Write short note on Magnetron Oscillator.
The magnetron cross section view and field distribution inside the magnetron are presented in Fig. 38(a) and 2 respectively. The reason behind the development of magnetron tube was the requirement of high power during the time of 2nd world war though it was invented by Hull in 1921. The basic structure of a magnetron is a number of identical resonators arranged in a cylindrical pattern around a cylindrical cathode.
Magnetron is a crossed – field device, in which the magnetic field is applied parallel to the axis of the cathode and electric field is applied perpendicular to the axis of the cathode. The space between cathode and the anode is the interaction space of the electron (emitted by the cathode) and cross field.
Under zero magnetic field electron emitted by the cathode moves radially in the outward direction towards anode. However the applied magnetic field crosses the electrons to curve in their paths. Then for a critical magnetic field, the electrons would describe arcs with just graze the anode and bend back forward the cathode. At higher values, the electron would spiral in the interaction space never stays far from the cathode.
A portion of the field distribution of the interaction region near to adjacent cavities under π- mode operation is as shown in the Fig. 38(b). As the phase difference between adjacent cavities is π- radians, hence it is called π- mode of operation of magnetron. Different phase angles between the eight coupled resonators give rise of different modes of operation of a magnetron.
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Fig. 38(a) Magnetron cross – section
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Fig. 38(b) Field distribution of a magnetron
Mode jumping in Magnetron:
If the frequencies of the different modes of operation are far apart, the magnetron has a tendency of mode jumping during the operation. On way to force then to operate in a particular mode is by strapping technique as shown in Fig. 38(c). The resonant modes of magnetron are very close to each other and there is always a possibility of mode jumping. The weaker modes have frequencies differing very little from the dominant mode and the purity of vibration may be lost. Mode jumping hence must be a avoided. A magnetron in which no effort is made to separate the dominant mode (mostly the π-mode) from other modes is said to be unstrapped. A method commonly employed to avoided mode jumping known as strapping.
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Fig. 38(c) Strapped magnetron
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Fig. 38(d) Strapping scheme for π-mode
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Fig. 38(e) rising sun magnetron
Strapping means to connect alternate anode plates with to conducting rings of heavy gauge touching the anodes poles at the dots as shown in Fig. 38(d). This is done in order to make the 2π anode poles together (0, 2π, 4π and 6π anode cavites). Strapping helps in achieving only the dominant mode (here π-mode) in the magnetron. However strapping may cause power loss in the conducting space and strapping may introduce stray effects.
A magnetron which needs no strapping is the rising sun magnetron shown in Fig. 38(e). Here the anode cavities are designed to be dissimilar and only the dominant mode with 2π phase will be effective. The adjacent cavities oscillate at widely different frequencies and hence separation will be quite effective.
