Coaxial Diode Research Articles

Theory and experiments of a power-ratio tunable dual-wavelength Nd:YVO4/Nd:GdVO4 laser by varying the pump wavelength

We demonstrate the theory and experiments of a power-ratio tunable dual-wavelength laser with coaxial diode end-pumping configuration by varying the pump wavelength, which is realized by controlling the working temperature of the pump laser diode (LD). Composite laser gain media containing an Nd : YVO4 crystal and an Nd : GdVO4 crystal was used for example. The dynamics of the dual-wavelength laser generation based on practical input operating parameters were simulated, and in the experiment, a total power of 3.72 W was obtained under the maximum LD pump power of 8 W, corresponding to the optical–optical conversion efficiency of 46.5%. The characteristics of the power-ratio tuning and output power agreed well with the theoretical predictions.

Simulation of High-Voltage Ion Diode with Wire Cathode at Atmospheric Pressure of Nitrogen

Physic-topological simulation of a high-voltage coaxial ion diode with a wire metal cathode at atmospheric nitrogen pressure in the hydrodynamic drift-diffusion approximation is performed. The reactions of nitrogen ionization by electrons, attachment of electrons to nitrogen molecules with the formation of negative ions, recombination of charged particles with opposite signs of charge, secondary ion-electron emission of the cathode were taken into account. The distribution of potential and density (concentration) of charged particles in the interelectrode gap, the density of ionic and electron currents at the electrodes were calculated within the self-consistent problem with the following parameters: diameter of wire metal cathode 0.01-0.16 mm, diameter of tubular anode 6 or 20 cm, voltage 20-40 kV, gas temperature 300 or 600K. The influence of geometry, voltage and gas temperature on the discharge parameters has been determined. The obtained calculated data on the discharge current are consistent with the experiment. It is shown that two zones are formed in the discharge between the electrode gap – one is with a width of about 1 mm with a strong and rapidly changing electric field near the cathode and the second long zone with the drift of charged particles towards the anode with a smaller but constant field strength. This is a characteristic feature of negative corona discharges. In the cathode zone there is an intensive ionization of nitrogen with the generation of positive ions and electrons. In the second zone, the density of positive ions decreases sharply due to recombination and weak ionization. The reaction of attachment of electrons to nitrogen molecules begins almost near the cathode surface and continues throughout the cathode zone, in the drift zone the concentration of negative ions gradually decreases. Moreover, the role of electronic conductivity is greatly reduced as we approach the anode. Due to the low mobility of negative ions and, accordingly, the high electrical resistance of the drift zone, the voltage drop on this space part represents a significant portion of the discharge voltage (~1.5 kV on the cathode zone and 18.5 kV on the drift space, at the total voltage of 20 kV). The fact that the highest concentration of positive ions is formed near the cathode, and negative – along the entire interelectrode gap, it can be used, respectively, in the processes of ionic nitriding of wire cathode metal materials and for processing materials and biological substances (bacteria, viruses, fungi), sensitive to negative ions, at the location of the carriers of these substances near the anode. To implement the latter, it is advisable to modify the design of the external anode for efficient extraction of nitrogen ions into the environment. It is also advisable to continue research in the direction of increasing the energy efficiency of ion generation by determining the method of the maximum allowable reduction of the voltage drop on the space of drift of charged particles.

Microwave breakdown in an overmoded relativistic backward wave oscillator operating at low magnetic field

Microwave breakdown has always been a huge challenge to the development of high-power microwave (HPM) sources. Some unique breakdown phenomena in a novel and powerful overmoded relativistic backward wave oscillator (RBWO) operating at low magnetic field are demonstrated. Three different breakdown mechanisms are utilized to explain these phenomena through detailed electromagnetic field calculation and particle-in-cell demonstration and effective methods are applied or suggested to mitigate the breakdown. The breakdown in the slow wave structure (SWS) mainly results from bombardment by the main electron beam under the intense radial electric field. Increasing the span between the main electron beam and the SWS or applying a coaxial extraction structure operating at coaxial TM01 mode might decrease the radial electric field and lessen the bombardment. The breakdown in the internal reflector originates with the field-induced emission in the inner ring under the intense axial electric field of the TM01 and TM02 mode and the subsequent electron-triggered emission in the outer ring. Removing the central part or constructing a complex reflector surface can suppress the emission. The breakdown in the slot retained for the Rogowski coil results from low pressure gas discharge initiated by microwave leakage from the RBWO into the diode region. Pasting microwave absorbing material into the coaxial diode region helps to obtain normal beam current waveforms measured by the Rogowski coil.

