Voltage Pulse Front Research Articles

Электронный механизм распространения канала наносекундного пробоя в жидких органических диэлектриках

The mechanism of anode-initiated breakdown in liquid organic dielectrics with long molecular chains is proposed on the basis of the experimental data on high velocities of the breakdown channel propagation in organosilicon and organofluorine liquids (~10^{7} cm/s), which is comparable to that obtained earlier in crystals in the same conditions. The high velocities of the anode-initiated breakdown channel are satisfactory explained within the model of the cascade Auger transitions, developed for the crystalline materials. According to this model, velocity of the breakdown channel propagation is proportional to the electrical field strength. The time delay in breakdown channel formation relative to the voltage pulse front does not exceed ~5·10^{-10} s within the margin of error.

Effect of the Voltage Pulse Front Steepness on the Electric Strength of Polymers

We have studied the effect of the leading edge steepness of a high-voltage pulse front on the dielectric strength of 3-μm-thick polyethylene terephthalate film. It is found that its breakdown electric field strength increases logarithmically from 550 to 700 MV/m upon an increase in the voltage growth rate (pulse front steepness) from 2 to 70 GV/s. It is shown that the experimental data can be described using the concept of the ionization mechanism of polymer breakdown, which is not associated with the evolution of impact ionization in polymers, and accounting for the decrease in the overvoltage coefficient in a polymer dielectric upon an increase in the pulse front steepness.

Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.

Modification of water, aqueous solutions, and dielectric films in diffuse discharge formed by fast electron preionization at the short front of a voltage pulse

Formation of a diffuse discharge at a short front of voltage pulse (less than 1 ns), as well as when placing water, water solutions, and dielectric film inside the discharge, is studied. A volumetric discharge was observed in water on the anode when placing dielectric film (polyethylene, vinyl chloride-methyl methacrylate (VCMMA), etc.), water, or aqueous solutions of 3% hydrogen dioxide or 18% KMgO4 inside the discharge. The change in the absorption spectrum of substance was found after multiple discharge switching on. Fourier analysis of the infrared absorption spectra showed the difference between the spectra of exposed and unexposed polyethylene film. The changes in the absorption spectra of water, 3% hydrogen dioxide and 18% KMgO4 aqueous solutions are related with expansion of the band of valence oscillations of OH-groups without clearly expressed maxima (for the studied liquids). Therefore, volumetric diffuse discharge in molecular and atomic gases can be used in modifying dielectrics, water and aqueous solutions.

Multimodule System for Fast-Rising High-Voltage Pulse Formation Based on Explosive Current Opening Switches

Matching devices in the form of electro-exploded current opening switches (EEOSs) or explosive current opening switches (EOSs) are used for transforming current pulses of magnetocumulative generators (MCGs) into voltage pulses of hundreds of kilovolts at high-impedance loads. Current pulse sharpening is accompanied by significant losses of the stored energy. Using EOSs are more preferable to EEOSs from the point of view of the energy losses since it does not lead to efficiency decrease in the MCG operation. This paper considers magnetocumulative multimodule systems consisting of several synchronously operating and serially connected small helical MCGs. Each generator is equipped with an explosive current opening switch. The voltages generated at each opening switch are summarized into the load. Fast operation of MCGs allows using very thin foils in the opening switches and obtaining the short voltage pulse front. Results of the works on creating high-voltage devices based on helical MCGs of a 60-mm diameter and disk EOSs of a 200-mm diameter are presented in this paper. In the experiment, the device consisting of two modules provides a voltage pulse of 370 kV with a rise time of ~170 ns into a load of 29 Ω and an inductance of 0.5 μH. Maximum power in the load is ~ 4.3 GW, and the emitted energy is ~1.1 kJ. The four-module device forms the pulse with the characteristic current rise time of ~100 ns in the similar load at twice smaller current.

Influence of the voltage pulse front shortening on the pulse repetition rate in a copper vapour laser

The lasing characteristics of a copper vapour laser are investigated in the regime of a pulse train excited in the internal-heating tube with the diameter of and length of . Two power supply schemes are compared: a conventional scheme with a storage capacitor discharged through a thyratron connected to a peaking capacitor and the scheme in which the peaking capacitor is connected to the laser active element through a kivotron – a fast switch based on the ‘open discharge’ with a turn-on time of less than . It is shown that in the considered range of the pulse repetition rates in the first case we deal with a typical energy dependence on frequency having a maximum near . In the second case, the lasing energy is frequency-independent; hence, the average power in this range is proportional to . The results obtained are explained by the neutralised influence of the initial electron concentration on energy characteristics of the copper vapour laser.

Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm2, and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.

Generation of gigawatt 10-GHz pulses with stable phase

The generation of microwave pulses in a 10-GHz range has been studied in a nonstationary relativistic backward wave oscillator (BWO) operating at a pulse train repetition rate of up to 300 Hz. Regimes with a stabilized phase of the high-frequency filling of pulses with respect to the accelerating voltage pulse front have been observed at a BWO peak output power of ∼1 and 3 GW. In pulse trains with a length of 10–100 s, the average output microwave power reached ∼1 kW.

Picosecond-controlled switching of high-voltage gas discharge

It is experimentally demonstrated that a nitrogen-filled discharge gap of a high-voltage oscillator can be switched by an electron beam with a time spread no exceeding ∼25 ps relative to the accelerating voltage pulse front. The regime of high-precision control is obtained in the case of a homogeneous potential distribution in the gap, at a microsecond-long voltage buildup to a level of ∼90% of the self-induced breakdown. The period of induced conductivity in the discharge gap corresponds to the beam current pulse duration.

