Electric Discharge in Gases

the passage of electric current through a gaseous medium under the action of an electric field, accompanied by changes in the state of the gas.

Because of numerous existing factors, there are many kinds of electric discharges in gases. Among these factors are the varied conditions that determine the initial state of the gas (for example, composition and pressure), the numerous external effects that affect the gas, the many shapes, materials, and arrangements of electrodes, and the varied geometry of the electric field arising in the gas. Moreover, the laws applying to such discharges are much more complex than the laws governing the passage of current through metals and electrolytes. Electric discharges in gases obey Ohm’s law only in those cases where the externally applied potential difference is quite small (seeOHM’S LAW). Consequently, the electric properties of electric discharges in gases are usually represented by a volt-ampere characteristic (Figures 1 and 3).

Figure 1. Volt-ampere characteristic of a silent discharge

Gases become electrically conductive upon ionization (seeIONIZATION). If an electric discharge in a gas occurs only when some external effect initiates and maintains ionization (that is, upon the action of an external ionizer), the discharge is a nonself-sustained, or non-self-maintained, discharge. An electric discharge that continues even after an external ionizer ceases to operate is called a self-sustained, or self-maintained, discharge.

A “silent” discharge is initiated when the gas is ionized by a continuously operating external ionizer and if there is only a small potential difference across the gas-filled space between the anode and the cathode. As the potential difference (voltage) increases, the current of the silent discharge initially increases proportionally to the voltage (segment O A of the curve in Figure 1). Then the increase in current caused by the increase in voltage slows down (segment AB of the curve), and when all charged particles arising per unit time under the action of the ionizer depart in that time to the cathode and the anode, an increase in voltage no longer causes an increase in current (segment BC). The further increase in voltage causes an increase in current, and the silent discharge undergoes a transition to the Townsend discharge (segment CE in Figure 1), which is non-self-sustained. In this case, the current strength is determined not only by the intensity of ionization achieved by the ionizer but also by the gas amplification, which depends on the gas pressure and the electric field intensity in the space where the discharge occurs (seeFIELD INTENSITY, ELECTRIC).

A silent discharge is observed if the gas pressure is close to atmospheric pressure. Phenomena that may act as external ionizers include natural radioactive radiation, cosmic rays, photon streams (strong irradiation by light sources), and beams of fast electrons. Ionizers of the last two types are used in gas lasers, primarily for pulsed operation.

The transition of the non-self-sustained type of electric discharge to the self-sustained type is characterized by a sharp increase in current (E on the curve in Figure 1); this transition is the electric breakdown of the gas. The voltage that corresponds to the breakdown point is called the firing voltage U_f (seeFIRING VOLTAGE). If the field is homogeneous, the value of U_f depends on the gas used and on the product of the gas pressure p and the interelectrode distance d (seePASCHEN’S LAW and Figure 2). After the occurrence of an avalanche breakdown, the discharge assumes the form of a glow discharge if the pressure is sufficiently low, of the order of several millimeters of mercury (mm Hg). At higher pressures such as atmospheric pressures, an avalanche increase in the discharge gives rise to a space charge, which alters

Figure 2. Paschen curves for various gases. The products p · d, in mm Hg, are laid out on the axis of the abscissas, and the breakdown potentials U_f, in volts, are laid out on the axis of the ordinates.

the character of the breakdown process (see). One or several narrow, plasma-filled conducting channels, called streamers, now emanate from one of the electrodes (seeSTREAMER). The formation time of the streamers is very short, about 10^–7sec.

A self-sustained gas discharge achieves a steady state after a short transitional process. Such a discharge is usually carried out within a closed insulated (glass or ceramic) vessel. The current in the gas flows between two electrodes: the negative cathode and the positive anode.

The glow discharge (seeGLOW DISCHARGE), which occurs, as a rule, at low pressures and weak currents, is one of the principal types of discharges in gases (region c in Figure 3). There are four main regions of the discharge space that are characteristic of the glow discharge: (1) cathode dark space, (2) negative glow, (3) Faraday dark space, and (4) positive column. Regions (1) through (3) are located near the cathode and form the cathode part of the discharge space, where a sharp fall in potential (seeCATHODE DROP) occurs, associated with a high concentration of positive ions at the boundary between regions (1) and (2). Electrons that had been accelerated in region (1) produce an intensive impact ionization in region (2). The luminescence of the glow is caused by the recombination of ions and electrons into neutral atoms or molecules. The positive column of the discharge is characterized, owing to its high and constant concentration of electrons, by a low drop in potential, by high electrical conductivity, and by luminescence, the last caused by the return of excited gas molecules (or atoms) to the ground state (the state of lowest possible energy).

The steady state of the positive column can be ascribed to the mutually compensating processes of formation and loss of charged particles. The formation of such particles occurs during the ionization of atoms and molecules as a result of their collisions with electrons. The loss of charged particles occurs as a result of ambipolar diffusion toward the wall of the vessel, which forms the boundary of the discharge space, and the subsequent recombination process. Diffusion streams that are directed away from the wall, in the direction of the discharge current, often cause the formation of unique, usually moving, “layers” in the positive column.