Suppression of backward current in a low-magnetic-field foilless diode

An annular cathode is generally fixed on a cylindrical cathode holder in a magnetically insulated coaxial foilless diode. Electrons emitted from cathode plasmas or the cathode holder may easily move backward under the action of the quasi-static electric field and axial magnetic field, leading to backward current loss, which will seriously reduce the efficiency of high power microwave sources, especially under the condition of a low guiding magnetic field. A novel storage electrode immersed in the uniform magnetic field is proposed to suppress the backward current. Reverse electrons will be decelerated and blocked by the storage electrode. Under the guiding magnetic field of 0.66 T, over 66% of the backward current is suppressed (from 3.6 kA to 1.2 kA) when the storage electrode is used, and furthermore, the waveforms of the diode voltage and current are improved significantly.

Research of a pulse magnetic system for eliminating electron conductivity in an ion diode for the neutron generation

The work presents the results of modeling the process of suppressing an electronic component in a small-sized coaxial ion diode by a pulsed magnetic field of a spiral electrodynamic line. Information was obtained on the features of this process, which is necessary for designing a diode with pulsed magnetic insulation.

Influence of oscillating magnetic field on the keyhole stability in deep penetration laser beam welding

The stability of the keyhole decreases for deep penetrated high-power laser beam welding. The keyhole tends to collapse with increasing laser power and e.g. keyhole induced porosity can occur. This study deals with the observation of the keyhole during high-power laser beam welding in partial penetration mode by means of a high-speed camera. A butt configuration of 25 mm thick structural steel and transparent quartz glass was used for the experiments. An oscillating magnetic field was applied perpendicular to the welding direction on the root side of the steel plate. The keyhole was highlighted with a coaxial diode laser. It was ascertained that the stability of the keyhole and the weld penetration depth were increased by applying an oscillating magnetic field with an oscillating frequency of 1.2 kHz and a magnetic flux density of 50 mT.

Resolution Improvement in a Multi-Point Fiber Refractometer Based on Coherent-OFDR

In this letter, we propose a signal processing method for improving the resolution of a multi-point fiber refractometer from 10 -4 to 10 -6 . The fiber refractometer uses the Fresnel reflection at the fiber tip to measure the refractive index (RI), based on the coherent optical frequency domain reflectometry (C-OFDR) technique; using a standard four-pin coaxial pigtailed DFB diode laser as the optical source. The proposed method is divided into two steps; first the raw signal is processed to reduce the nonlinearity generated by the optical frequency sweeping laser, and then a pattern that repeats itself in the sensing signal is identified, extracted and replicated several times to improve the sensing signal used to calculate the RI. In the experimental results, using 30 ms of effective acquisition time, and the proposed method, an improvement of two orders of magnitude in the resolution of the system was obtained.

DETERMINATION OF THE GEOMETRIC SIZE OF THE COMMUNICATION HOLE OF TWO CYLINDRICAL WAVEGUIDES

It is always relevant to improve performance in electrodynamic systems. When solving problems on the electrodynamic characteristics of hollow coupled systems, the question is often asked about the relationship between them, about the form of the communication hole with a certain orientation of the guide axes in the system elements and its geometric dimensions. Such a system is a generator (small-sized local oscillator8 mmrange). The inclusion of a high-Q stabilizing resonator in the Gunn diode generator significantly improves its characteristics. The use of a low-quality coaxial chamber as a diode section increases the generation stability. However, this complicates the numerical calculations of the electrodynamic system of the generator due to the uncertain configuration of the communication hole, since it arises as a result of the intersection of two cylindrical volumes of a coaxial waveguide and a high-quality cylindrical resonator. In the present work, the task is determination of the shape and size of the intersection figure of two unequal radii of cylindrical volumes with axes orthogonally located in relation to each other at a distance. The resulting shape of the intersection figure in a planar approximation forms a flat ellipse. The larger diameter of the coupling ellipse depends on the diameter of the resonator, the smaller on the inner diameter of the coaxial chamber, depending on the distance between their axes. It is necessary to determine the equivalent rectangular hole of the connection. Its presence simplifies the construction of a tangent electric field at the communication hole, which is necessary for numerical calculations of the electrodynamic characteristics of the system. In this case, with constant diameters of the cylindrical resonators, the geometrical dimensions of the hole depend only on the distance between the axes. It is with this circumstance that they are dealing with the study of the connection between a cylindrical coaxial diode section and a high-Q stabilizing resonator. Unlike other circuits, where the diode is included in the waveguide section, in this case, its inclusion is made in a coaxial line.

