Explosive Emission Centers Research Articles

Theoretical Model of the Retrograde Motion of Cathode Spots of Vacuum Arc

The phenomenon of cathode spot (CS) retrograde motion has been attracting extensive research in the field of vacuum arc. Although a number of theoretical models have been proposed to explain this phenomenon, up to now, the first-principle understanding is still missing. In this work, based on the generalized Ohm’s law, it is proposed that there is a transport current in the explosive emission center when a transverse magnetic field (TMF) is applied. The investigation of the influence of the transport current on the strength of the electric field in the cathode sheath suggests that the transport current affects the ignition of new CS in the retrograde direction. In the framework of the ecton model, combined with the dynamics of explosive emission plasma, the relationship between the strength of applied TMF and the ignition probability of new CS in different directions is established, and the retrograde motion characteristics of CS are simulated using a statistical model. The simulation results show that the directionality of retrograde motion of CS is enhanced with an increase in TMF, and the retrograde motion speed of CS increases with the increase of the applied TMF. When the strength of TMF is small, the motion speed of a single CS has a linear relationship with the strength of TMF, but with the increase of the strength of TMF, the motion speed of a single CS is gradually saturated, which is consistent with experimental facts. The feasibility of the model is verified by comparing the simulation results with relevant experimental results.

Modification of the cathode material around the explosive electron emission centers in the spark stage of vacuum breakdown

On the basis of numerical calculations estimates for the fields of mechanical stresses around the explosive electron emission centers of the cathode during the development of pulsed vacuum breakdown are given. Expansion of material due to conduction loss was taken into account. It is shown that the sizes of regions with broken crystalline structure is much larger than the corresponding of explosive emission centers. Keywords: vacuum breakdown, explosive electron emission, mechanical stresses.

Модификация материала катода вокруг центров взрывной электронной эмиссии в искровой стадии вакуумного пробоя

On the basis of numerical calculations estimates for the fields of mechanical stresses around the explosive electron emission centers of the cathode during the development of pulsed vacuum breakdown are given. Expansion of material due to conduction loss was taken into account. It is shown that the sizes of regions with broken crystalline structure is much larger than the corresponding of explosive emission centers.

Runaway of electrons and initiation of explosive electron emission during pulse breakdown of high-pressure gases

We propose a scenario of the initiation of explosive electron emission on the boundary of the electrode and a high-pressure gas. According to this scenario, positive ions are formed due to the gas ionization by field-emission electrons and accumulated in the vicinity of protrusions of micron size at the cathode. The distance between the ion cloud and the emitting surface decreases with increasing pressure which results in a growth of the local field. As a consequence, an explosive growth of the emission current density occurs for a dense gas (the gas with the pressure of tens of atm). As a result, explosive-emission centers can be formed in dozens of ps. These centers give a start to plasma channels expanding towards the anode. Runaway electron flow generated near the channel heads ionizes the gas gap, causing its subnanosecond breakdown.

Generation of runaway electrons in plasma after a breakdown of a gap with a sharply non-uniform electric field strength distribution

The paper is devoted to the study of the initiation and formation of a negative streamer in a sharply inhomogeneous electric field and the generation of runaway electrons (REs) in air and helium at atmospheric pressure and below, as well as in sulfur hexafluoride at low pressure. Nanosecond voltage pulses of negative polarity with an amplitude of 18 kV were applied across a point-to-plane gap 8.5 mm long. The studies were carried out using broadband measuring sensors and equipment with picosecond time resolution, as well as using a four-channel ICCD camera. Using a special method for measuring the dynamic displacement current caused by the redistribution of the electric field during streamer formation, the waveforms of voltage, discharge current, RE current, and dynamic displacement current were synchronized to each other, as well as to ICCD images. Data on the generation of REs with respect to the dynamics of streamer formation were obtained. It was found that REs are generated not only during the breakdown of the gap, but also after that. It has been found that the formation time of explosive emission centers affects the generation of REs after breakdown. Based on the measurement data of the voltage, discharge current, and dynamic displacement current, the electron concentration in the plasma channel after breakdown and the electric field strength near the surface of the grounded electrode were calculated.

