Metal-dielectric Cathode Research Articles

Large size shielded metal-ceramic cathodes

The technology for manufacturing large size shielded metal-ceramic cold cathodes is described. It has been shown that the use of a shielded metal-ceramic cathode allows to significantly increase the impedance of the vacuum diode and to form an electron beam of constant cross-sectional size or of the required size (up to 400 mm wide) with a fairly uniform electron beam current density distribution (≤15%).It has been found that the emission decline of the shielded metal-ceramic cathode, like that of the metal-dielectric cathode operating at a high repetition rate (≥100 pps), is significant at the beginning of the process and then slows down, allowing continuous operation for up to 40 h. Both cathodes require periodic cleaning of the plates, which is more difficult for metal-ceramic plates.At the same time, a metal-ceramic cathode works better at low vacuum, allowing twice as much current to be generated (in the same geometry) as a metal-dielectric cathode, while reducing the accelerating voltage by ∼15%. However, metal-ceramic plates are about 100 times more expensive to manufacture than dielectric plates for a metal-dielectric cathode.Therefore, the choice of cathode type for vacuum diode accelerators is not a trivial problem, but depends on many factors that must be taken into account in each case and for each radiation technology.

Investigating emission characteristics and plasma properties in a long-pulse magnetically insulated coaxial diode with different cathode materials

Magnetically insulated coaxial diodes are widely used in O-type high power microwave device for intensive annular electron beam generation. This study reports on a series of electrical and optical measurements on a long-pulse magnetically insulated coaxial diodes in which the emission characteristics and the plasma properties were observed. The impact of cathode material, blade angle, diode voltage and guiding magnetic field on the emission characteristics and the plasma properties of six types of annular cathodes were studied. Two configurations of these cathodes have been used, one with a sharp edge for aluminum, bronze, graphite, stainless-steel, silicon-aluminum alloy and another metal–dielectric. The impressing diode performance was achieved with the metal-dielectric cathode characterized on the excellent emission characteristics and plasma properties. Moreover, high voltage and small blade angle are beneficial to the emission capability and emission uniformity. For the cathode with smaller blade angle, its axial plasma expansion velocity is smaller during the pulse plateau. The strong magnetic field or the large diode voltage is confirmed to reduce the axial and radial expansion velocities.

Effect of a high-voltage mesh electrode on the volume and surface characteristics of pulsed dielectric barrier discharges

Electrical breakdown in a pulsed asymmetric dielectric barrier discharge between a glass-covered mesh electrode and a grounded metal electrode in the air at atmospheric pressure is investigated. Volume discharge forms between the metal tip and the dielectric surface and spreads over the dielectric surface. Breakdown and discharge behaviors depend on the polarity of the charged electrode covered with glass compared to the metal rod electrode. In the case of the dielectric cathode (covered mesh), volume discharge features a stronger and longer-lasting emission. Volume discharge is weaker with outstretched surface discharge developing on the opposite glass electrode sustained by the embedded mesh when the metal rod functions as a cathode. The development and spatial distribution of the surface discharge depend on the relative polarity of the dielectrics caused by the charge deposition of the preceding discharge and is independent of the polarity of the applied high voltage. The discharge emission is brighter for the metal cathode and dielectric anode than for the metal anode, with a branching discharge developing and spreading in a star-like structure along the embedded grid, while a ring-like structure was observed for the metal anode and dielectric cathode. The duty cycle influences the discharge development and properties through the effects of the gas phase and surface pre-ionization.

Metal–ceramic cathode for nanosecond electron accelerators

The technology for producing metal ceramic (MC) plates is described. It was shown that application of the MC cathode allows one to increase the beam power by a factor ∼1.5 and the current by a factor of 2, compared to metal–dielectric cathodes. It has been found that the emission properties of the MC cathode can be controlled by changing the composition of the MC plate as well as by changing the geometric form of the MC plate.Namely, the rectangular MC plate produces a uniform potential distribution on its surface and it results in faster ignition of the surface flash-over which uniformly covers the cathode surface. This leads to a more uniform electron emission distribution. The latter results in significantly higher (>2 times, up to 20 A/ns) rate of the current increase. In addition, no visible damage of the MC surface was found after 106 shots indicating a potentially long lifetime of such type of cathodes.

