A magnetron is a device that converts high voltage DC electrical power into microwave power. The internal arrangement of a magnetron is shown below.
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.
Different anode designs are used in e2v magnetrons. Each type has its own particular advantages that make it suitable for use in specific applications. The following table shows general characteristics of these anode designs.
- The Strapped Vane Anode offers good efficiency, low starting jitter, excellent mode control and low cost
- The Long Anode gives the ultimate high power capability while retaining excellent frequency stability and mode control
- The Rising Sun Anode is particularly suited to millimetre-wave magnetrons and has superb short-pulse capability
The operational life of a well-designed and properly used magnetron is ultimately dependent upon the cathode. e2v uses a wide range of cathode technologies, in order to achieve optimum device performance and life. These include:
- Nickel mush, oxide cathodes, for high voltage and high power applications such as LINACs
- Patented ”2-part” barium aluminate cathodes and BA cathodes with metal overlays for demanding high current applications in millimetre magnetrons
- Bright emitter tungsten cathodes for continuous wave industrial heating magnetrons
- e2v’s patented long-life ”ridge” oxide cathode that has helped to more than treble the average life of its marine magnetrons
- Directly-heated cathodes that start with less than two seconds preheating and with the capability to operate at duty cycles of up to 25%.
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.
AlNiCo magnetic materials are used where accurate setting of operating voltage and good temperature stability of field is required.
e2v pioneered the use of samarium-cobalt magnets in magnetrons and has been using this material in production magnetrons for some years. The advantages of samarium-cobalt over AlNiCo include:
- Dramatic reduction in size and weight
- Virtual immunity to accidental demagnetisation due to magnets being brought too close to ferrous material
- Much higher magnetic field – this is particularly important for millimetre-wave magnetrons.
Frequency agility for ECCM and anti-glint has been a requirement of many radar designers. This has prompted us to develop several agile mechanisms to offer tuning rates above those that are possible with simple mechanical cam-actuated tuners. Examples of these include the multipactor-tuned magnetron, the tuning-fork tuned magnetron and the piezo-tuned magnetron. In each case, the mechanism has been proved by volume production experience.
Phase priming and injection locking
Magnetron oscillation builds up from random noise at a frequency determined by the anode. A low power signal injected into the magnetron before it begins to oscillate will control the phase, but not the frequency of oscillation. This is known as phase priming.
Higher levels of injected signal lock both the phase and oscillation frequency. In general, it is most convenient to inject the locking signal into the output of the magnetron, via a circulator. Magnetron ”chains” are used to give the system a higher overall gain.
One example of this type of system is the PLM5800 series of Ku-band, 2-magnetron amplifier chains incorporating circulators. These offer:
- Output power up to 400 W in Ku-band
- Duty cycle in excess of 10% possible
- Three-second starting
- Weight less than 1 kg
- Gain in excess of 20 dB (total system gain).