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A new metal-dielectric cathode with long lifetime
Physics of Plasma
Abstract
Explosive emission cathodes are widely used in high power microwave generation. Conventional metallic cathodes have the disadvantages of bad emission uniformity and short lifetime. In order to improve the emission property of metallic cathodes, a new metal-dielectric cathode is fabricated with the plasma spraying technology. Unlike previous metal-dielectric cathodes, our metal-dielectric cathode adopts a ferroelectric ceramic which possesses a large permittivity. The results of lifetime experiments show the metal-dielectric cathode presents just slight performance deterioration within 2.5 × 10⁵ pulses and thus has a much longer lifetime than the normal copper cathode. The morphology observation demonstrates that the good emission property of the metal-dielectric cathode may benefit from the appearance of irregularities on the dielectric surface, which will have large microscopic field enhancement factors with the help of large permittivity of the ferroelectric material.
... Therefore, to achieve L-EECs with low plasma drift rates, long lifetimes, satisfactory emission uniformity, and low emission thresholds, cathode materials have been made considerable development in the past decades. [8][9][10][11][12][13] Graphite is one of the most commonly used EEE materials due to its long lifetime and simple preparation process. 8,14 Graphite also has better stability in repetitive frequency operation compared to velvet or metallic materials. ...
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.
... Due to the high emission threshold and long emission delay [14], the metal EEE cathode has hardly appeared in recent studies. The metalceramic cathode has a long life, but the emission current is low [15], and its practicability in the RM has yet to be developed. The low lifetime limits the application of velvet cathodes in RM, despite its high current density and low emission threshold [16]. ...
Flake graphite has excellent explosive electron emission performance for its plenty surface micro-bulges and high graphitization degree, but its advantages have rarely been studied in relativistic magnetrons. In this paper, flake graphite cathodes with different fineness were used in coaxial diodes and relativistic magnetrons, and the influence of the surface microstructure of the flake graphite cathode on the relativistic magnetron performance is demonstrated for the first time. The result showed that the graphite microstructure mainly affected the initiation stage of explosive emission. Moreover, the delay time is determined by the degree of graphitization and the surface microstructure of graphite, and the surface microstructure plays a major role. Under the applied voltage of ~330kV-~490kV, The finer flake graphite possessed a shorter emission delay and facilitated the microwave excitation. The excitation time of finer flake graphite is ~3ns, and ~5ns for lager flake graphite. After several operations of the relativistic magnetron, the edges of flake graphite were blunted, and the surface of the flake graphite appeared droplet-like areas formed by anode atom sputtering. This work reveals the advantages of the finer flake graphite cathode in the fast priming of the relativistic magnetron, thus shedding new light on cathode design of the relativistic magnetron.
... In Refs. 4 and 22, annular copper-glass EECs are fabricated and show preferable emission properties, which are much better than those of a pure copper cathode. In Ref. 23, a copper-BaTiO 3 EEC is fabricated by spraying a layer of a ferroelectric material on the side of an annular copper base and the results show that the copper-BaTiO 3 cathode possesses a better emission ARTICLE scitation.org/journal/adv property and a much longer lifetime than the normal copper cathode. ...
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.
Time-and-space resolved cathode plasma expansion velocities in magnetically insulated coaxial diodes (MICD) with edged cathodes made of bronze dielectric, bronze, and graphite have been compared through optical diagnosis. The voltage and current of the diode pulse of 260 ns are about 400 kV and 2.8 kA, respectively. The axial and radial expansion velocities of the cathode plasma were obtained through the angled lateral view by utilizing high-speed framing camera combined with the digital image processing methods. In addition, the evolution of the cathode and collector plasmas was visible firstly on the front view at the same time. The bronze-dielectric cathode not only showed low expansion velocity of about 0.5 cm/
but also had other superior characteristics indicated by the strong emission capability, the fast plasma formation and the uniform plasma distribution. Thereby, the metal–dielectric cathode is a good candidate for applications in a long-pulsed MICD.