High-Current Pulsed-Repetitive Electron Accelerator “Sinus-320”: Formation and Diagnostics of a Wide-Aperture Beam

A wide-aperture (16 cm × 27 cm) source of e-beams for sterilization of plastic packages based on a“SINUS- 320” high-current accelerator with 450 keV energy, 9 kA current, 12 ns pulse duration, and repetition rate up to 100 Hz is described. A metal-dielectric cathode forms e-beam by explosive emission during a few nanoseconds of the voltage pulse. A high degree of e-beam uniformity can be achieved because of a large number of triple points (~800) and of electron scattering in a foil and in a 3 cm air layer for characteristic electron free paths in air up to half a meter.

Time-and-Space Resolved Cathode Plasma Expansion Velocities in Magnetically Insulated Coaxial Diode With a Metal–Dielectric Cathode

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/μs 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.

Russian Physics Journal (Former Soviet Physics Journal) and its Contribution to the Development of High-Current Pulse Electronics and Electrophysics

This article assesses the impact that the journal has had on the research and development in high-current pulse electronics and electrophysics, gas electronics, physics of gas lasers, relativistic microwave electronics, plasma electronics, etc. All these fields of research involve achievements in three areas of science and technology, namely, the creation of high-power nanosecond pulse generators; fundamental studies of nanosecond high-current discharges in gases, which have led to the discovery of the phenomenon of multielectron initiation of discharges, and studies of pulsed vacuum discharges, which have resulted in the discovery of explosive electron emission. The author began his research activities in these areas in 1958 at Tomsk Polytechnic Institute and continued them at the Institute of High Current Electronics of the Siberian Branch of the USSR Academy of Sciences, which was opened in 1977. A number of articles published in the journal were devoted to priority issues such as the studies of nanosecond discharges in gases, in vacuum, and in water; the description of the properties of explosive electron emission; the invention of metal-dielectric cathodes with ferroelectrics, and the proposal of gas compression in devices based on electrically exploded cylindrical conductors.

Investigation of cold cathode for nanosecond electron accelerators

In many technological applications, electron beams of large size are required on one of the coordinates. The cold cathode used for this purpose has to possess high stability of characteristics, durability and be able to generate a uniform distribution of the electron beam current density. Comprehensive research into the properties of the metal dielectric (MD) cathode on the URT-1M accelerator was conducted. The influence of quantity and the relative positioning of the emission elements on the beam parameters and distribution of the electron beam current density were established. The influence of the shape of the cathode case on stability of operation at a high repetition rate was also investigated. A measurement technique using pulsed cathode luminescence (PCL) of phosphorus to estimate the distribution of the electron beam current density was developed. This PCL technique was tested by a dosimetry technique using plastic detectors. The qualitative compliance of the results from both techniques was established. By selecting the number and arrangement of MD plates, it was possible to obtain a more or less uniform (15%) electron beam current density distribution on the output foil.It was set so that falling of emission MD cathode by operation at high repetition rates (>100 pps) occurs rather slowly, thus ensuring continuous functioning for 40 h.

Repetitive nanosecond electron accelerators type URT-1 for radiation technology

The electron accelerator URT-1М-300 for mobile installation was created for radiation disinfecting to correct drawbacks that were found the URT-1M electron accelerator operation (the accelerating voltage up to 1 МV, repetition rate up to 300 pps, electron beam size 400 × 100mm, the pulse width about 100ns). Accelerator configuration was changed that allowed to reduce significantly by 20% tank volume with oil where is placed the system of formation high-voltage pulses, thus the average power of the accelerator is increased by 6 times at the expense of increase in pulses repetition rate. Was created the system of the computerized monitoring parameters (output parameters and thermal mode) and remote control of the accelerator (charge voltage, pulse repetition rate), its elements and auxiliary systems (heat of the thyratron, vacuum system), the remote control panel is connected to the installation by the fiber-optical channel, what lightens the work for service personnel. For generating an electron beam up to 400mm wide there are used metal- ceramic] and metal-dielectric cold cathodes of several emission elements (plates) with a non-uniform distribution of the electron beam current density on the output foil ± 15%. It was found that emission drop of both type of cathodes, during the operation at the high repetition rate (100pps) is substantial at the beginning of the process, and then proceeds rather slowly that allows for continuous operation up to 40h. Experiments showed that linear dependence of the voltage and a signal from the pin-diode remains within the range of the charge voltage 45–65kV. Thus, voltage increases from 690 to 950kV, and the signal from the pin-diode – from (2,8–4,6)*104Gy/s. It allows to select electron energy quite precisely with consideration of the radiation technology requirements.