Inverted Polarity Effect at the Subnanosecond High-Voltage Breakdown of Air

Inverted polarity effect was studied at the breakdown of point-to-plane discharge gap filled with air at a pressure range of 0.013–0.3 MPa. High-voltage nanosecond pulses were applied to the electrode with a small radius of curvature. Pulser was operated at both negative and positive polarities in the single-pulse mode. Waveforms of voltage pulse, discharge current, and intensity of the discharge plasma radiation near the electrode with a small radius of curvature were registered. It was obtained that at the breakdown of point-to-plane discharge gap filled with air, the breakdown voltage at positive polarity of the pointed electrode was higher than that at negative one. The inverted polarity effect was shown to come to naught with an increasing of air pressure up to 0.3 MPa. It is suggested that the inverted polarity effect is related to the impeded formation of the explosive emission centers on the flat electrode (cathode) after arrival of the ionization wave front to the one. In addition, both the inverted and the ordinary polarity effects were not observed at the breakdown of nitrogen at pressures of 0.4 and 0.7 MPa.

Numerical simulations of stable explosive-emission center plasma expansion in vacuum

We present the results of our numerical simulations of the stable cathode-flare expansion stage in the vacuum-breakdown switching period. We used the model of two-fluid hydrodynamics with corrections that allowed it to be applied to a consistent description of the electron and ion transport in plasma and the electron transport in a plasma-anode vacuum gap without imposing any joining conditions at the expanding plasma boundary. We have analyzed the dynamics of the density, directed velocity, and temperature profiles for both components and the influence of various factors on the plasma-cloud expansion velocity.

On the mechanism of operation of a cathode spot cell in a vacuum arc

The erosive structures formed on a tungsten cathode as a result of the motion of the cathode spot of a vacuum arc over the cathode surface have been examined. It has been found that the average mass of a cathode microprotrusion having the shape of a solidified jet is approximately equal to the mass of ions removed from the cathode within the lifetime of a cathode spot cell carrying a current of several amperes. The time of formation of a new liquid-metal jet under the action of the reactive force of the plasma ejected by the cathode spot is about 10 ns, which is comparable to the lifetime of a cell. The growth rate of a liquid-metal jet is ∼104 cm/s. The geometric shape and size of a solidified jet are such that a new explosive emission center (spot cell) can be initiated within several nanoseconds during the interaction of the jet with the dense cathode plasma. This is the underlying mechanism of the self-sustained operation of a vacuum arc.

Development mechanism of cathode surface plasmas of high current pulsed electron beam sources for microwave irradiation generation

AbstractThis paper presents the development mechanism of surface plasmas of carbon-fiber-cathode electron beam source and its effects on the operation of a high-power microwave source, reflex triode vircator powered by about 400 kV, 9 kA, about 350 ns pulsed power accelerator. Based on the current and voltage characteristics of diodes using carbon fiber cathode, the axial expansion velocity is 1.2 cm/μs and the delay time of explosive emission is 2 ns. Further, the comparison of carbon fiber and stainless steel cathodes is made. It was found that the threshold electric field for carbon fiber cathode is about 25 kV/cm, and the delay time of explosive emission and threshold electric field for stainless steel cathode is, respectively, 4.5 ns and 40 kV/cm. The radial expansion velocity of individual emitting centers is estimated to be 1.2 cm/μs, equal to the axial expansion velocity, and this shows the cathode plasma spots spherically expand. In the optimal diode gap for microwave irradiation or at the average current density of 230 A/cm2using carbon fiber cathode, the screening radius was 0.67 cm, the lifetime of cathode emitting centers was about 60 ns, the cathode plasma density was 5 × 1015 cm−3, and the Debye radius of cathode plasma was <3 × 10−5 cm−3. The self-quenching behavior of explosive emission centers occurs, due to the process of cathode surface material release and cooling. The generation and self-quenching of emitting centers, and screening effect of cathode plasmas determine the increase and decrease of cathode emitting area, which is independent of the current density and background pressure. The relation between the lifetime of virtual cathode and background pressure was discussed. It was found, both theoretically and experimentally, that a lower background pressure indicates a longer microwave pulse or a better microwave waveform. It was observed by comparison that the temporary behavior of cathode emitting area is similar to the development process of microwave pulse. The changes of emitting area affects the stability of beam current injected into the virtual cathode region, further leading to the fluctuation of microwave pulse of vircator.