The lifetime of explosive emission cathodes is important for high power microwave generators operating in the repetitive regime. For normal metallic cathodes, micropoints on the cathode surface with large field enhancement factors may be gradually consumed in explosive electron emission, which will lead to a limited lifetime. In this paper, a metal-ferroelectric cathode made of stainless steel and BaTiO3 is manufactured. Under voltage close to 1 MV and current near 10 kA, this cathode presents a much longer lifetime than the normal stainless steel cathode, demonstrating the lifetime advantage of the metal-ferroelectric cathode. Nevertheless, in the lifetime experiment of 1.28 × 10⁵ pulses, this metal-ferroelectric cathode also presents obvious lifetime phenomena, one of which is the microwave duration generated by a relativistic backward wave oscillator decreasing from 27 ns to 19 ns. Observation of the cathode surface morphology shows that the emission property deterioration of the metal-ferroelectric cathode may originate from severe ablation of the ferroelectric ceramic layer, which leads to shortening of the ceramic layer relative to the metallic layer. Therefore, choosing the metallic material properly and decreasing the blade thickness of the metallic layer moderately may suppress the relative shortening of the ceramic layer and thus can further lengthen the lifetime of the metal-ferroelectric cathode.
The design goals of a 20 GW/100 Hz repetitive pulsed accelerator are: output power exceeding 20 GW, pulse width about 40 ns, and repetition rate ranging from 1 to 100 Hz. General design of the device based on Tesla technology is described in this paper. For charging the forming line, a Tesla transformer with open magnetic core is used, whose efficiency exceeds 75% by optimization. The output power of the device reaches 22.6 GW at 100 Hz repetition rate, and operation experiments have verified reliability. The device is was mainly applied to generating intense current. It can also be applied to mechanism researches of the intense current diode and high power microwave.
A repetitively pulsed diode capable of generating intense electron beams of 100 kA currents with electron energy of 500 keV was developed and a ceramic honeycomb slab was placed between the cathode and the anode. It was observed that the electron and ions were confined within the ceramic pores, redistributing the electric field by reducing it within the ceramic pores and increasing it on the cathode surface. It was also observed that no surface plasma was detected outside the pores inside the diode vacuum. The results show that the cathode may satisfy the requirement for KrF laser inertial fusion energy research.
Cesium-iodide (CsI)-coated graphite cathodes are promising electron sources for high power microwave generators, but the mechanism driving the improved emission is not well understood. Therefore, an ab initio modeling investigation on the effects of thin CsI coatings on graphite has been carried out. It is demonstrated that the CsI coatings reduce the work function of the system significantly through a mechanism of induced dipoles. The results suggest that work function modification is a major contribution to the improved emission seen when CsI coatings are applied to C.
We present results of the investigation of a metal–ceramic cathode suitable for technological applications. The cathode was tested in electron diodes powered by two different high-voltage generators 500 kV, 50 ns, 300 , 200 Hz and 300 kV, 250 ns, 84 , 5 Hz. The metal–ceramic cathode which was made in a form of a disk was composed of TiO 2 ceramics with stainless steel spherical particles uniformly inserted inside. It was shown that already at relatively low accelerating fields (E5 kV/cm) fast cathode plasma formation occurs as a result of surface flashover. This surface flashover is initiated in triple points located in micropores which were formed during the process of cathode preparation due to the different thermal expansion coefficients of the ceramic and the metal. Experimental data concerning the uniformity of the light emission from the cathode surface and inside the anode–cathode gap and divergence of the generated electron beams are presented. Also, it is shown that the uniformity of the generated electron beam depends strongly on the duration of the accelerating pulse. © 2002 American Institute of Physics.
