Thyratron
a gas-filled electron tube, usually having three electrodes, with a hot or cold cathode and grid control of the initiation time of a non-self-sustaining arc discharge in the former case or a glow discharge in the gas filling the tube in the latter case.
After initiation, the grid of a thyratron can no longer control the anode current. In contrast to a tacitron, therefore, the discharge in a thyratron can be extinguished only by reducing the anode voltage to a value less than the rated discharge voltage.
With the development of semiconductor electronics, thyratrons designed for use as relays, in current rectifiers, and in transducers (seeCONVERSION TECHNOLOGY) have been replaced almost entirely by semiconductor devices, mainly thyristors. Pulse-forming thyratrons, however, are widely used—mainly in circuits for shaping high-power pulses of electric current (primarily as switching devices in the modulators of radar transmitters).
When a pulsed voltage with an amplitude of 100–300 volts is supplied to the grid of a pulse-forming thyratron, an auxiliary discharge is initiated in the space between the grid and the cathode. When the grid current and, correspondingly, the concentration of charged particles near the grid (in the region penetrated by the anode field) rise to critical values, the rapid process of formation of an arc-discharge plasma between the anode and cathode commences. In this process, which takes only a few tens of nanoseconds, the anode current increases rapidly, the voltage decreases, and the thyratron switches from the fired to the unfired state.
During the operation of pulse-forming thyratrons (for example, in a linear modulator; see Figure 1), the discharge is usually initiated periodically at the repetition rate of the grid pulses. Each time the thyratron discharge is initiated, the pulse-forming line discharges through the load, for example, a magnetron. In the process of discharge, the voltage across the thyratron decreases from ≈ 2 Ea to a value less than the arcing potential, and the discharge is extinguished. As a result, periodically repeated current pulses flow through the load.
Existing types of pulse-forming thyratrons make it possible to obtain current pulses ranging in amplitude from 1 to 5,000 amperes and in length from 0.1 to 6 microseconds or more at a repetition rate of up to 30 kilohertz (for short pulse lengths). The thyratrons, whose efficiency can be as high as 95–98 percent, are distinguished by high stability of the initiation time (the spread in the length of the leading edges of the pulses does not exceed 3 × 10–9 sec), a short recovery time, and high reliability. The anode voltage of high-power thyratrons may be as high as 100 kilovolts. Most pulse-forming thyratrons are filled with hydrogen at a pressure of 25–95 newtons per square meter; deuterium and mixtures of hydrogen and deuterium are used less often.
At low currents (10–50 milliamperes) and with low anode voltages (150–300 volts), glow-discharge thyratrons with one or more grids and with current control, as in pulse-forming thyratrons, or electrostatic control are also used. Electrostatic control requires an additional electrode, known as the priming grid. The considerable recovery time (thousands of microseconds) and slow response of glow-discharge thyratrons limit their range of application
mainly to low-frequency devices used in computer technology, automatic control, and physical experiments. For example, such thyratrons are used as sawtooth voltage generators. A promising variety of glow-discharge thyratrons is the indicator glow-discharge thyratron, which is used in devices for visual data display (seeGAS-DISCHARGE INDICATORS). A specific feature of indicator glow-discharge thyratrons is the possibility of controlling their initiation by low-voltage signals (a few volts). This feature makes it possible to use such thyratrons in combination with transistorized and integrated-circuit devices.
Thyratrons are produced industrially in glass, metal-glass, and metal-ceramic enclosures.
REFERENCES
Kaganov, I. L. lonnye pribory. Moscow, 1972.Fogel’son, T. B., L. N. Breusova, and L. N. Vagin. Impul’snye vodorodnye tiratrony. Moscow, 1974.
T. A. VORONCHEV
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