thumb|Magnetron with section removed to exhibit the cavities. The cathode in the center is not visible. The antenna emitting microwaves is at the left. The magnets producing a field parallel to the long axis of the device are not shown.
thumb|A similar magnetron with a different section removed. Central cathode is visible; antenna conducting microwaves at the top; magnets are not shown.
thumb|Obsolete 9 GHz magnetron tube and magnets from a Soviet aircraft radar. The tube is embraced between the poles of two horseshoe-shaped [[alnico magnets (top, bottom), which create a magnetic field along the axis of the tube. The microwaves are emitted from the waveguide aperture (top) which in use is attached to a waveguide conducting the microwaves to the radar antenna. Modern tubes use rare-earth magnets, electromagnets or ferrite magnets which are much less bulky. ]]
The cavity magnetron is a high-power vacuum tube used in early radar systems and subsequently in microwave ovens and in linear particle accelerators. A cavity magnetron generates microwaves using the interaction of a stream of electrons with a magnetic field, while moving past a series of cavity resonators, which are small, open cavities in a metal block. Electrons pass by the cavities and cause microwaves to oscillate within, similar to the functioning of a whistle producing a tone when excited by an air stream blown past its opening. The resonant frequency of the arrangement is determined by the cavities' physical dimensions. Unlike other vacuum tubes, such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier for increasing the intensity of an applied microwave signal; the magnetron serves solely as an electronic oscillator generating a microwave signal from direct-current electricity supplied to the vacuum tube.
The use of magnetic fields as a means to control the flow of an electric current was spurred by the invention of the Audion by Lee de Forest in 1906. Albert Hull of General Electric Research Laboratory, USA, began development of magnetrons to avoid de Forest's patents, but these were never completely successful. Other experimenters picked up on Hull's work and a key advance, the use of two cathodes, was introduced by Habann in Germany in 1924. Further research was limited until Okabe's 1929 Japanese paper noting the production of centimeter-wavelength signals, which led to worldwide interest. The development of magnetrons with multiple cathodes was proposed by A. L. Samuel of Bell Telephone Laboratories in 1934, leading to designs by Klaas Posthumus in 1934 and Hans Hollmann in 1935. Production was taken up by Philips, General Electric Company (GEC), Telefunken and others, limited to perhaps 10 W output. By this time the klystron was producing more power and the magnetron was not widely used, although a 300 W device was built by N.F. Alekseev and D.D. Malairov in the USSR in 1936 (published in 1940). Their first working example produced hundreds of watts at 10 cm wavelength, an unprecedented achievement. New radars appeared for night-fighters, anti-submarine aircraft and even the smallest escort ships, The magnetron remains in use in some radar systems, but has become much more common as a low-cost source for microwave ovens. In this form, over one billion magnetrons are in use.
Construction and operation
Conventional tube design
In a conventional electron tube (vacuum tube), electrons are emitted from a negatively charged, heated component called the cathode and are attracted to a positively charged component called the anode. The components are normally arranged concentrically, placed within a tubular-shaped container from which all air has been evacuated, so that the electrons can move freely (hence the name "vacuum" tubes, called "valves" in British English).
If a third electrode (called a control grid) is inserted between the cathode and the anode, the flow of electrons between the cathode and anode can be regulated by varying the voltage on this third electrode. This allows the resulting electron tube (called a "triode" because it now has three electrodes) to function as an amplifier because small variations in the electric charge applied to the control grid will result in identical variations in the much larger current of electrons flowing between the cathode and anode.
Hull or single-anode magnetron
The idea of using a grid for control was invented by Philipp Lenard, who received the Nobel Prize for Physics in 1905. In the USA it was later patented by Lee de Forest, resulting in considerable research into alternate tube designs that would avoid his patents. One concept used a magnetic field instead of an electrical charge to control current flow, leading to the development of the magnetron tube. In this design, the tube was made with two electrodes, typically with the cathode in the form of a metal rod in the center, and the anode as a cylinder around it. The tube was placed between the poles of a horseshoe magnet arranged such that the magnetic field was aligned parallel to the axis of the electrodes.
With no magnetic field present, the tube operates as a diode, with electrons flowing directly from the cathode to the anode. In the presence of the magnetic field, the electrons will experience a force at right angles to their direction of motion (the Lorentz force). In this case, the electrons follow a curved path between the cathode and anode. The curvature of the path can be controlled by varying either the magnetic field using an electromagnet, or by changing the electrical potential between the electrodes.