The operation of cold explosive-emission cathodes having a current density of ∼104 A/cm2, fabricated using various materials, was investigated under a large number of switching cycles. The cathode voltage was ∼500
kV, the maximum current ∼5 kA, and the pulse duration ∼20 ns. It is shown that when the number of switchings is small (⩽103 pulses), cathodes having similar geometry exhibit similar emission properties. For most of the materials studied, as the
number of switching cycles increases (⩾103 pulses), the current rise time increases (as far as the pulse duration) and the maximum vacuum diode current decreases. When
a graphite cathode was used, the maximum current remained unchanged up to 108 switching cycles. The mass removed from the cathode was determined for various materials. The results were used to achieve
continuous operation of a relativistic 3 cm backward-wave tube having an output power of 350–400MW and an almost constant
power level during 108 pulses at a repetition frequency of 100–150 Hz.
It has been shown experimentally that electron current densities of more than 30 A/cm2 can be achieved from a cathode made of ferroelectric ceramic, when applying a field of order 0.1 MV/m. This current exceeds the Child–Langmuir current by two orders of magnitude. The current in the diode varies linearly with the applied voltage, provided that the latter is positive. In this theoretical study we show that the ferroelectric material plays a crucial role in the emission process. When a voltage is applied to the ferroelectric, the internal polarization field varies and the amount of screening charge required decreases. As a result, the electrons distribution near the cathode changes, forming a cloud which fills part of the diode gap. If now a positive voltage is applied to the anode, electrons are readily transferred through the diode gap. The qualitative and quantitative results of the theory are in good accordance with the experiment.
Strong pulsed electron emission has been observed from 12/65/35 lead lanthanum zirconate titanate ceramic composition in two different nonswitched phases at room temperature and at the temperature 100 °C. The electron emission parameters of this composition appear to be independent of phase for the two phases investigated. Fast photography and direct observation show that the strong electron emission occurs from the surface discharge plasma. The new experimental data make it possible to demonstrate the validity of the Child–Langmuir law for this electron emitter. A pulsed plasma lead lanthanum zirconate titanate ceramic cathode with burst frequency up to 100 kHz and collector current density up to 10 A/cm2 is developed. © 1996 American Institute of Physics.
Experiments have proven that surface contaminants on the cathode of an electron beam diode influence electron emission current and impedance collapse. This letter reports on an investigation to reduce parasitic cathode current loss and to increase high voltage hold off capabilities by reactive sputter cleaning of contaminants. Experiments have characterized effective radio frequency (rf) plasma processing protocols for high voltage anode–cathode (A–K) gaps using a two-stage argon/oxygen and argon rf plasma discharge. Time-resolved optical emission spectroscopy measures contaminant (hydrogen) and bulk cathode (aluminum) plasma emission versus transported axial electron beam current turn on. Experiments were performed at accelerator parameters: V = −0.7V=−0.7 to −1.1 MV,−1.1MV, I(diode)=3–30 kA,I(diode)=3–30kA, and pulse length=0.4–1.0 μs.length=0.4–1.0μs. Experiments using a two-stage low power (100 W) argon/oxygen rf discharge followed by a higher power (200 W) pure argon rf discharge yielded an increase in cathode turn-on voltage required for axial current emission from 662±174 kV662±174kV to 981±97 kV.981±97kV. The turn-on time of axial current was increased from 100±22100±22 to 175±42 ns.175±42ns. © 1999 American Institute of Physics. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/69353/2/APPLAB-75-1-31-1.pdf
The emission threshold of explosive emission cathodes (EECs) is an important factor for beam quality. It can affect the explosive emission delay time, the plasma expansion process on the cathode surface, and even the current amplitude when the current is not fully space-charge-limited. This paper researches the influence of the emission threshold of an annular EEC on the current waveform in a foilless diode when the current is measured by a Rogowski coil. The particle-in-cell simulation which is performed under some tolerable and necessary simplifications shows that the long explosive emission delay time of high-threshold cathodes may leave an apparent peak of displacement current on the rise edge of the current waveform, and this will occur only when the electron emission starts after this peak. The experimental researches, which are performed under a diode voltage of 1 MV and a repetitive frequency of 20 Hz, demonstrate that the graphite
cathode has a lower emission threshold and a longer lifetime than the stainless steel cathode according to the variation of the peak of displacement current on the rise edge of the current waveform.
