What Are Magnetrons?

Magnetrons are crossed-field vacuum electron devices that generate microwave radiation by exploiting the interaction between a radial electric field and a coaxial magnetic field to cause electrons to spiral in a circular path between a central cathode and a surrounding anode. The anode is machined from solid copper into a cylindrical block containing an even number of resonant cavities arranged symmetrically around the cathode gap; as the spiraling electron cloud passes the cavity openings, it excites resonant oscillations that radiate coherent microwave power through a coupling port. The cavity magnetron was developed at the University of Birmingham in 1940 by John Randall and Henry Boot, who demonstrated 400 watts of continuous power at a 10-centimeter wavelength, a performance far beyond earlier magnetron designs. Magnetrons deliver high peak and average power in compact packages and remain the dominant high-power microwave source at frequencies from roughly 1 to 10 GHz.

The device draws on electron physics, electromagnetic resonator theory, and precision mechanical fabrication. At very high applied voltages, the electron trajectories are modified by relativistic mass increase, an effect that must be accounted for in the design of high-power pulsed magnetrons used in radar transmitters.

Operating Principle and Structure

The central cathode emits electrons thermionically and is held at a large negative potential relative to the grounded anode. The permanent magnet surrounding the tube applies an axial magnetic field parallel to the cathode axis. Electrons emitted from the cathode experience both the radial electric field (which accelerates them outward) and the axial magnetic field (which deflects them circumferentially via the Lorentz force), causing them to follow cycloidal trajectories rather than radial paths. When the rotating electron space charge synchronizes with the electromagnetic mode of the resonant cavities, energy transfers efficiently from the electrons to the microwave field, a process called phase focusing. The output frequency is determined primarily by the cavity geometry and the applied magnetic field, and can be tuned over a narrow range by mechanical or magnetic adjustment. As described in IEEE Spectrum's account of the cavity magnetron's wartime and peacetime impact, roughly 250,000 cavity magnetrons were manufactured for Allied radar systems during World War II, and the same basic design later became the power source in domestic microwave ovens operating at 2.45 GHz.

Radar and High-Power Applications

Magnetrons are used as transmitter tubes in pulse radar systems for air traffic control, weather surveillance, shipboard navigation, and military targeting. In a pulsed radar magnetron, a high-voltage modulator delivers short bursts of energy to the cathode, producing peak power levels from tens of kilowatts to several megawatts with pulse widths of microseconds. The high peak power is essential for detecting small targets at long range. Medical linear accelerators also use magnetrons as the microwave source to accelerate electrons for radiation therapy, where the magnetron drives a waveguide accelerating structure at frequencies around 3 GHz.

Magnetron Sputtering

The term "magnetron" also applies to a class of physical vapor deposition (PVD) devices that use the same principle of magnetically confined electron orbits to sustain a high-density plasma near a solid target. In magnetron sputtering, a planar or cylindrical cathode target is placed in a vacuum chamber with a permanent magnet array behind it; the magnetic field traps secondary electrons near the target surface, increasing the ionization efficiency of the working gas at pressures below one millitorr. As reviewed in the physics and technology of magnetron sputtering discharges, operating modes include direct-current (dc) magnetron sputtering, radio-frequency (rf) sputtering for insulating targets, and high-power impulse magnetron sputtering (HiPIMS), which produces dense metal ion fluxes useful for hard coating deposition.

Applications

Magnetrons have applications in a wide range of fields, including:

• Pulse radar systems for air traffic control, weather sensing, and maritime navigation
• Domestic and industrial microwave ovens, where the magnetron generates the 2.45 GHz heating field
• Medical radiation therapy, as the microwave source in linear electron accelerators
• Thin-film deposition in semiconductor fabrication, optical coating, and hard surface treatment via magnetron sputtering PVD systems
• Industrial drying and dielectric heating processes requiring high continuous microwave power