Ion Devices

(also called gas-discharge devices), vacuum-tube devices whose operation is based on the use of various types of electric discharges in a gas (for example, inert gases or hydrogen) or in metal vapors.

The simplest ion device is a diode whose vessel is filled with an inert gas or mercury vapor. The properties of ion devices are determined by the interaction of the electron flux with the gaseous medium and the electric field between the electrodes (the anode and the thermionic or cold cathode). As they move from the cathode to the anode, the electrons ionize the atoms and molecules of the gas with which they collide; electrons and positively charged ions are formed in the space between the electrodes of the device. As a result of compensation for the space charge of the electrons by the positive ions in an ion device, very high current intensities may be produced at small potential differences (a voltage drop) between the electrodes; this is unattainable in other types of vacuum-tube devices. Additional electrodes, such as grids and secondary anodes, are used to control the moment of appearance of the discharge in the ion device. In most cases electric discharges are accompanied by radiation of light (glow) with a spectral composition characteristic for a given gas. There are more than 50 classes of ion devices, whose operation is based on the use of certain properties of various types of discharge, primarily glow, arc, spark, and corona discharge.

Glow-discharge devices (signal lamps, stabilitron tubes, cold-cathode thyratrons, decatrons, digital indicator lamps, and matrix display panels) constitute the largest and most important group of ion devices. The gas pressure in them is tens of newtons per sq m, the current intensity does not exceed several dozen milliamperes, and their service life is tens of thousands of hours. They have small dimensions and light weight. However, the speed of operation of such devices does not exceed hundreds of microseconds (at an operating frequency of less than 100 kilo-hertz).

In arc-discharge devices, principally with a heated cathode, the gas pressure is tenths of a newton per sq m. Such devices (discharge-tube rectifiers, thyratrons, clipper devices, and taci-trons) have low internal resistance (dozens of ohms), and the voltage drop is 10–20 V (in pulse mode, 100–200 V). Their service life is limited by the gradual erosion of the cathode and the decline in the pressure (hardening) of the filling gas. A liquid mercury cathode (as in mercury valves or ignitrons) is used to increase service life. Devices with such a cathode are able to pass a current of up to several thousand amperes and to withstand an inverse voltage of up to hundreds of kilovolts. There are also arc-discharge devices, called arkatrons, with a self-heating cathode.

In spark-discharge devices, when a voltage exceeding a certain value (the puncture voltage) is fed between two cold metal electrodes, an electric spark arises in the form of a thin, brightly glowing channel that is intricately curved and branched. The gas pressure in such devices is dozens or several hundred kilo-newtons per sq m. Mixtures of inert gases with oxygen, carbon dioxide, and other gases are frequently used. The time required to form a spark discharge is very short (fractions of a nanosecond). The ability of the discharge gap to change its electrical conductivity almost instantaneously within a broad range—the electrical resistance of the interval changes from fractions of an ohm to hundreds of megohms—is used in spark dischargers, both uncontrolled and controlled (trigatrons).

In corona-discharge devices, such as stabilitron tubes, the ionization of the gas takes place in the region of greatest field intensity (the corona region), under the necessary condition of an abrupt nonuniformity of the electric field between the two electrodes (for example, when the electrodes are coaxial). The gas pressure in such devices may be hundreds of newtons per sq m or more. The dependence of current intensity on the voltage applied to the electrodes is a straight line that is nearly parallel to the axis of the currents.

A separate group of ion devices is made up of gas-discharge light sources, most of which are arc-discharge devices that run at high gas pressure (several hundred kilonewtons per sq m); high-intensity lamps; erythemal lamps, which produce strong ultraviolet radiation; and gas lasers (atomic, ionic, and molecular), which are sources of coherent electromagnetic oscillations in the light wavelength band.

There is also a group of ion devices (such as attenuators, phase shifters, and dischargers) whose operation is based on the interaction of a superhigh-frequency field and an ionized region of the gas.