On the influence of the voltage pulse profile on the process of particle trapping in the adiabatic trap of the gyrotron

We analyze the process of stabilization of electron beam parameters in a gyrotron for different profiles of the front of the accelerating-voltage pulse and the case of a large space charge trapped in the adiabatic trap. The duration of the portion of the pulse front, within which the electrons are reflected from the magnetic mirror and trapped, is at least an order of magnitude shorter than the entire length of the pulse front and is comparable with the characteristic time of potential oscillation development in the trap at great values of the pitch factor. The results of numerical analysis of the electron beam within the framework of models with finite durations of the voltage pulse front and instantaneous switch-on of the voltage showed that the steady-state values of the beam parameters are only weakly dependent on the pro.le of the pulse front.

Pulsed discharge in nitrogen and argon under an elevated pressure in a nonuniform electric field

The characteristics of a discharge and radiation in nitrogen and argon under pressures of 10–760 Torr and the discharge formation without pre-ionization of the gap from an auxiliary source are considered. A peak is detected on the pressure dependence of the radiation power of the second positive system of nitrogen for E 0/p ∼ 270 V/cm Torr and nitrogen pressure p ∼ 70 Torr. In the pressure range 10–760 Torr and for a voltage pulse leading front duration of ∼ 10 ns, an electron beam is formed behind the grid anode with various half-amplitude pulse durations. It is shown that, under the given conditions, the electron beam is formed at the voltage pulse front both in the case of a discharge gap breakdown and in the absence of a clearly manifested breakdown, as well as for a 10-ns delay of breakdown at the leading front of a discharge current pulse.

Ultrashort electron beams generated on the flat part of a voltage pulse in nitrogen and helium

In the case of voltage pulses with a small amplitude, an ultrashort avalanche electron beam (UAEB) in a diode filled with nitrogen or helium is generated on a flat part of the pulse. UAEBs obtained at a voltage of 25 kV have a full width at half maximum (FWHM) of about 200 ps and are delayed relative to the voltage pulse front by a time reaching tens of nanoseconds. Waveforms of the electron beam current pulse with several peaks of subnanosecond duration have been observed. At elevated pressures in a gas-filled diode, the voltage across the gap decreases by 10–20% during the electron beam generation.

Volume x-ray emission in gas diodes at atmospheric pressure

The generation of x-rays and high-energy electron beams in gas diodes filled with air and nitrogen at atmospheric pressure has been studied by experimental and theoretical methods. It is established that soft x-ray radiation is not only generated in the region of dense discharge, but is predominantly emitted from a weak-current discharge region. For a high-energy electron beam formation in the gap, the role of the voltage pulse front is not less important than that of the voltage amplitude; the electric field strength at the cathode has an optimum value for the electron beam formation.

Carrier injection as a cause of THz lasing excitation in SiGe/Si QW structures

AbstractThe evolution of THz emission, current and probe voltage in SiGe‐QW laser structures at the leading front of voltage pulse was studied. The excitation of stimulated THz emission is shown to be the result of electron injection through contacts and/or n‐Si substrate. The mechanism of intra‐center population inversion caused by carrier injection is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A high-power periodic nanosecond pulse source of coherent 8-cm electromagnetic radiation

The generation of short electromagnetic pulses excited in an extended slow-wave system (SWS) of a relativistic backward wave tube (BWT) operating in the so-called superradiance regime with a carrier frequency of 3.7 GHz has been simulated and experimentally studied. At a decreased magnetic field (about 0.2 T) in the SWS, the BWT generated 2.5-ns microwave pulses with a power of up to 800 MW. At a pulse repetition rate of 100 Hz, the working life of the system was limited by the lifetime of an explosive emission cathode (106 pulses). The possibility of phase synchronization of the high-frequency field of the relativistic microwave oscillator with respect to the voltage pulse front is demonstrated for the first time.

On the nature of the Vorob’evs effect in pulse breakdown of solid dielectrics

The Vorob’evs effect consists in certain features of the discharge observed when a solid dielectric in contact with two rodlike electrodes is placed in a liquid dielectric medium and a voltage pulse with increasing front is applied to the electrodes. When the pulse front slope is small, the discharge develops in the liquid over the solid dielectric surface; whereas the discharge at a sufficiently large slope of the pulse front penetrates into the solid and produces its fracture with cleavage of the surface fragments. In order to explain this phenomenon, it is suggested that, at a sufficiently high voltage buildup rate, a displacement current that is related to the motion of the surface discharge plasma passes through a microprotrusion occurring on the electrode surface at the contact site and causes the electric explosion of this microprotrusion. The metal plasma jet generated as a result of this explosion penetrates into the solid dielectric and forms a discharge channel in depth of this material. The surface discharge plasma formed at a small slope of the voltage pulse front closes the electrode circuit, thus preventing the discharge penetration in depth of the solid.

Investigations of factors influencing the current density distribution of a large cross section electron beam

Investigations are presented of the initial phase of operation of multi-edge cathodes for large-aperture electron guns. The influence of the form and amplitude of the supply voltage on the spatial distribution of the electron beam current density is shown. It was demonstrated that whilst the voltage is rising the cathode plasma is generated by the groups of operating edges and has a spot structure. An approximate analytical dependence between the number of operating edges and the voltage applied to the gun electrodes was determined. Quantitative results illustrating the dependence of the current distribution uniformity on the steepness of the voltage pulse front were obtained.