Arc Discharge

one of the types of stationary electric discharge in gases. It was first observed between two carbon electrodes in air by V. V. Petrov in 1802 and independently by H. Davy in 1808–09. The luminous current channel of this discharge is bent into the shape of an arc; hence the name “arc discharge.”

The formation of an arc discharge is preceded by a short transient process in the space between the electrodes (the discharge gap). The duration of the transient process (stabilization time of the discharge) is usually about 10^-6 to 10^-4 sec, depending on the pressure and the type of gas, the width of the discharge gap, and the condition of the electrode surfaces. The arc discharge is produced by ionizing the gas in the gap (for example, by means of an auxiliary, so-called triggering, electrode). In other cases it is produced by heating one or both electrodes to a high temperature or by moving them apart after they have been touching for a short time. An arc discharge may also be developed as a result of an electrical breakdown of the discharge gap during a brief, sharp increase of the voltage between the electrodes. If the breakdown occurs when the gas pressure is close to atmospheric pressure, then the transient process that precedes the arc discharge is called a spark discharge.

Typical parameters. Arc discharges are characterized by a great variety of forms. A discharge can take place at virtually any gas pressure, from less than 10^-5 mm of mercury up to hundreds of atmospheres; the potential difference between the electrodes may have any value between several volts and several thousands of volts (a high-voltage arc discharge), and it may be DC or AC. However, AC half-cycle is usually much greater than stabilization time of the arc discharge, which makes it possible to regard each electrode as a cathode during one half-cycle and as an anode in the next. The distinctive features of all forms of arc discharge (which are closely associated with the nature of the electron emission from the cathode in this type of discharge) are the small cathode fall and the high current density at the cathode. The cathode fall is generally of the order of the ionization potential for the working gas, or even lower (1–10 volts), and the current density at the cathode ranges from 10² to 10⁷ amperes per sq cm (A/cm²). At such a high current density the current strength in an arc discharge is usually also high (on the order of 1–10 A or more), in some forms it reaches many hundreds and thousands of amperes. However, arc discharges exist that have a low current strength, such as a discharge with a mercury cathode, which operates on currents of 0.1 A or less.

Electron emission. A fundamental difference between an arc discharge and other types of stationary electrical discharges in a gas is the nature of the elementary processes that occur at the cathode and in the region near it. Whereas secondary electron emission takes place in a glow discharge and in a negative corona discharge, in an arc discharge electrons are emitted from the cathode as a result of thermionic and autoelectronic emission processes (the latter is also called tunnel emission). When only the first of these processes occurs in an arc discharge, the discharge is called thermionic. The intensity of the thermionic emission is a function of the cathode temperature; consequently, in order to have a thermionic arc discharge, the cathode or individual portions of it must be heated to a high temperature. This is achieved by connecting the cathode to an auxiliary energy source (an arc discharge with an external heater or with artificial preheating). A thermionic arc discharge takes place when the cathode temperature is raised sufficiently by the impact of the positive ions that form in the discharge gap and are accelerated by the electric field toward the cathode. However, in an arc discharge without artificial preheating, the thermionic emission intensity is frequently too small to maintain the discharge, and the autoelectronic emission process plays an important role. The combination of these two forms of emission is known as thermoautoelectronic emission.

Autoelectronic emission from a cathode requires the presence of a strong electric field near the cathode’s surface. Such a field is created by the space charge of positive ions at a distance from the cathode of the order of the mean free path for the ions (10^-6 to 10^-4 cm). Calculations indicate that autoelectronic emission cannot maintain an arc discharge independently and is always accompanied to some extent by thermionic emission. Because of the complexity of studying the processes in the thin layer near the cathode at high current densities, sufficient experimental information regarding the role of autoelectronic emission in an arc discharge is not yet available. Theoretical analysis cannot yet explain satisfactorily all the phenomena observed in the various forms of arc discharges.

Relation between arc-discharge characteristics and emission processes. The layer in which the electrical field that produces autoelectronic emission is developed is so thin that it does not create a large potential drop at the cathode. However, for the field to be strong enough, the density of the space charge of the ions at the cathode—and thus the density of the ion current—must be great. Thermionic emission also can take place with ions of low kinetic energy at the cathode (that is, with a small cathode fall), but under these conditions a high current density is required, since the greater the number of ions bombarding the cathode, the more intensely it is heated. Thus, the distinctive features of arc discharges (the small cathode fall and the high current density) are due to the characteristics of the processes near the cathode.