At very high magnetic field settings the electrons are forced back onto the cathode, preventing current flow. At the opposite extreme, with no field, the electrons are free to flow straight from the cathode to the anode. There is a point between the two extremes, the critical value or Hull cut-off magnetic field (and cut-off voltage), where the electrons just reach the anode. At fields around this point, the device operates similar to a triode. However, magnetic control, due to hysteresis and other effects, results in a slower and less faithful response to control current than electrostatic control using a control grid in a conventional triode (not to mention greater weight and complexity), so magnetrons saw limited use in conventional electronic designs.
It was noticed that when the magnetron was operating at the critical value, it would emit energy in the radio frequency spectrum. This occurs because a few of the electrons, instead of reaching the anode, continue to circle in the space between the cathode and the anode. Due to an effect now known as cyclotron radiation, these electrons radiate radio frequency energy. The effect is not very efficient. Eventually the electrons hit one of the electrodes, so the number in the circulating state at any given time is a small percentage of the overall current. It was also noticed that the frequency of the radiation depends on the size of the tube, and even early examples were built that produced signals in the microwave regime.
Early conventional tube systems were limited to the high frequency bands, and although very high frequency systems became widely available in the late 1930s, the ultra high frequency and microwave bands were well beyond the ability of conventional circuits. The magnetron was one of the few devices able to generate signals in the microwave band and it was the only one that was able to produce high power at centimeter wavelengths.
Split-anode magnetron
thumb|upright=1.3|Split-anode magnetron (c. 1935). (left) The bare tube, about 11 cm high. (right) Installed for use between the poles of a strong [[permanent magnet ]]
The original magnetron was very difficult to keep operating at the critical value, and even then the number of electrons in the circling state at any time was fairly low. This meant that it produced very low-power signals. Nevertheless, as one of the few devices known to create microwaves, interest in the device and potential improvements was widespread.
The first major improvement was the split-anode magnetron, also known as a negative-resistance magnetron. As the name implies, this design used an anode that was split in two—one at each end of the tube—creating two half-cylinders. When both were charged to the same voltage the system worked like the original model. But by slightly altering the voltage of the two plates, the electrons' trajectory could be modified so that they would naturally travel towards the higher voltage side. The plates were connected to an oscillator that reversed the relative voltage of the two plates at a given frequency. Remembering that in an AC circuit the electrons travel along the surface, not the core, of the conductor, the parallel sides of the slot act as a capacitor while the round holes form an inductor: an LC circuit made of solid copper, with the resonant frequency defined entirely by its dimensions.
The magnetic field is set to a value well below the critical, so the electrons follow curved paths towards the anode. When they strike the anode, they cause it to become negatively charged in that region. As this process is random, some areas will become more or less charged than the areas around them. The anode is constructed of a highly conductive material, almost always copper, so these differences in voltage cause currents to appear to even them out. Since the current has to flow around the outside of the cavity, this process takes time. During that time additional electrons will avoid the hot spots and be deposited further along the anode, as the additional current flowing around it arrives too. This causes an oscillating current to form as the current tries to equalize one spot, then another.
The oscillating currents flowing around the cavities, and their effect on the electron flow within the tube, cause large amounts of microwave radiofrequency energy to be generated in the cavities. The cavities are open on one end, so the entire mechanism forms a single, larger, microwave oscillator. A "tap", normally a wire formed into a loop, extracts microwave energy from one of the cavities. In some systems the tap wire is replaced by an open hole, which allows the microwaves to flow into a waveguide.
As the oscillation takes some time to set up, and is inherently random at the start, subsequent startups will have different output parameters. Phase is almost never preserved, which makes the magnetron difficult to use in phased array systems. Frequency also drifts from pulse to pulse, a more difficult problem for a wider array of radar systems. Neither of these present a problem for continuous-wave radars, nor for microwave ovens.
