Plasmatron

a gas-discharge device for the production of a “low-temperature” (T ≃ 10⁴°K) plasma. Plasmatrons are used mainly in industry for processing purposes, but devices similar to the plasmatron are used as plasma engines. The term originated in the late 1950’s and early 1960’s, when plasmatrons came into wide use in industry and in laboratories. In that period efficient methods, from the engineering point of view, were developed for the stabilization of the high-frequency discharge and the arc discharge, and ways were found to insulate the walls of the chambers where the discharges occur from the thermal action of the discharges. For this reason, high-frequency and arc plasmatrons have received the most widespread acceptance.

Arc plasmatron. An arc plasmatron operating on direct current consists of the following basic components: one (cathode) or two (cathode and anode) electrodes, a discharge chamber, and a device for feeding the plasma-forming material. The discharge chamber may be combined with the electrodes to form a hollow-cathode plasmatron. Arc plasmatrons operating on alternating voltage are used more rarely; when the voltage alternates with a frequency of ≃ 10⁵ hertz, such devices are classed as high

Figure 1. Schematics of arc plasmatrons: (a) axial, (b) coaxial, (c) with toroidal electrodes, (d) with bilateral efflux, with an external plasma arc, (f) with consumable electrodes (erosion plasmatron); (1) power supply, (2) discharge, plasma jet, (4) electrode, (5) discharge chamber, (6) solenoid, (7) object being processed

frequency plasmatrons. Various types of arc plasmatrons exist, including those with axial or coaxial arrangements of electrodes, with toroidal electrodes, with bilateral plasma efflux, and with consumable electrodes (Figure 1). The opening through which the plasma flows out of the discharge chamber is called the nozzle of the plasmatron; in some types of arc plasmatrons a circular or toroidal anode is the boundary of the nozzle.

Arc plasmatrons can be used (1) to generate an external plasma arc (usually called a plasma arc) or (2) to generate a plasma jet. In plasmatrons of the first group, the arc discharge occurs between the cathode of the plasmatron and the body being processed, which serves as the anode. These plasmatrons may contain either just a cathode or a cathode and a second electrode—an auxiliary anode that ignites the main arc by a low-power discharge between it and the cathode; this discharge may be either continuous or of short duration. In plasmatrons of the second group, plasma generated in a discharge between the cathode and the anode issues from the discharge chamber in the form of a long narrow jet.

Stabilization of the discharge in arc plasmatrons is achieved by a magnetic field, streams of gas, and the walls of the discharge chamber and of the nozzle. One widely used method of magnetic stabilization of a plasma jet plasmatron with a circular or toroidal anode coaxial with the cathode involves the generation by a solenoid of a strong magnetic field perpendicular to the plane of the anode; the field causes the flow channel of the arc to rotate continuously, circling the anode. The anode and cathode spots of the arc thus move around in a circle, thereby keeping the electrodes from melting or, if the electrodes are made of refractory materials, from being intensively eroded.

The methods of gas stabilization, thermal insulation, and arc compression include whirling, in which the gas is fed into the discharge chamber through spiral channels with the resulting formation of a gas eddy, which blows around the arc column and the generated plasma jet; a layer of cooler gas is thus positioned at the chamber walls by centrifugal forces, protecting the walls from contact with the arc. When strong compression of the plasma flow is not required—for example, in some plasma arc plasmatrons used for melting metals—the stabilizing gas flow is not whirled but is directed parallel to the arc column, and is not constricted by the nozzle; the cathode is located right at the opening of the nozzle. The stabilizing gas is often at the same time the plasma-forming material. Stabilization and compression of the arc is also performed by a stream of water with or without whirling.

The plasma of arc plasmatrons inevitably contains particles of the electrode material owing to erosion of the electrodes. When for engineering reasons this erosion process is desirable, it is intensified through the use of consumable electrodes. In other cases, it is minimized by fabricating the electrodes from refractory materials—such as tungsten, molybdenum, or special alloys —or by cooling the electrodes with water, a process that in addition extends the service life of the electrodes; sometimes both methods are used. A plasma of greater “purity” is produced by high-frequency plasmatrons.

Plasmatrons with plasma jets are usually employed for such purposes as the heat treatment of metals, the application of coatings, and the production of powders with spherical particles; they are used in plasma chemical technology and other areas. Plasmatrons with external arcs are used in the processing of electrically conductive materials. Plasmatrons with consumable electrodes are used with aggressive plasma-forming materials— such as air or water—and when it is necessary to generate metallic, carbon, or other types of plasma from the electrode materials —for example, in the carbothermal reduction of ores.

The outputs of arc plasmatrons are 10₂–10₇ watts. The jet temperature at the opening of the nozzle is 3,000°-25,000°K, and the velocity of the jet efflux is 1–10⁴ m/sec. The net efficiency is 50–90 percent, and the operating life, which is determined by the erosion of the electrodes, is as long as several hundred hours. The plasma-forming materials used include air, N₂, Ar, H₂, NH₃, O₂, H₂O, liquid and solid hydrocarbons, metals, and plastics.

High-frequency plasmatron. The components of a high-frequency plasmatron are an electromagnetic coil (inductor) or electrodes that are connected to a high-frequency energy source, a discharge chamber, and a device for the introduction of the plasma-forming material. The types of high-frequency plasmatrons include induction plasmatrons, capacitive plasmatrons, torch plasmatrons, corona discharge plasmatrons, plasmatrons with high-frequency coronas, and microwave plasmatrons (Figure 2).

The industrially most widespread plasmatrons are high-frequency induction plasmatrons, in which the plasma-forming gas is heated by eddy currents. Since the high-frequency induction discharge is electrodeless, these plasmatrons are used for heating reactive gases (such as O₂, Cl₂, and air), vapors of aggressive materials (chlorides, fluorides, and other materials), and inert gases, if the plasma jet is subject to stringent purity requirements. Induction plasmatrons are used to produce finely divided and highly pure powdered materials based on borides, nitrides, carbides, and other chemical compounds. In plasma chemical processes, the reaction zone may be combined within the discharge

Figure 2. Schematics of high-frequency plasmatrons: (a) induction, (b) capacitive, (c) torch, (d) microwave; (1) electric power supply, (2) discharge, (3) plasma jet, (4) inductor, (5) discharge chamber, (6) electrode, (7) wave guide

chamber of such plasmatrons. The output of these plasmatrons can reach 1 megawatt. The temperature in the center of the discharge chamber and in the initial section of the plasma jet is approximately 10⁴°K. The velocity of the plasma efflux is 0–10₃ m/sec. The frequencies range from several tens of thousands of hertz to tens of megahertz. The net efficiency is 50–80 percent, and the working life is about 3,000 hours. The working frequencies in microwave plasmatrons are thousands and tens of thousands of megahertz. Magnetrons are used as power-supply generators for such plasmatrons.

With the exception of induction plasmatrons, high-frequency plasmatrons are presently (1970’s) used chiefly in laboratory applications. High-frequency plasmatrons, like arc plasmatrons, often make use of gas whirling, which insulates the discharge from the chamber walls. This makes it possible to fabricate chambers of high-frequency plasmatrons from materials of low thermal stability, for example, from simple or organic glass.

Techniques used to start plasmatrons—that is, to initiate the electrical discharge—include closing of the electrode gap, ignition of an auxiliary arc discharge, high-voltage sparkover of the gap between the electrodes, and injection of plasma into the discharge chamber. The principal trends of further evolution of plasmatrons include the development of specialized plasmatrons and plasma reactors for the metallurgical and chemical industries, increasing the output of a single unit to 1–10 megawatts, and increasing the operating life.