A new metal-dielectric cathode with long lifetime

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.

Preliminary investigation of an improved metal-dielectric cathode for magnetically insulated transmission line oscillator.

In order to explore the cathode with good repetition quality for the magnetically insulated transmission line oscillators, an improved metal-dielectric cathode is proposed and investigated experimentally. The cathode is designed to be step-like shape, and thin copper plated boards are periodically and compactly arrayed on the surface of the cathode base, which ensures the uniformity of the electrons emitted from the cusps of the copper plated boards. According to the numerical simulation results, the step-like shape is beneficial to convert the kinetic energy of the magnetic insulating current partially and enhance the beam-wave interaction efficiency. Finally, a preliminary experiment of an L-band magnetically insulated transmission line oscillator (MILO) with the improved metal-dielectric cathode is carried out. A high power microwave (HPM) with an average power of 1.95 GW is generated from the MILO, with an efficiency of 13.5%. Under the same experiment condition, the output power and main frequency with the presented cathode are almost the same to those with the velvet cathode. Apart from that, metal-dielectric cathode has the merits of small outgassing and long lifetime, and all of these make the improved metal-dielectric cathode significantly promising for the MILO repetition operation.

Current characteristics of quasi-planar vacuum diodes with explosive-emission cathodes made of various materials at a high-voltage pulse duration of a few nanoseconds

The currents of 5-ns pulsed high-current electron beams produced in a planar vacuum diode with explosive-emission cathodes made of various materials with no external magnetic field at an average electric field strength in the gap of about 300 kV/cm have been measured and time-integrated observation of the optical luminescence of the cathode surface have been performed. Cathodes with a ceramic bushing and spring metal contacts, with ceramic plates set in a magnetic iron matrix, with blades made of stamped exfoliated graphite (Graflex), with blades made of foil fiberglass plastic, and a composite cathode made of crystalline boron and copper powders were tested. The current carried by one emission center has been estimated to range between 5 and 20 A for various cathodes. For the metal-dielectric cathode, the velocity of expansion of the cathode plasma over the ceramic surface has been estimated as 2·107 cm/s. The lifetimes of the cathodes at a pulse repetition rate of 50 Hz have been investigated.

Effects of Different Cathode Materials on Submicrosecond Double-Gap Vircator Operation

The operation of a double-gap S-band vircator has been investigated at submicrosecond duration of a high-current electron beam generated in a planar diode. The experiments were performed at accelerating voltages of les550 kV and diode currents of up to 17 kA using a radio-frequency cavity with a wide coupling window between its two sections. Three types of cathodes have been studied, namely, metal-dielectric, carbon fiber, and velvet cathodes. The main features of the operation of the vircator using each cathode are analyzed. The microwave pulse duration with the metal-dielectric and carbon fiber cathodes reached ~250 ns at the peak power level of ~100 MW; with the velvet cathode, a duration of ~400 ns was achieved. It has been found that, in addition to the common limitations of the microwave pulse duration related to the dynamics of the diode impedance governed by the cathode plasma expansion, there is another factor, namely, the anode-cathode gap, which determines the delay at the beginning of the microwave generation. The latter effect is explained by the role of electrons oscillating between the virtual and real cathodes in the generation process. The issue of radiated microwave frequency behavior is discussed as well.

High-current relativistic electron beam transport in a drift tube in the absence of an external magnetic field

The current of an intense relativistic electron beam emitted from a metal-dielectric multiedge cathode that passes through an anode grid and is transported in a smooth-wall drift tube in the absence of an external magnetic field has been measured at the collector. In these experiments, the electron flux was transported over a distance twice as large as the drift tube diameter. An optimum cathode and storage electrode configurations have been found that allowed the efficiency of the relativistic Cherenkov microwave oscillator without a guiding magnetic field to be increased up to 10% at a maximum output pulse radiation power of 1.3 GW and a working frequency of 4.03 GHz.

