathode have been investigated. The multichannel initiation of explosive electron emission from the (copper disk) cathode occurs due to the use of simultaneously operating resistively uncoupled arc plasma sources built in the cathode. Three gun operation options were tested, namely with a vacuum, a gas-filled, and a plasma-filled diode. In the last case, a plasma anode was formed using a high-current reflective (Penning) discharge or a hybrid discharge combining a Penning discharge with vacuum arcs. The cathode unit has shown a high emissive power (a factor of 1.5–2 higher than that of a conventional gun with a plasma anode and a multiwire copper explosive emission cathode). Stable operation of the gun was observed at an accelerating voltage of amplitude 5–30 kV, in contrast to at least 17–20 kV required for stable operation of the conventional gun. The gun operation with a vacuum and a gas-filled diode provided a considerably more efficient energy transfer from the power supply to the electron beam.

Cold atmospheric pressure plasma jet prepares TiO2 coating on carbon fibre for field emission and explosive electron emission

As the ‘heart’ of high-current vacuum electronic devices, explosive emission cathodes play an important role in their applications. In this work, a titanium dioxide (TiO2)/carbon fibre cathode prepared by a cold atmospheric plasma jet is reported. TiO2 on the carbon fibre surface prepared by an atmospheric plasma jet is at the mixture of anatase and rutile phases. The field enhancement factor of the TiO2/carbon fibre cathode is 6681, which is over ten times higher than that of the carbon fibre cathode. Furthermore, the TiO2 coating slows the diode impedance collapse and decreases the cathode plasma expansion velocity from 1.5–2.0 to 1.0–1.5 cm µs−1, which is explained by a cathode plasma model. The surface of the TiO2/carbon fibre cathode is covered by 1.6 cm2 plasma, far exceeding that of the carbon fibre cathode (1.1 cm2), which enables uniform explosive electron emission. The obtained results show that TiO2/carbon fibre cathodes prepared by cold atmospheric plasma jets will be potential candidates as high-current explosive electron emitters.

Investigation of Annular Explosive-Emission Cathodes of the Conductor–Insulator Structure

Research is devoted to investigation of properties of pulsed explosive emission cathodes for X-ray tubes. Three types of annular explosive emission cathodes were studied: metal ceramic with slits (of a comb-shaped kind), ring metal ceramic, and carbon ceramic. A pulsed high-voltage generator used in experiments is characterized with the following parameters: voltage pulse full-width at half-maximum is about 20 ns, pulse voltage amplitude is up to 150 kV, and pulse repetition rate is up to 200 Hz. The installation makes it possible to study demountable models of pulsed X-ray tubes of different designs. Current and voltage waveforms corresponding to different operation modes and rates of rise of X-ray tube anode voltage were analyzed. The obtained characteristics of explosive cathodes depend on the cathode’s material, the shape, and the supply voltage of the primary energy storage. The current pulse duration for the ring carbon-ceramic cathode is considerably lower than that for the other cathodes, although it generates a higher radiation dose rate.

Токовые характеристики сильноточной электронной пушки с многоканальным инициированием взрывной эмиссии пробоем по поверхности диэлектрика

Current characteristics of a plasma-filled, high-current electron gun with a new cathode assembly which can be named as a controlled explosive-emission cathode have been investigated. Multi-gap initiation of explosive emission is performed with the use of parallel operated, resistively decoupled arc plasma sources which electrodes and tube ceramic isolators are built-in to the explosive-emission cathode. The initiation of explosive emission was performed using 69 operating in parallel arc plasma sources whose electrodes and tubular ceramic insulators are inserted to the disk explosive-emission cathode. Experiments have demonstrated operability of high-current electron gun with a new cathode assembly at accelerating voltage up to 30 kV and its good emissivity (average in area current density) which was approximately 1.4–2.4 times higher than an emissivity of traditional electron gun with plasma anode and multi-wire copper cathode.

Suppression of the Parasitic Oscillations in anX-Band Relativistic Coaxial Gyrotron Driven by an Intense Beam Current

In 100-MW level relativistic gyrotrons driven by explosive emission cathodes (EECs), significant parasitic oscillations are caused by the high beam current and the large beam velocity spread. In this study, the parasitic oscillations in an X-band coaxial relativistic intense current gyrotron were explored. The oscillations were suppressed in particle-in-cell (PIC) simulations through modifications of design. Lossy ceramics were inserted to suppress the parasitic modes in the beam tunnel. The beam collector was optimized to keep the electron beam from the cyclotron resonance region of the parasitic modes. In the 3-D PIC simulation of the whole tube, the output power of 35 MW and efficiency of 20.6% were achieved under the voltage of 300 kV, the beam current of 567 A, the pitch factor of 1.24, and the velocity spread of 25.6%. A single TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode operation was achieved in this relativistic coaxial gyrotron.