The magnetron

Physics of Magnetron

1. The Magnetron

2. THE MAGNETRON The microwave radiation of microwave ovens and some radar applications is produced by a device called a magnetron.

3. Magnetron Principles A magnetron is a device that converts high voltage DC electrical power into microwave power .

4. Construction The magnetron is constructed from a circular arrangement of microwave cavities that form the anode. The cathode is arranged so that it is concentric to the anode vane tips. When the air is extracted, and the cathode is hot, electrons are emitted, from its surface, into the space between the cathode and the anode vane tips. If a DC voltage is applied between the cathode and the anode and a magnetic field is applied at right angles to the electric field, then the electrons will follow a curved path as shown above. Microwave power is generated as the electrons interact with the anode resonator structure.

5. Magnetic Circuit Technology The applied magnetic field sets the operating voltage of the magnetron. Very high power magnetrons use water-cooled solenoids to provide a uniform magnetic field in the interaction space between the cathode and the anode. In some applications, such as LINACs , the solenoid current is varied to enable stable magnetron operation over a wider power range.

6. Physics Behind Magnetron

7. Physics Behind Magnetron (Continued)

8. The magnetron is called a " crossed-field " device in the industry because both magnetic and electric fields are employed in its operation, and they are produced in perpendicular directions so that they cross. The applied magnetic field is constant and applied along the axis of the circular device illustrated. The power to the device is applied to the center cathode which is heated to supply energetic electrons which would, in the absence of the magnetic field, tend to move radially outward to the ring anode which surrounds it. Physics Behind Magnetron (Continued)

9. Applications Radar In radar devices the waveguide is connected to an antenna . The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated. As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen. Several characteristics of the magnetron's power output conspire to make radar use of the device somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability is noted not only as a frequency shift from one pulse to the next, but also a frequency shift within an individual transmitter pulse. The second factor is that the energy of the transmitted pulse is spread over a wide frequency spectrum, which makes necessary its receiver to have a corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the received radar echoes, thereby reducing overall radar performance. The third factor, depending on application, is the radiation hazard caused by the use of high power electromagnetic radiation. In some applications, for example a marine radar mounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent advances in aviation weather avoidance radar and in marine radar have successfully implemented semiconductor transmitters that eliminate the magnetron entirely.

10. Heating In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking chamber.

11. Lighting In microwave-excited lighting systems, such as a sulfur lamp , a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur, metal halides, etc.).

12. LINAC’s Magnetrons are also used in LINAC’s to accelerate charged particles. In accelerator, high voltage flat topped DC pulses of a few microseconds duration are generated by the modulator section. These pulses are delivered to the magnetron and simultaneously to the electron gun. Pulsed microwaves produced in magnetron are injected into the accelerator tube or structure via a waveguide system. At the proper instant electrons produced by the electron gun are also pulse-injected into the accelerator structure. These electrons (of about 50 keV initial energy) interact with electromagnetic field of the microwaves. The electrons gain energy from the sinusoidal electric field by an acceleration process analogous to that of a surf rider.

13. Drawback of Magnetron The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube . This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used.