A metal-dielectric cathode

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 wide-aperture x-ray source with high intensity and uniformity of radiation

The results of experimental research on an x-ray source intended for preionizing the laser mixture in wide-aperture electric-discharge excimer lasers are presented. An experimental prototype of the source in which an x-ray radiation pulse is formed in an inverted-type vacuum diode during braking of accelerated electrons in a tantalum foil has been developed. It is shown that the use of a metal-dielectric cathode allows generation of x rays with nonuniformity of the intensity distribution within limits of 10% at an energy of quanta no higher than 55 keV and a radiation exposure dose 160 mR.

Improving the Operating Characteristics of an УPT-0.5 Accelerator

An УРТ-0.5 electron accelerator intended for use in radiation technologies was updated after tests were run. To increase the service life of the pulse transformer’s insulation, additional inductance was introduced into the supply circuit. The heating regime of the foil of the vacuum diode’s output window was simplified substantially after the metal-dielectric cathode was replaced with a metal-ceramic one. A metal screen was used on the metal-ceramic cathode to increase the impedance of the vacuum diode.A line for the sterilization of individually packaged disposable medical items was developed on the basis of the updated accelerator.

A Vacuum Diode for the Two-Sided Electron Irradiation of Objects

A vacuum diode for the two-sided electron irradiation of objects makes it possible to reduce the irradiation nonuniformity and increase the thickness of the irradiated layer of the object with a uniform distribution of the absorbed dose over the layer thickness. The cathode holder of the diode is constructed in the form of two symmetrical arms connected to a common current lead. Two cathodes are attached to the arms' ends opposite each other. The electron beam is extracted to the atmosphere through two windows–anodes positioned across from one another. The vacuum diode with metal–dielectric cathodes was tested on a УРТ-0.5 nanosecond electron accelerator (the electron energy is ∼0.5 MeV and the output beam power is ∼1 kW). The results obtained have shown that the diode ensures the two-sided irradiation of objects. Depending on the irradiated surface, the irradiation conditions can be varied by changing the cathode–anode distance in one of the arms of the diode.

Emission uniformity and emittance of explosive field-emission cathodes

Explosive field-emission cathodes as well as plasma-flashover cathodes are important for high-power microwave tubes. These cathodes have the advantage of being lightweight as well as requiring no heater for electron emission. However, this class typically suffers from large amounts of outgassing, nonuniform emission, and very high emittance. In this paper, we review research into the uniformity and emittance of tufted carbon-fiber cathodes that have been coated with cesium iodide (CsI) salt. The CsI cathode is compared to polymer velvet, metal-dielectric, and carbon-slat cathodes. We find the uniformity and emittance are related for all of these cathodes. In general, the more uniform the electron emission, the lower the emittance of the cathode. This article shows that given proper diode design, the tufted carbon-fiber cathode with CsI has some promise as an electron emitter producing a good quality electron beam.

Cathode and anode plasmas in short-pulse explosive field emission cathodes

Explosive field emission cathodes have been a subject of research for a number of years. These cathodes offer high current densities and total current without requiring a heater for the production of electrons. Generally these cathodes consist of some structure with a series of tips or metal–dielectric regions in which a large electric field enhancement can occur. A cathode plasma is then formed from these discharge points that then supplies the electrons necessary for space charge limited emission. This article reports on a series of optical measurements in which the cathode and anode plasmas of explosive field emission cathodes are observed. Three types of cathodes are investigated. These types are a polymer velvet cathode, a metal–dielectric cathode, and a tufted carbon fiber cathode in which the fibers have been coated with a cesium iodide salt. Cesium iodide coated carbon fiber cathodes have shown a great deal of promise for various field emitter applications. From these high speed photos, the evolution of the plasmas on both cathode and anode can be qualitatively ascertained. Experimentally we find that not only does cathode plasma behavior depend on the type of cathode, but the anode plasma behavior does also. Further, we find that the best performing cathode shows the most rapid plasma formation on both anode and cathode, yet without a rapid plasma expansion across the anode–cathode gap.