A high voltage pulse generated by changing the temperature of a pyroelectric crystal was used to trigger a strong ferroelectric electron emission from a ferroelectric cathode. Different configurations such as a positive or negative pulse being applied to the front or back of the ferroelectric cathode were investigated. Negative pulse applied to the front of the cathode was found to generate the largest emission current and total charges. These differences in emission properties are interpreted using the mechanism of surface plasma assisted electron emission.
Investigation of relativistic backward wave oscillator with high efficiency and power capacity is presented in this paper. To obtain high power and high efficiency, a TM{sub 021} mode resonant reflector is used to reduce the pulse shortening and increase power capacity to about 1.7 times. Meanwhile, an extraction cavity at the end of slow wave structure is employed to improve the efficiency from less than 30% to over 40%, through the beam-wave interaction intensification and better energy conversion from modulated electron beam to the electromagnetic field. Consistent with the numerical results, microwave with a power of 3.2 GW, a frequency of 9.75 GHz, and a pulse width of 27 ns was obtained in the high power microwave generation experiment, where the electron beam energy was configured to be {approx}910 kV and its current to be {approx}8.6 kA. The efficiency of the RBWO exceeds 40% at a voltage range of 870 kV-1000 kV.
A low-impedance transit-time oscillator without foils has been proposed in our previous work. Recently, the correlated experiments have been carried out in our laboratory. In the experiments, two kinds of explosive-emission cathodes made of copper dielectric and polymer velvet were employed, and the electron collectors mainly consist of stainless steel and graphite. The experiments show that the cathode and collector materials have great influence on microwave generation in the device. This paper presents the experimental results, along with comparisons with the simulation results. It was found that the combination of the polymer-velvet cathode and the graphite collector is much more advantageous to the generation of high-power microwaves.
It is known that the Curie point theta of the ferroelectric BaTiO3 shifts to higher temperatures when a dc bias field is applied. If the crystal shows a sharp transition, we expect by applying an ac field at the Curie temperature that the crystal would become alternately ferroelectric and nonferroelectric in the cycle of the ac field. This can be seen in the shape of the hysteresis loop at temperatures slightly above theta. In the center of the polarization P versus field E plot, we observe a linear behavior corresponding to the paraelectric state of BaTiO3 above theta. At both high voltage ends, however, we observe a hysteresis loop corresponding to the ferroelectric state. A change in temperature causes a change in size and shape of the double hysteresis loops, ranging from a line with curves at the ends (higher temperature) to two overlapping loops (lower temperature). The results obtained allow us to calculate the different constants in the free-energy expression of Devonshire and Slater. One of the results shows that the transition is of the first order since the P4 term turns out to be negative. The properties of the hysteresis loops are discussed, especially the large spontaneous electrical polarization and the low coercive field strength.
High-resolution electron energy distribution measurements of individual microscopic sites have been used to develop a detailed theoretical model of the field-induced hot-electron emission mechanism responsible for pre-breakdown currents at vacuum-gap fields of 10-30 MV m-1. The model, which is based on the formation of conducting channels in a metal-insulator-vacuum (m.i.v.) micro-emission regime, has provided experimentally verified quantitative relations for the current-voltage characteristic, spectral shape, and the field-dependence of the spectral full width at half maximum (f.w.h.m.) and shift.
The anomalous behaviour of powdered barium titanate is confirmed by the use of the formula for mixtures. Systematic measurements of the dielectric properties of BaTiO3 have been done for two cases: embedded grains and fine-grained pressed powder. The influence of the size and shape of the grains, and of the method of preparation of the samples, has been studied.
This paper presents a physical description of the cathode plasma process of an explosive emission cathode (EEC) and experimental results on a type of oil-immersed graphite EEC. It is believed that the generation of a cathode plasma is mainly dependent on the state of the cathode surface, and that adsorbed gases and dielectrics on the cathode surface play a leading role in the formation of the cathode plasma. Based on these ideas, a type of oil-immersed graphite EEC is proposed and fabricated. The experiments indicate that the oil-immersed cathodes have improved emissive properties and longer lifetimes.
