In this paper, we review the state of the art in studying the physical processes that occur in the cathode spots of vacuum arcs. The now available experimental data are interpreted in the context of ...the ecton mechanism of the operation of vacuum arc cathode spots. Central in this mechanism is the explosive electron emission, a phenomenon discovered by the author and his co-workers in the mid-1960s while studying high-voltage pulsed vacuum breakdown. In the light of the ecton mechanism, the cathode spot of a vacuum arc consists of individual cells which are explosive emission sites each emitting a portion of electrons termed an ecton. The cathode spot processes are cyclic in nature due to the finiteness of the ecton lifetime. It is shown that an arc is self-sustained due to the explosive emission processes initiated on the interaction of the cathode plasma either with nonmetal inclusions present in the cathode surface (first-type spots) or with liquid metal jets ejected from the zone of an active cathode spot (second-type spots). Attention is focused on the physical processes occurring during the operation of a cathode spot cell. A statistical model of a vacuum arc is used to interpret the effect of the spontaneous extinction of an arc. It is shown that an increase in the arc current is accompanied by a slight increase in the number of simultaneously operating ectons; therefore, as observed in the experiments, the parameters of a vacuum do not greatly depend on the current up to the kiloampere level.
The paper deals with the temporal structure of the explosive electron emission cell of the vacuum arc cathode spot. Essential features of the cathode spot cells operation - the ignition, burning and ...extinction have been considered in frames of the ecton model.
For the first time, we demonstrate experimentally the possibility of Cherenkov superradiant generation with a phase imposed by an ultrashort seed microwave pulse. The phases of seed and initiated ...Ka-band microwave pulses were correlated with the accuracy of 0.5-0.7 rad for the power ratio down to -35 dB. Characteristics of such a process were determined in the frame of a basic theoretical model that describes both spontaneous and stimulated emission of an electron beam moving in corrugated waveguides. The obtained results open up opportunities of reaching extremely high radiation power density in phased arrays of short-pulse coherently operating microwave generators.
We demonstrate both theoretically and experimentally the possibility of correlating the phase of a Cherenkov superradiance (SR) pulse to the sharp edge of a current pulse, when spontaneous emission ...of the electron bunch edge serves as the seed for SR processes. By division of the driving voltage pulse across several parallel channels equipped with independent cathodes we can synchronize several SR sources to arrange a two-dimensional array. In the experiments carried out, coherent summation of radiation from four independent 8-mm wavelength band SR generators with peak power 600 MW results in the interference maximum of the directional diagram with an intensity that is equivalent to radiation from a single source with a power of 10 GW.
Abstract
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.
A mechanism for the initiation of explosive electron emission at the interface between the cathode and a dense gas is proposed. It is based on the accumulation of positive ions, which appear in the ...gas ionized by field-emission electrons, near natural microprotrusions. The distance at which ions appear decreases with increasing gas density, which leads to an increase in their Coulomb field on the emitting surface. As a result, the emission current density in a high-pressure gas (tens of atmospheres) increases explosively, leading to the formation of many explosive-emission centers in tens of picoseconds. They initiate the development of plasma channels growing toward the anode. Runaway electrons are generated at the tops of the plasma tips and ionize the gas, providing its subnanosecond breakdown. This scenario of the breakdown development can be implemented at a critically low reduced electric field (i.e., the ratio of its strength to the pressure), when the characteristic time of avalanche multiplication of thermal electrons is longer than the duration of the voltage pulse.
As was shown earlier for pulsed discharges that occur in electric fields rising with extremely high rates (10
18
V/(cm s)) during the pulse rise time, the electron current in a vacuum discharge is ...lower than the current of runaway electrons in an atmospheric air discharge in a 1-cm-long gap. In this paper, this is explained by that the field emission current from cathode microprotrusions in a gas discharge is enhanced due to gas ionization. This hastens the initiation of explosive electron emission, which occurs within 10
–11
s at a current density of up to 10
10
A/cm
2
. Thereafter, a first-type cathode spot starts forming. The temperature of the cathode spot decreases due to heat conduction, and the explosive emission current ceases. Thus, the runaway electron current pulse is similar in nature to the ecton phenomenon in a vacuum discharge.
−
It is shown that, in addition to the well-known Townsend and streamer discharges in gas, there is a third type of discharge, namely nanosecond diffuse-channel discharge. It occurs in a highly ...overvolted gas gap. The study is carried out on the example of air at normal conditions in a uniform electric field. In this case, the ratio
, where
d
is the gap spacing and
x
c
is the electron avalanche critical length. The electric field at the head of such an avalanche reaches 10
6
V cm
–1
and higher, therefore, it emits runaway electrons, which create new electrons ahead of the old ones. An avalanche chain is formed, formally similar to a streamer but with low electrical conductivity. The runaway electrons and ultraviolet photoemission from the cathode contribute to the accumulation of secondary electrons in the gap. This leads to the appearance of a diffuse glow discharge, which then turns into a channel discharge and in an arc. The dependence of the overvoltage coefficient η on the product
pd
is calculated, where
p
is the gas pressure at
d
/
x
c
= 10. It is compared with the well-known curve that separates Townsend and streamer discharges in air.
A two-dimensional axisymmetric model has been developed to describe the formation of liquid-metal jet and the droplet pinch-off. These processes occur during the extrusion of the melt from the crater ...by the pressure of the cathode spot plasma of a vacuum arc. The jet formation has been numerically simulated for a copper cathode in the "inertial" mode of the melt splashing until the first droplet pinch-off. In this case, a jet with a longitudinal velocity gradient is formed. This gradient decreases the diameter of the jet and causes its elongation, resulting in droplet pinch-off. It has been shown that the mechanism of the droplet pinch-off is based on Rayleigh-Plateau instability. The droplet pinch-off time decreases with increasing jet velocity and increases for droplets of larger diameter. The simulation predicted the electrical explosion of the droplet-jet neck at the current density on the droplet surface ≥ 107 A·cm−2.
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