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A two-dimensional magnetohydrodynamic unsteady two-temperature model of the initial stage of operation of the explosion emission center is constructed using a wide-range equation of state of matter, taking into account the processes of ionization and electronic thermal conductivity. Numerical simulation within the framework of this model showed that in the initial stage of functioning of the explosion emission center, the conditions under which the electron current from the cathode to the plasma flows through a continuous metal-plasma transition, the conductivity of which is ohmic in nature, are realized. The crater formed as a result of the action of the explosion center is formed mainly due to the extrusion of the substance by inertia acquired from the explosion of the micro-island, and not due to evaporation. As a result of extrusion, a parapet is formed on the edge of the crater. The current flowing through the explosion emission center is concentrated in this parapet, thereby contributing to the preservation of a continuous metal-plasma transition in the area of the parapet, when a metal-plasma transition with a sharp boundary is already formed in the center of the crater as a result of a decrease in the current density. In the framework of the two-dimensional formulation of the zone of transition of the current from the metal into the plasma takes the form of rings coinciding with the parapet. During the operation time of the explosion center, the velocity and ion composition of the plasma flowing out of it changes. At the beginning of the cycle, the plasma consists mainly of three-charged ions and its speed is high, by the end of the cycle the share of single-charge ions increases, and the plasma speed drops. In an average cycle speed of the ions is greater, the more they charge.
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Current density is 108 A/cm2 |
Concentration of heavy component 1021 cm-3 |
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| Time is 0.3 ns |
| Time is 0.7 ns |
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Distribution of the current density module and the concentration of the heavy component for different moments of time. Full current is 7 A. the Numbers on the axis are the distances in microns. It is clearly seen that as the crater grows, the current is concentrated in the parapet at the edge of the crater.
The calculations allowed us to propose a new mechanism for the movement of the cathode spot. Traditionally, the movement of the cathode spot is interpreted as the successive birth and death of emission centers. On the clean surface of the cathode, an overlapping chain of craters is obtained, i.e. a new emission center arises at the boundary of the crater formed by the action of the previous emission center. It is the discrete nature of erosion traces that is the main reason for the assumption of the abrupt movement of the cathode spot. According to calculations, the current of the explosion center is not distributed evenly over the crater area, but is collected at its border with the subsequent concentration in the growing parapet. The appearance of a sharp boundary inside the crater increases the current concentration in the parapet, where the increased current density helps to maintain the ohmic metal-plasma transition. In the calculations, with further growth of the crater, the current density in the parapet still falls below the density necessary to maintain the ohmic transition. But this is due to the two-dimensional cylindrical geometry, in which the height of the parapet and the current density do not depend on the third coordinate - j. Judging by the photos of craters, the uniformity of the shape and height of the parapet by j is disturbed during the expansion of the crater. Hence, it can be assumed that the homogeneity of the current density in j is not preserved in the real situation. This will lead to a rupture of the continuous ring of the current transfer region into several fragments and, consequently, the current density will not decrease so quickly and the ohmic transition will remain longer than in the axially symmetric case. The result is a local metal inhomogeneity with a high concentration current, i.e. the same as the initial condition for the development of the previous explosion emission center. It is logical to assume that there will be an explosion similar to the original explosion, resulting in a new crater exactly on the border of the old crater. Thus, the CS moves, it leaves discrete footprints, but does not go out and not jump, but moves continuously. A simple illustrative model was created to demonstrate this movement mechanism. The results of calculations for this model are shown in the figure below.
Schematic representation of the motion (and division in this case) of the cathode spot, top view.
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