Cathode spot

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

Equation 16.9 tells us that the protective oxide film on iron will be preserved in alkaline media, weakened in neutral water, and lost in acidic environments. Indeed, in very acidic solutions, the distinction between extended anodic and cathodic sites will be lost along with the oxide film, although local anodic and cathodic spots will persist, and so dissolution of iron with accompanying hydrogen evolution becomes general across the surface of the specimen. 

The principal method of introducing metal impurities into early pinch discharges was considered to be arcing. The externally applied voltages, although low, still permit the occurrence of unipolar arcing between the plasma and the wall when driven by the sheath potential. Local electron emission from a cathode spot is balanced by a uniform flow back to the surface of energetic electrons in the tail of the Maxwellian distribution. 

Arcs with hot cathode spots. If the cathode is made from lower-melting-point metals like copper, iron, silver, or mercury, the high temperatme required for emission caimot be sustained permanently. Electric current flows in this case through hot spots that appear, move fast, and disappear on the cathode surface. Current density in the spots is extremely high (10" -10 A/cm ), which leads to intensive but local and short heating and evaporation of the cathode material while the rest of the cathode actually stays cold. The mechanism of electron emission from the hot spots is thermionic field emission. Cathode spots appear not only on the low-melting-point cathodes but also on refractory metals at low currents and low pressures. 

Vacuum arcs. This type of low-pressure arc, operating with cathode spots, is special because the gas-phase working fluid is provided by erosion and evaporation of the electrode material. This type of arc is of importance in high-current electrical equipment, high-current vacuum circuit breakers, and switches. 

The cathodes spots are the localized current centers, which appear on the cathode when significant current should be provided but the cathode carmot be heated enough as a whole. The most typical cause of cathode spots is the application of metals with relatively low melting points. The cathode spots can also be caused by low arc currents, which are only able to provide the necessary electron emission when concentrated to a small area. The cathode spots also appear at low gas pressures (<1 Torr), when metal vapor from the cathode provides atoms to generate positive ions bringing their energy to the cathode to sustain the electron emission. To provide the required evaporation, cmrent is concentrated in spots at pressures <1 Torr and currents 1-10 A, such spots occur even on refractory metals. 

Table 4-8. Typical Characteristics of Cathode Spots of Are Diseharges...

Several problems related to the cathode spot phenomenon are not thus far completely solved. For example, the cmrent-voltage characteristics of vacuum arcs are not decreasing but increasing also there is no complete explanation of the cathode spot motion and splitting. The most intriguing cathode spot paradox is related to the direction of its motion in an... 

Arc Cathode Spots. The minimum current though a cathode spot for different nonferromagnetic materials can be found using the empirical formula (4-54). Using this relation, calculate the minimum current density for copper and silver electrodes and compare the results with data presented in Table 4-8. 

Cathode spot Highly mobile, minute luminous area on the negative electrode surface of an arc that emits electrons, jets of plasma, and metallic vapor. 

Cathode-spot track Erosion marks left on the negative electrode of an arc by the passage of a cathode spot. 

The vacuum arc is shown in idealized form in Fig. 1. A slow-motion color movie of such an arc is a beautiful sight to behold. One sees a cold cathode surface covered with isolated, small, brilliant spots. These cathode spots move erratically over the cathode surface, sometimes dividing into two or more fragments or extinquishing and reforming elsewhere on the cathode. Associated with the cathode spots are luminous jets that shoot off into space and constantly change direction. Between the electrodes one sees a diffuse glow whose color is characteristic of the electronically excited metal vapor of the electrodes. If the arc... 

Upon arc ignition (Section II. A), the space between the electrodes quickly fills with a diffuse plasma consisting of partially iottized metal vapor. At high currents this plasma expands into the volume surrounding the electrodes and their supports. At low currents the positive electrode collects electron current from the plasma uniformly over its surface. The metal shield or vacuum envelope that surrounds the arc also collects charges from the plasma and metal vapor. At high currents one or more distinct anode spots may appear. These spots always form on the end of the anode, which faces the cathode, in contrast with the cathode spots, which some-times wander off the end of the cathode and move about on the sides of the electrode. The anode spot also tends to remain in one position, in contrast with the mobility of its counterparts on the cathode. 

FIGURE 2 Photograph of a vacuum arc showing anode spot and cathode spots with plasma jets. (Photograph courtesy of Dr. Gerhard Frind, Corporate Research and Development, General Electric Company.)... 

The cathode spot, which is essential for the veiy existence of a vacuum arc, is the least understood of all vacuum-arc phenomena. Clearly, it is the source of electron emission for the arc, but it also provides plasma and metal vapor. The cathode spot is a highly efficient electron emitter. The current carried by a single spot depends on the cathode material and may vary from several amperes to a few hundred for most metals. When the arc current exceeds the current that a single spot normally carries, additional spots will form, sometimes by the enlargement or division of the original spot and also by the formation of new spots. Cathode spots do not respond instantly to a demand for an increase in current when the voltage applied across the arc is increased. It would appear that the spots normally operate at maximum current for the available heated emitting area and require a thermal response time of several microseconds to meet a demand for increased current. 

Sometimes what appears to the eye as a single cathode spot is actually, on closer examination, numerous, small, active areas. This cellular substmcture is found more frequently on mercury cathodes and may consist of a cluster of 4-12 cells. 

Because cathode spots have a finite lifetime, they often extinguish and reform elsewhere on the cathode while the are is burning. These spots are seldom stationary and move about on the eathode surface in a random, erratic way, sometimes reaching speeds of 30 m/sec on copper. It would appear that they tend to repel one another and also move in reverse to the direction expected for a conductor... 

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