Technology
Magnetron principles
The magnetron is an oscillator which converts DC pulsed power into microwave power by means of a standing wave structure. Pulsed operation enables the very high peak power required by the accelerator guide to be generated while limiting the average power demanded.
Electrons from a heated cylindrical cathode are accelerated radially by a high voltage pulse applied between the cathode and anode. A magnetic field parallel to the axis of the cathode causes electrons to follow a curved path. With the correct magnetic field, the majority of electrons return to the cathode where they release secondary electrons which repeat the process.
Anode structure
The anode resonators terminate in slotted segments or vane tips which form the inside diameter of the anode. As the electrons travel close to the anode, they induce electrical charges in the vane tips. When these transient charges resonate at the natural frequency of the anode, there is a build-up of energy stored in the anode. The magnetron is designed to operate in the “π mode” in which alternate tips carry transient charges of the same polarity.
Temperature changes cause the anode resonator sizes to alter which changes the frequency of oscillation. Frequency changes due to variations in output power are minimised by control of the temperature of the coolant water. The magnetron frequency tuner allows for correction of thermal drift by an automatic frequency control (AFC) circuit.
Cathode and heater
Electrons circulating between cathode and anode finally fall back to the cathode with an energy proportional to the output power of the magnetron.
To avoid overheating the cathode, filament heater power should be reduced at higher power levels. Fluctuating heater current may cause unwanted vibration of the cathode or fluctuation of the magnetron frequency. Therefore a DC heater supply is recommended to avoid this problem.
