The Theory of Current Switching in Vacuum
If contacts carrying current disconnect, an electric arc is created between them, which supports a high conductivity of the space between the contacts and allows the current leaking between the contacts as if they were still closed. This causes the element between the contacts to heat to a temperature high enough to dissociate (break-up) its molecules and ionize the space. The existence of an electric arc is characterized by the quasi-equilibrium state, when the arc voltage self regulates at a level sufficient to maintain the conductivity of the formed plasma, where the diameter of the arc column shrinks or expands depending on the changes of the value of the passing current.
The energy produced at this moment goes into the surrounding environment due to thermal conductivity, convection and radiation. This energy balance must be destroyed if the arc is to be suppressed and current is to be disconnected. When the current reaches natural zero, the energy stops coming from the network. By intensively cooling the space between the contacts at this moment, it is possible to break the arc, disturb the space conductivity and restore its electric stability and by doing this to accomplish the disconnection.
Cooling must be very quick, so that the speed of suppressing the remaining arc and restoring electrical stability of the space between the contacts exceeds the speed of the transient recovery voltage (TRV). If the contacts carrying current open in a vacuum, all the current is rushed to the last remaining point of contact, causing intensive local heat at that spot. With further contact separation a bridge is formed of melted metal that, due to the high density of current in it, instantaneously heats up and explodes, creating an arc in the environment of ionized metal steam formed as a result of the explosion. The ionized metal steam is a perfect current conductor so a stable arc charge is formed in the space between the electrodes. Thus a vacuum arc is in fact an arc in a metal steam environment. Current carriers are created in the space between the electrodes from the cathode through multiple point sources of current called cathode spots.
A current with the value of 60-100 amps passes through each spot. With the cathode spot sizes ranging from a few microns to several dozens of microns, this creates a current density up to a hundred million amperes per square centimeter. This huge current density heats up the metal of the electrode in the cathode spots. It boils and evaporates. The pressure in these points reaches dozens of atmospheres and the temperature several thousand degrees. At such temperature and pressure, streams of dense and strongly ionized plasma flow out off cathode spots and close the circuit to anode. When monitoring cathode spots it seems as through they are in constant chaotic movement on the surface of the cathode. In reality the effect of the spots’ movement is created by the constant process of disappearance (dieing off) of some spots and appearance of others. Each spot has a limited “life span”. New spots are born in place of an “old” spot that has died. Very often new spots appear by means of division of an already existing spot in half or more. The amplitude of the current arc and the cathode material determines the number of spots existing on a cathode at a given moment in time. For instance each spot on a copper electrode carries a current of about 100 Amps. This way an arc formed on a copper electrode by a 1000 Amps current creates about 10 cathode spots.
If we consider half a period of alternating current, we can notice that the number of cathode spots will grow simultaneously with the growth of the current level. Further with the reduction of current, the number of spots will reduce until right before the current transfer through natural zero there will be only one spot left. When current reaches its ultimate minimum value, which is called the chopping current, which mainly depends on the cathode material, the last cathode spot ceases to exist. At this time current stops passing through the space between the electrodes and metal steam is condensed on the electrodes in about 10 microseconds. When the current disappears, network voltage begins to be restored on the separated electrodes. This process takes about 50-60 microseconds, i.e. by the time voltage restoration begins, the charge carriers at the space between the electrodes are absent and its dielectric characteristics have been completely restored.
The presence of cathode spots is vital for the existence of the vacuum arc because they are the source of plasma without which the arc cannot exist. The anode, as opposed to cathode, acts as a positively charge probe, extracting substantial current from the plasma that is required to satisfy the needs of the outside circuit. Plasma between the electrodes provides the conductive environment required to transport current from cathode to anode. With a further increase of current in the arc, plasma, instead of smoothly washing the anode as it was described earlier, centers on a small area of this electrode. This anode spot, usually located at the sharp contact edge in a melted condition plays a key part in breaking through the space in the attempt of voltage restoration. During this process, cathode spots have a tendency to group and the arc itself acquires the appearance of a brightly shinning braid. The arc causes great erosion of both electrodes, the numeric value of which depends on the amplitude of the current and its duration.
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