Improving the efficiency of suppression of electron conductivity in the vacuum accelerator neutron tubes with coaxial diode system

Discussed two options for increasing the magnetic isolation efficiency of the electron component in a vacuum neutron tube with coaxial geometry for the acceleration of deuterons. Computer analysis of the previously developed diode systems with magnetic insulation has shown that the suppression ratio does not exceed 50%. The proposed new options of suppression systems used the current ring and diaphragm, located near the edge of the cylindrical anode. Computer analysis showed that for these acceleration geometries the suppression factor could exceed 97%.

Investigation on Adjustable Magnetic Pulse Compressor in Power Supply System

Magnetic pulse compressors, based on magnetic switches, are considered as competitive candidates for high-voltage pulse generation. It is generally believed that the lack of operating-parameter tunability of these devices is currently one of the crucial factors limiting their applications. In this paper, an adjustable high-power magnetic pulse compressor, which is deemed as the key component of a power supply system, is investigated theoretically and experimentally. Dominant parameters of the compressor, including the compression ratio, the saturation time, and the peak voltage, can be regulated, which is beneficial for widening the application areas of power supply systems. Specifically, the prepulse of the magnetic pulse compressor is studied as one of the key parameters. A two-stage magnetic pulse compressor was then setup and when the charging time of the primary source was alternated between 19 and 25 μ s, pulses with rise-time of 1 μ s were achieved with good repeatability. This in turn verified the tunable capacity of the device. When applied in a solid-state high-voltage pulse generator, the pulse compressor successfully enabled the generation of intense electron-beam with peak power over 5 GW, peak voltage of 380 kV, pulse width over 170 ns, and repetitive rate of 20 Hz. The device has been successfully used for research on plasma science and for driving the sources of high-power microwaves where the requirements on the device often vary substantially. In the latter application, based on the high reliability of the solid-state pulse generator, optical diagnosis has been conducted of the plasma expansion characteristics of the magnetically insulated coaxial diodes cathode, in order to analysis the mechanism of the pulse shortening phenomenon. Experimental results show reasonable agreements with the analyses.

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.

Toward Nanowire HBT: Reverse Current Reduction in Coaxial GaAs/InGaP n(i)p and n(i)pn Core-Multishell Nanowires

In this work the reduction of reverse currents in Au-catalyzed, MOVPE grown coaxial GaAs nanowire diodes are reported. The reduction is achieved by introducing an interstitial, lattice-matched i-InGaP shell (spacer) as tunneling barrier inside the junction, which also functions as a selective etch stop. With increasing spacer thickness, rectification ratios of >1.57 × 106 at ± 1.65 V, ideality factors of 1.3, and dark saturation current densities as low as 20 pA cm−2 are extracted, which are related to a reduced tunneling probability. Temperature-dependent DC measurements of junctions with thin spacers show a correlation to a simple (trap-assisted) tunneling model. With absolute reverse currents in the pA range down to −3 V bias, the improved diode is implemented as a collector-base junction in a coaxial n(i)pn nanowire structure by growing an additional, outer n-doped InGaP shell as the emitter layer in a nanowire HBT.

The Low-Energy State of an Electron Beam in a Coaxial Diode with a Homogeneous Anode and Inhomogeneous Magnetic Field Profile

The possible formation of an extended low-energy state of electron beam in a coaxial diode with homogeneous cylindrical anode and moderate magnetic field with inhomogeneous profile is demonstrated for the first time. It is established that, depending on the magnetic field configuration, virtual cathodes (VCs) of two types can be formed: (i) a stationary VC with a localized reflection plane and (ii) a moving VC with a two-stream low-energy state of the electron beam.

Effect of a submicrosecond-advanced voltage pulse on the formation of a high-current electron beam in a magnetically insulated coaxial diode

The effect of a double accelerating voltage pulse on the formation of a high-current electron beam in a magnetically insulated coaxial diode under fore-vacuum pressures has been investigated. High voltage pulses of durations <0.5 and ∼1 ns and amplitudes (modulo) <120 and ≥ 160 kV, respectively, were applied to a graphite cathode with a submicrosecond delay. When the time delay between the pulses was increased to a certain value at a residual gas pressure of 5 × 10−3 Torr, the current of the second beam increased about fourfold and then decreased. The current reached a maximum of 4.5 kA, which was greater than that carried by the matched load of the high voltage generator. The observed effect can be accounted for by the neutralization of the beam charge in the plasma expanding from the cathode. The plasma channel could be formed due to explosive electron emission from the cathode or due to impact ionization of the residual gas.

