Magnetron-Type Devices

a class of superhigh-frequency (SHF) electron-tube devices (300 megahertz to 300 gigahertz) in which electrons move in crossed constant electric and magnetic fields and an SHF electromagnetic field. They are used to generate and amplify oscillations in radar, navigational, and space-communications equipment; linear accelerators; medical apparatus; and SHF heaters.

In magnetron-type devices a constant electric field is generated in the anode-cathode gap (the interaction space), and a constant magnetic field is generated perpendicular to the lines of force of the constant electric field and the direction of motion of the electrons (in devices of cylindrical design, the magnetic field is parallel to the axis of the cathode). The conditions for feedback between the electromagnetic field and the electron flux required for self-excitation of oscillations in magnetron-type devices are readily satisfied. Because of the feedback, the electrons that interact with the electromagnetic field and give up to it a portion of the energy they acquired from the DC source are shifted to the anode and finally strike it, whereas the electrons that pick up energy from the electromagnetic field return to the cathode and bombard it. The phenomenon of electron bombardment is used in some high-power magnetron-type devices to maintain the required cathode temperature. To achieve efficient and sustained interaction between the electrons and the electromagnetic field, their motion must be synchronized—that is, the velocity v_e of the translatory motion of the electrons must be equal to the phase velocity of the traveling wave of the field.

Magnetron-type devices are multifunctional in nature: they operate efficiently in various electrical modes and under various service conditions with high efficiency (up to 90 percent). They are capable of generating and amplifying oscillations over a very wide range of electromagnetic wavelengths (from meter to millimeter waves), generating oscillations with high power (up to several hundred kilowatts of continuous power and up to several dozen megawatts of pulsed power) with relatively low anode voltages (up to 50 kilovolts), frequency tuning over a broad range (up to 20 percent mechanically and up to 100 percent electrically), and amplifying oscillations over a broad frequency band (20 percent and more) with fairly high gain (20 decibels and more).

The prototype of all magnetron-type devices is the multi-resonator magnetron, which is the best-known device of this class.

A great variety of oscillators and amplifiers have been developed on the basis of the magnetron principle of the interaction of an electron stream with an electromagnetic field; they differ in the design of the delay systems (slow-wave circuits) and the units for forming the electron flux. Three families of magnetrons are distinguished according to these attributes: (1) magnetrons with a closed, annular slow-wave circuit and electron flux (with the cathode in the interaction space); (2) magnetrons with an electrically open slow-wave circuit and a closed, annular electron flux (with the cathode in the interaction space); (3) magnetrons with closed or open slow-wave circuits and an injected electron flux (with the cathode outside the interaction space).

The first family includes mainly the multiresonator magnetron, or traveling-wave magnetron, in which the slow-wave circuit has clearly defined resonance properties (that is, the oscillations are excited at discrete frequencies), the operational type of oscillations is the π type or the π/2 type, and the oscillation frequency may be tuned mechanically or electrically over a small range (3-10 percent); the coaxial magnetron (a variety of multiresonator magnetron) with frequency tuning (up to 20 per-cent) and stabilization by means of an external or internal high-Q cavity resonator that is coaxial with the resonator system of the magnetron and is excited by an H₀₁₁ type of wave; the regenerative-amplifier magnetron, in which oscillations of the π type are stimulated and their frequency controlled by a low-power external signal, which is usually injected through a circulator into a heavily loaded resonator system; the voltage-tunable magnetron (mitron), in which a heavily loaded oscillatory system (usually of the interdigital type) has weakly defined resonance properties and the emission current of the cathode is limited, so that under low-power conditions the frequency is tuned over a broad range (up to about 1 octave and more) by means of the voltage.

The second family includes the carmatron, a backward-wave generator in which a bar-type slow-wave circuit (most frequently of the interdigital type) is usually used, with an internal energy absorber and tuning of the oscillation frequency by means of voltage; the amplitron, a powerful backward-wave amplifier with matched input and output devices and an amplifying frequency band of up to 10 percent of the average frequency (with SHF reflections at the input and output and with limiting of the emission current by means of temperature, an amplitron can operate as a self-excited oscillator with frequency tuning); the stabilotron, a very stable generator with mechanical frequency tuning, consisting of an amplitron, a reflecting power divider, a phase shifter, and a high-g stabilizing resonator (the term “platinotron” is often found in the literature as a general designation for the amplitron and stabilotron); and the ultron, a forward-wave amplifier with a broader amplifying frequency band (up to 20 percent) and higher gain (up to 30 decibels) than that of the amplitron.

The third family includes the magnetron-type backward-wave tube, with voltage frequency tuning of the generated oscillations over a broad range (up to 20 percent), and the traveling-wave tube of the magnetron type having a broad amplifying frequency band (up to 20 percent) and high gain (up to 20 decibels).