Plasma of an arc discharge. The discharge gap of an arc discharge is filled with a plasma that consists of electrons, ions, and neutral and excited atoms and molecules of the working gas and of the electrode materials. The mean energies of the various kinds of particles in the plasma may differ. Consequently, in speaking of the temperature of an arc discharge, a distinction is made between the ion temperature, the electron temperature, and the temperature of the neutral component. When these temperatures are all equal, the plasma is called isothermal.

Non-self-sustaining arc discharge. An arc discharge with artificial preheating of the cathode is called non-self-sustaining, since the discharge cannot be maintained by its own energy: when the external heat source is disconnected, the arc is extinguished. A discharge is easily ignited without auxiliary ignition electrodes. An increase in the voltage of this arc discharge at first increases its current to a value determined by the intensity of the thermionic emission from the cathode at a given heater temperature; then, up to some critical voltage, the current remains almost constant (the so-called free mode). When the voltage exceeds the critical value, the nature of the cathode emission changes: the photoelectric effect and secondary electron emission begin to play an essential role in it (the energy of the positive ions becomes high enough to dislodge electrons from the cathode). This leads to an abrupt increase in the discharge current, and the discharge passes into the restricted mode.

Under certain conditions an arc discharge with artificial preheating continues stable operation when the voltage between electrodes decreases to values that are not only smaller than the ionization potential of the working gas but also lower than its minimum excitation potential. This form of arc discharge is a low-voltage arc. Its existence is due to the development close to the cathode of a potential maximum that exceeds the anode potential and is close to the first excitation potential of the gas, as a result of which step ionization becomes possible.

Self-sustaining arc discharge. Self-sustaining arc discharge is maintained by the energy of its own discharge. On refractory cathodes (tungsten, molybdenum, or graphite) the arc discharge is purely thermionic in nature; the bombardment of the positive ions heats the cathode to a very high temperature. In such an arc discharge, the material of a fusible cathode is rapidly vaporized; the vaporization cools the cathode, and its temperature does not attain the values at which the discharge can be maintained by thermionic emission alone—autoelectronic emission takes place along with it.

A self-sustaining arc discharge can exist both at extremely low gas pressures (the so-called vacuum arcs) and at high pressures. The plasma of a self-sustaining low-pressure arc discharge is notable for its nonisothermal quality: the ion temperature is only slightly above the temperature of the neutral gas in the space surrounding the discharge region, whereas the electron temperature reaches tens of thousands of degrees and, in narrow tubes and at high currents, hundreds of thousands of degrees. This is explained by the fact that the more mobile electrons, receiving energy from the electrical field, are not able to transfer it to the heavy particles in their rare collisions.

In a high-pressure arc discharge the plasma is isothermal (more accurately, it is quasiisothermal, because although the temperatures of all the components are equal, the temperatures in different parts of the arc-discharge column are not the same). This form of arc discharge is characterized by a substantial current strength (10 to 10³ A) and a high plasma temperature (of the order of 10^4° K). The highest temperatures are achieved in such an arc discharge when the arc is cooled by a stream of liquid or gas—the current channel of a “cooled arc” becomes thinner and is heated more intensively at the same current strength. It is this form of an electric discharge that is called an electric arc—the arc discharge is bent by the action on the current channel of streams of gas that are either directed from outside or are convection streams produced by the discharge itself.

Cathode spots. A self-sustaining arc discharge on fusible cathodes is distinguished by the fact that thermoautoelectronic emission of electrons occurs only in small portions of the cathode (the so-called cathode spots). The small dimensions of these spots (less than 10^-2 cm) are caused by the pinch effect, which is the constriction of the current channel by its own magnetic field. The current density in a cathode spot depends on the cathode material and may reach tens of thousands of A/cm². Therefore intensive erosion takes place in cathode spots and jets of vaporized cathode materials fly out of them at a velocity of the order of 10⁶ cm/sec. Cathode spots are also formed during arc discharge on refractory cathodes if the pressure of the working gas is less than about 10² mm of mercury (mm Hg). At higher pressures the thermionic emission of an arc discharge with chaotically shifting cathode spots is transformed into a thermionic arc discharge without a cathode spot.

Applications. Arc discharge is widely used to smelt metals in arc furnaces, in gas-discharge light sources, for electric welding, and as a plasma source in plasmatrons. Various forms of arc discharge occur in gas-filled and vacuum current converters (mercury current rectifiers and gas and vacuum electrical switches). An arc discharge with artificial preheating of the cathode is used in fluorescent lamps, dischargetube rectifiers, thyratrons, ion sources, and electron-beam sources.