Theory of a High-Voltage Pulse Discharge in a High-Pressure Gas: Hydrodynamic and Kinetic Approaches

Comparative results of modeling of the phenomenon of a high-voltage discharge in nitrogen at atmospheric pressure based on two theoretical approaches: hydrodynamic and kinetic descriptions of electrons in discharge plasma are presented. The detailed spatiotemporal pattern of the development of coaxial gas-filled diode breakdown with formation of runaway electrons in it is described. It is demonstrated that the switching characteristics of the subnanosecond discharge are reliably modeled within the limits of both hydrodynamic and kinetic descriptions of electron components, whereas the parameters of runaway electrons can reliably be calculated only using the kinetic description.

Experimental research on time-resolved evolution of cathode plasma expansion velocity in a long pulsed magnetically insulated coaxial diode

Unlike planar diodes, separate research of the axial and radial plasma expansion velocities is difficult for magnetically insulated coaxial diodes. Time-resolved electrical diagnostic which is based on the voltage-ampere characteristics has been employed to study the temporal evolution of the axial and radial cathode plasma expansion velocities in a long pulsed magnetically insulated coaxial diode. Different from a planar diode with a “U” shaped profile of temporal velocity evolution, the temporal evolution trend of the axial expansion velocity is proved to be a “V” shaped profile. Apart from the suppression on the radial expansion velocity, the strong magnetic field is also conducive to slowing down the axial expansion velocity. Compared with the ordinary graphite cathode, the carbon velvet and graphite composite cathode showed superior characteristics as judged by the low plasma expansion velocity and long-term electrical stability as a promising result for applications where long-pulsed and reliable operation at high power is required.

Current loss of magnetically insulated coaxial diode with cathode negative ion

Current loss without an obvious impedance collapse in the magnetically insulated coaxial diode (MICD) is studied through experiment and particle-in-cell (PIC) simulation when the guiding magnetic field is strong enough. Cathode negative ions are clarified to be the predominant reason for it. Theoretical analysis and simulation both indicate that the velocity of the negative ion reaches up to 1 cm/ns due to the space potential between the anode and cathode gap (A–C gap). Accordingly, instead of the reverse current loss and the parasitic current loss, the negative ion loss appears during the whole pulse. The negative ion current loss is determined by its ionization production rate. It increases with diode voltage increasing. The smaller space charge effect caused by the beam thickening and the weaker radial restriction both promote the negative ion production under a lower magnetic field. Therefore, as the magnetic field increases, the current loss gradually decreases until the beam thickening nearly stops.

Низкоэнергетическое состояние электронного пучка в коаксиальном диоде с однородным анодом и неоднородным профилем магнитного поля

AbstractThe possible formation of an extended low-energy state of electron beam in a coaxial diode with homogeneous cylindrical anode and moderate magnetic field with inhomogeneous profile is demonstrated for the first time. It is established that, depending on the magnetic field configuration, virtual cathodes (VCs) of two types can be formed: (i) a stationary VC with a localized reflection plane and (ii) a moving VC with a two-stream low-energy state of the electron beam.

Simulative research on reverse current in magnetically insulated coaxial diode

The reverse current tends to occur in the transition region of the guiding magnetic field in a magnetically insulated coaxial diode (MICD). Influence of the guiding magnetic field on characteristics of the MICD especially on the reverse current is studied by the particle-in-cell (PIC) simulation in this paper. The reverse current is confirmed to be irrelevant with the guiding magnetic field strength. However, the reverse current is clarified quantitatively to depend on the electric and magnetic field distribution in the upstream of the cathode tip. As the MICD has been widely employed in microwave tubes, a simple approach to suppress the reverse current on the premise of little change of the original diode is valuable and thus proposed. The optimum matching point between the cathode and the magnetic field is selected in consideration of the entrance depth tolerance, the diode impedance discrepancy and the reverse current coefficient.

Variation of the beam parameters of runaway electrons in a gas discharge under the conditions of nonuniform preliminary ionization

Within the theoretical model of a high-pressure hybrid nanosecond discharge with runaway electrons, a strong dependence of the electron beam amplitude, duration, and energy spectrum on the conditions of the preliminary ionization of a gas in the discharge gap is demonstrated. The conditions with uniform and nonuniform distributions of initial electrons in a coaxial diode filled with sulfur hexafluoride at atmospheric pressure are simulated. It is shown that the amplitude and current pulse profile of the electron beam substantially change upon the variation of the initial distribution of the electrons in the discharge gap.