ECIA - TEP 42
Application Notes for Pulse Magnetrons
|Publication Date:||1 November 1963|
DEFINITION AND FUNCTION OF THE TUBE .
Magnetrons are defined as :* electron tubes "characterized by the interaction of electrons with the electric . field of circuit element in crossed steady electric and magnetic fields to produce a-c power output." This type of tube is often used when there is need for a device that will generate high power at microwave frequencies, as, for example, in the transmitter of a microwave radar system. Although such a transmitter operate only in very short pulses and at a low duty factor, the peak power required during this operation maybe very high. For such applications, the magnetron-type oscillator is well suited.
The choice of transmitter tube f or a radar system under design largely influences the ultimate capability of that system. It determines the peak and average power available, the pulse shape, the spectral purity that can be achieved, and many other of the limiting characteristics of the system. It does not, however, determine. Whether or not, nor to what extent, these limiting features are actually achieved. To obtain the desired reliability in performance of a system being designed, it is necessary not only that a tube with the required inherent capability be chosen, but also that the system be so designed and operated that this capability can be realized,
A magnetron is an extremely nonlinear device, its characteristics being functionally dependent on the instantaneous values of the imposed conditions, It is responsive t o many phenomena which characterize its electrical and mechanical environment, some of the relations between response and stimulus being quite subtle. Some of the stimuli have in fact not yet been firmly kdentified, and ther-efore all cause and effect relationships cannot be precisely defined. Al - though magnetrons of many different designs have been used with considerable success, the designer incorporating these in to systems being designed to meet new requirements must relate all available knowledge of their properties to his equipment requirements if he is to avoid the many possible pitfalls .
DESCRIPTION AND GENERAL CONSTRUCTION OF TUBE
Structurally, a magnetron is a vacuum diode on which a magnetic field is imposed. The genera of magnetrons to be discussed. here, utilize a cylind.rica1 cath0d.e coaxial with an annular anode in which a number of resonant cavities are formed. Electrons emitted by the cathode move in the inter - action space between cath0d.e and. anode under the combined influence of the radial d-c electric field and an axial, externally supplied, static magnetic field. Unlike electron tube types of more general applicability, such as triodes and tetrodes, the magnetron, is a complete energy-converting package, requiring only a power supply, usually direct current, and a load to absorb the r-f output. With most lower-frequency tybe types, the design of the associated circuitry is largely under the control of the circuit designer. The magnetron, like certain other microwave tubes providing in one package a source of electrons, a means of influencing their motion, an electromagnetic resonator, an interaction mechanism for converting resonator, an interaction mechanism for converting d-c power to r- f power, and a means for coupling the r-f power out, removes many details of the overall circuitry design of a magnetron system from the reach of the circuit designer.
* IRE Standards on Electron Tubes: Definitions of Terms, 1957, (57IRET.S2), Proc. IRE, 524 983-3.010(1957).