Figure 3. Volt-ampere characteristic of a discharge: (ab) Townsend discharge, (bcd) glow discharge, (de) arc discharge

Upon increasing the discharge current, the normal glow discharge becomes abnormal (Figure 3) and a contraction of the positive column ensues. The column separates from the walls of the vessel, and there begins within the column an additional process, known as volume recombination, which contributes to the loss of charged particles. A prerequisite for this phenomenon is a high density of charged particles. If the discharge current is increased still further, the gas heats up to such a degree that its thermal ionization becomes possible. The collisions between atoms and molecules in this case occur with such a force that electrons are released. This type of discharge is called arc discharge (seeARC DISCHARGE). A continuing increase in current increases electrical conductivity of the column and causes the volt-ampere characteristic of the arc discharge to fall (Figure 3). It should be noted that although an arc discharge can be maintained under a wide range of gas pressures and other conditions, most arc discharges occur at pressures close to atmospheric pressures.

In all cases, the transition regions between the discharge columns and the electrodes are of prime importance, the conditions at the cathode being more complex than those at the anode. For glow discharge, the uninterrupted connection between the cathode and the positive column is assured by the large cathode drop. In self-sustained arc discharges, cathode spots appear as a result of strong local heating of the cathode. In these spots there occurs either thermionic emission or a more complex emission of electrons from a cloud of evaporating cathode material (seeTHERMIONIC EMISSION). The process of emission from the cathode of an arc discharge is not as yet fully understood (1978), but it is being intensively studied.

All types of electric discharges in gases discussed thus far occur under the action of direct-current voltages. However, discharges in gases can also occur under the action of alternating-current voltages. Such discharges exhibit steady-state characteristics if the frequency of the alternating-current voltage is sufficiently high (or, conversely, so low that a half-period of the alternating voltage is many times greater than the time required to establish the discharge; in this case, each electrode simply serves, in turn, as the cathode and the anode). The high-frequency electrjc discharge in gases is a typical example of an alternating-current discharge, and it can be maintained even without electrodes (seeELECTRODELESS DISCHARGE). The changing electric field generates a plasma in some part of the space and imparts to the electrons an energy of such a magnitude that the ionization achieved by the field overcomes the loss of charged particles incurred by diffusion and recombination. The external appearance and the characteristics of a high-frequency discharge depend on the kind of gas used, on the frequency of the changing field, and on the power input. Elementary processes occurring on the surfaces of solids, such as metals or insulators of the discharge chamber, play a certain role only during the initial “firing” of the discharge. A stationary high-frequency discharge is similar to the positive column of a glow discharge.

In addition to stationary discharges, whose characteristics do not depend on time, there also exist nonstationary (pulsed) discharges, which arise in fields that are either strongly nonhomogeneous or that change with time, for example, at pointed or curved surfaces of conductors and electrodes. The intensity and nonhomogeneity of such fields are so great near such bodies that the impact ionization of gas molecules by colliding electrons occurs. The two important types of nonstationary discharges are the corona discharge and spark discharge (seeCORONA DISCHARGE and SPARK DISCHARGE).

In corona discharges, ionization does not lead to a breakdown, since the strong nonhomogeneity of the electric field exists only in the immediate proximity of wires and sharp points. A corona discharge is essentially a firing or ignition process repeated many times. The length of the distance from the conductor in which this process can occur is limited, since beyond a certain distance the intensity of the field becomes too low to maintain the discharge. In contrast to the corona discharge, a spark discharge does lead to a breakdown. The spark type of electric discharge in gases has the appearance of intermittent, bright, zig-zag, branching filament channels, which are filled with ionized gas (plasma); these filaments permeate the interelectrode space, disappear, and are replaced by new ones. Spark discharge is accompanied by the evolution of a large quantity of heat and by a bright luminescence. It passes through the following stages: first there occurs a sharp increase in the number of electrons in the strongly nonhomogenous field near the conductors (electrodes) as a result of a series of successive ionizing events, initiated by a few random free electrons. The next stage is the formation of electron avalanches, followed by the transition of the avalanches to streamers under the action of a space charge when the density of charged particles at the head of each avalanche exceeds a certain critical value. The combined effects of space charge and ionizing electrons and photons in the “head” of the streamer accelerate the buildup of the discharge. Lightning is one example of a naturally occurring spark discharge; it can be several kilometers long and can produce maximum currents of hundreds of thousands of amperes (seeLIGHTNING).

At the present time (1970’s), all types of electric discharges in gases are being investigated and applied in many areas of science and technology. Glow, arc, and pulsed discharges are used to excite gas lasers. Plasmatrons, in which arc discharges or high-frequency discharges constitute the basic operating process, have found important applications in several technological processes, such as the manufacture of extremely pure semiconductors and metals. In plasma chemistry, powerful plasmatrons are used as reactors (seePLASMA CHEMISTRY). Precision methods of electric spark machining are based on spark discharge. In focusing the light emission of lasers, an air breakdown occurs at the focal point and an electrode-less discharge arises, called the laser spark, which is similar to a high-frequency discharge or a spark. High-power, high-current discharges in hydrogen constitute the first steps in the quest to achieve controlled thermonuclear fusion (seeCONTROLLED FUSION).

In the natural sciences, the study of electric discharges in gases is classified as a branch of plasma physics. A low-temperature plasma, characterized by a low degree of ionization, is formed during a discharge. In low-temperature plasma, the atoms and molecules of neutral gases play an important role, which is not the case in high-temperature, fully ionized, plasma. Electrons, ions, and neutral particles all interact “softly.” Consequently, a thermodynamically unbalanced situation can develop, in which the electrons, ions, and neutral gas all have different temperatures. This situation becomes even more complex if the emission of light cannot be disregarded in the energy balance of electric discharges in gases (this applies, for instance, to high-current arc discharges). In such cases, low-temperature plasma must be described in terms of the kinetic theory of plasma.