Question

Draw the construction of magnetron and describe it’s working.

Answer 1

Working/Operation: 1. Now assume RF oscillations are initiated due to some noise transient within the magnetron, the oscillations will be sustained by device operation.

2. Self-oscillations will be obtained if the phase difference between adjacent anode poles is nπ/4 (N=8), where n is an integer. n=4 results in π mode. Here the anode poles are π radians apart. 3. The dotted lines refer to the path of electrons in case of static field. The solid lines refer to the electron trajectories in the presence of RF oscillations in the interaction space. 4. The electron ‘a’ is seen to be slowed down in the presence of oscillations thus transferring energy to the oscillations during its longer journey from cathode to anode. Such electrons which participate in transferring energy to the RF field are called as favored electrons and these electrons are responsible for bunching effect. 5. An electron ‘b’ is accelerated by the RF field. Instead of imparting energy to the oscillations, it takes energy from the oscillations resulting in increased velocity. Hence bends more sharply, spends very little time in the interaction space and is returned back to the cathode. Such electrons are called un-favored electrons which do not participate in the bunching process; rather they are harmful as they cause back heating. 6. Similarly electron ‘c’ which is emitted little later to be in correct position moves faster and tries to catch up with electron ‘a’ and an electron emitted at d will be slowed down to fall back in step with the electron ‘a’. 7. This result in all favored electrons like a, c, d to form a bunch and are confined to electron clouds or spokes as shown in fig below. This process is called phase focusing effect corresponding to the bunch of favored electrons around the reference electron ‘a’. The spokes so formed in the π-mode rotate with an angular velocity corresponding to 2 poles/cycle.

8. The phase focusing effect of these favored electrons imparts enough energy to the RF oscillations so that they are sustained.