As a promising kind of high current cold cathode, the Ferroelectric Cathode (FEC) has several significant advantages, such as a controllable trigger time, lower vacuum requirement and large emitting area fabricability. The emitting current density of the FEC fabricated at Tsinghua University was more than 200 A/cm 2 . In order to make the ferroelectric cathode into practical applications, a high current density diode using a ferroelectric cathode was designed, based on the PIC simulation. The performance of the FEC diode was investigated experimentally. When the applied diode voltage was 60 kV, a current density of more than 250 A/cm 2 was obtained, and the current density distribution was also measured.
In order to improve the pulse repetition rate and the maintenance-free lifetime of an improved magnetically insulated transmission line oscillator (MILO) previously developed in our laboratory, a metal-dielectric cathode is investigated experimentally. It consists of three components: a stainless steel base, bronze foils, and double-sided printed boards. The experimental results show that the shot-to-shot reproducibility of the diode voltage and current is very good and the performances of the improved MILO are steady. In addition, no observable degradation could be detected in the emissive characteristic of the metal-dielectric cathode after 350 shots. The experimental results prove that the metal-dielectric cathode is a promising cathode for repetitively pulsed MILO operation. However, the leading edge of the radiated microwave pulse is gradually delayed during the repetition rate. A likely reason is that high pressure results in gas ionization in the beam-microwave interaction region, and plasma formation delays the time that the improved MILO achieves nonlinear steady state.
A two-stage 500 kV 200-A ferroelectric electron gun has been
designed, fabricated, tested, and used in a high power microwave
amplifier experiment. We report on the operational characteristics of
the gun including measurements of the beam dynamics. The optimum
conditions for application of the trigger and its timing are also
reported. Faraday cup measurement shows that the beam radius is 4.1 mm,
in good agreement with simulation. The gun is designed for use in
traveling-wave tube amplifiers, and testing of an X-band amplifier
driven by the gun is reported. A peak output power of 5.9 MW has been
observed from a single stage amplifier driven by a 100 A. 450 kV beam.
This corresponds to energy converging efficiency of 13.1% and is the
first observation of high power (~MW) microwave generation using the
beam generated from a ferroelectric cathode,
We present in this review an account of recent research into the
use of ferroelectrics as electron beam sources for pulsed power
applications. The work is reviewed according to the ferroelectric
material used and the switching process employed. Most of the current
research uses PLZT or PZT, which can be ferroelectric,
antiferroelectric, or paraelectric depending on the stoichiometry.
Switching is accomplished by the application of a fast-rising electric
field to the ferroelectric material. The most commonly used materials
are PLZT 2/95/5 and PLZT 8/65/35 or PZT (with no lanthanum): in the
former case, the applied field switches the material from the
antiferroelectric state to the ferroelectric state, and in the latter
cases, around a hysteresis loop. Results have been reported with
ferroelectric cathodes where current densities of up to a few hundred
amperes per square centimeter have been achieved, with pulse durations
of several microseconds. With shorter duration pulses and PLZT cathodes,
repetition rates of up to 2 MHz have been achieved. In this paper, we
focus on the results reported in the literature, and include a brief
account of the physical interpretation of the data. The possible use of
ferroelectric sources for pulsed power applications is indicated
The processes occurring in the diodes of high-current electron
accelerators which affect the electron beam structure are discussed. Two
types of diodes are analyzed: planar diodes producing large
cross-section beams, and magnetically insulated diodes producing
cylindrical beams. The effects considered are: the screening effect, the
effect of touches, the relay effect, the ring effect, etc. They are
entirely associated with the explosive electron emission occurring on
the cathode of a high-current diode. Methods which can be employed to
eliminate or control some of these effects are described-namely, the
application of an additional magnetic field, artificial initiation of
emission centers, and employment of metal-dielectric cathodes. Examples
of several types of cathode are given
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