High-current diode with ferroelectric plasma source-assisted hollow anode

The operation of a ferroelectric plasma source-assisted hollow anode (HA) electron source in a vacuum diode powered by an ∼200 kV and ∼400 ns pulsed generator was studied using time- and space-resolved laser induced fluorescence diagnostics. It was found that the plasma ion “temperature” in the vicinity of the HA output grid increases up to ∼15 eV during the accelerating pulse, which is consistent with a model of the potential screening of the grid by the randomly moving ions [Phys. Plasmas 13, 073506 (2006)]. Also it was shown that the increase in the HA plasma potential up to several kilovolts because of the appearance of a noncompensated ion charge in the HA bulk plasma due to electrons fast extraction, leads to explosive emission centers being generated at the HA grid and to nonuniformity in the cross-sectional electron beam current density. Finally, the plasma prefilled mode of diode operation was studied using a simple one-dimensional model of the plasma erosion and the HA plasma electron heating by energetic ions was considered.

Degradation and Recovery of the Emission From a Graphite Cathode in Relation to the Repetition Frequency of Nanosecond Accelerating Pulses

A study has been performed on the emissive characteristics of a cold graphite cathode in a magnetically insulated coaxial diode under the action of nanosecond accelerating pulses at a pulse repetition frequency (PRF) of up to 3.5 kHz. Emission was observed to degrade at PRF < 1 kHz and recover at PRF ~3.5 kHz. Estimates of the temperature conditions in the region of an explosive electron emission (EEE) center have shown that the pulse interval t~1 ms suffices for this region to cool down to 300 K. The cooling occurs predominantly by heat conduction. For t~0.3 ms, the residual heat is substantial. It has been proposed that there exists a frequency limit for the cathode microrelief polishing effect. The results of an experiment on the study of the mechanism of cathode emission recovery with increasing PRF are presented. Micrographs of the cathode taken after aging, photographs of the cathode in operation, and analyses of the fractional composition of the material removed from the cathode suggest that heating of some regions of the cathode emitting edge to the melting point of graphite plays an important role in the recovery of emission. This counts in favor of the hypothesized dominant contribution of thermoelectronic emission to the initiation of EEE due to the residual heat remaining in regions that have not cooled off during the pulse interval

The formation and structure of current cells in a vacuum arc cathode spot

A physical model describing the structurization and localization of electric current in the surface layer of a cathode in the initial stage of a vacuum arc discharge is proposed. According to this model, each current cell on the cathode surface represents a dislocation cell containing a ring electron vortex with the axis perpendicular to the electrode surface. It is shown that localization of the electric current at the vortex center leads to the formation of a spatial structure of hot spots acting as precursors of the explosive emission centers, the dynamics of which determines operation of the vacuum discharge.

Generation of droplets emitted by pool explosive-emission cathodes

The velocity spectrum of droplets emitted by a pool explosive-emission cathode was studied. Relation between sizes of droplets and its maximal velocities over the range of 103−18×104 cm/s have been found. The dependence obtained confirms the mechanism found earlier for the droplet emission under high pressures developed in explosive emission centers.

Investigation of the erosion drop fraction for liquid-metal explosive-emission cathodes

The velocity spectrum of drops emitted by a liquid-metal explosive-emission cathode has been investigated. A relation between the sizes of the drops and their maximum velocities has been found for the velocity range 10/sup 3/-8/spl times/10/sup 4/ cm/s. The relation obtained supports the earlier established mechanism for the emission of drops under the action of high pressures developing in explosive emission centers. The possibilities of an additional acceleration of drops due to the explosion of necks formed on breaking the drops off the cathode surface and at the expense of the kinetic energy of the ions of the expanding cathode plasma have been discussed.

Effect of the expanding explosive-emission centers plasma on the impedance of a high-current diode

Formulas for the space-charge-limited electron current in a planar diode with a small hemispherical or hemi-cylindrical plasma emitter have been obtained theoretically and by a numerical simulation. For a periodic cathode structure of hemispherical emitters, an approximate expression for current is derived. Based on these results, the increase in current with time in high current explosive-emission diodes is explained. A condition has been found under which the expanding cathode plasma has no effect on the diode impedance. This is related to the electric field pressure on the cathode. A configuration of the cathode surface has been proposed which provides realization of this condition.

Numerical study of the current-voltage characteristic of a plasma-filled high-current diode

A numerical study is performed of a one-dimensional model of a high current plasma-filled diode, connected in a circuit with a voltage generator. Flow of the plasma from an external source into the diode anode-cathode gap through the plane of the cathode grid proves to have a definite effect on the diode current-voltage characteristic. An estimate is made of the limiting current above which formation of a cathode double layer occurs. Generation of explosive emission at a high value of current or the time derivative of current causes rapid expansion of the cathode layer due to a lack of the majority, electron current component and the presence of a phase intensified by plasma erosion. Dependence of the current-voltage characteristic on density of cathode explosive emission centers is noted.