Common features
thumb|upright=1.6|Cutaway drawing of a cavity magnetron of 1984. Part of the righthand magnet and copper anode block is cut away to show the cathode and cavities. This older magnetron uses two horseshoe shaped [[alnico magnets, modern tubes use rare-earth magnets. ]]
All cavity magnetrons consist of a heated cylindrical cathode at a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is placed in the center of an evacuated, lobed, circular metal chamber. The walls of the chamber are the anode of the tube. A magnetic field parallel to the axis of the cavity is imposed by a permanent magnet. The electrons initially move radially outward from the cathode attracted by the electric field of the anode walls. The magnetic field causes the electrons to spiral outward in a circular path, a consequence of the Lorentz force. Spaced around the rim of the chamber are cylindrical cavities. Slots are cut along the length of the cavities that open into the central, common cavity space. As electrons sweep past these slots, they induce a high-frequency radio field in each resonant cavity, which in turn causes the electrons to bunch into groups. A portion of the radio frequency energy is extracted by a short coupling loop that is connected to a waveguide (a metal tube, usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar.
The size of the cavities determine the resonant frequency, and thereby the frequency of the 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.
Where there are an even number of cavities, two concentric rings can connect alternate cavity walls to prevent inefficient modes of oscillation. This is called pi-strapping because the two straps lock the phase difference between adjacent cavities at π radians (180°).
The modern magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1-kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Large S band magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW. In 1912, Swiss physicist Heinrich Greinacher was looking for new ways to calculate the electron mass. He settled on a system consisting of a diode with a cylindrical anode surrounding a rod-shaped cathode, placed in the middle of a magnet. The attempt to measure the electron mass failed because he was unable to achieve a good vacuum in the tube. However, as part of this work, Greinacher developed mathematical models of the motion of the electrons in the crossed magnetic and electric fields.
In the US, Albert Hull put this work to use in an attempt to bypass Western Electric's patents on the triode. Western Electric had gained control of this design by buying Lee De Forest's patents on the control of current flow using electric fields via the "grid". Hull intended to use a variable magnetic field, instead of an electrostatic one, to control the flow of the electrons from the cathode to the anode. Working at General Electric's Research Laboratories in Schenectady, New York, Hull built tubes that provided switching through the control of the ratio of the magnetic and electric field strengths. He released several papers and patents on the concept in 1921.
Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves. However, in 1924, Czech physicist August Žáček (1886–1961) and German physicist Erich Habann (1892–1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz. Žáček, a professor at Prague's Charles University, published first; however, he published in a journal with a small circulation and thus attracted little attention. Habann, a student at the University of Jena, investigated the magnetron for his doctoral dissertation of 1924. Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron. Most of these early magnetrons were glass vacuum tubes with multiple anodes. However, the two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency.
While radar was being developed during World War II, there arose an urgent need for a high-power microwave generator that worked at shorter wavelengths, around 10 cm (3 GHz), rather than the 50 to 150 cm (200 MHz) that was available from tube-based generators of the time. It was known that a multi-cavity resonant magnetron had been developed and patented in 1935 by Hans Hollmann in Berlin. However, the German military considered the frequency drift of Hollman's device to be undesirable, and based their radar systems on the klystron instead. But klystrons could not at that time achieve the high power output that magnetrons eventually reached. This was one reason that German night fighter radars, which never strayed beyond the low-UHF band to start with for front-line aircraft, were not a match for their British counterparts.
thumb|upright=1.22|[[John Randall (physicist)|Sir John Randall and Harry Boot's original cavity magnetron developed in 1940 at the University of Birmingham, England, now in the Science Museum, London.]]
thumb|upright=1.22|The electromagnet used in conjunction with Randall and Boot's original magnetron, in the Science Museum, London.
thumb|upright=1.22|right|The anode block which is part of the cavity magnetron developed by Randall and Boot
In 1940, at the University of Birmingham in the UK, John Randall and Harry Boot produced a working prototype of a cavity magnetron that produced about 400 W. (For an overview of early magnetron designs, including that of Boot and Randall, see.)
GEC at Wembley made 12 prototype cavity magnetrons in August 1940, and No 12 was sent to America with Bowen on the Tizard Mission, where it was shown on 19 September 1940 in Alfred Loomis’ apartment. The American NDRC Microwave Committee was stunned at the power level produced. However Bell Labs' director was upset when it was X-rayed and had eight holes rather than the six holes shown on the GEC plans. After contacting (via the transatlantic cable) Dr Eric Megaw, GEC’s vacuum tube expert Megaw recalled that when he had asked for 12 prototypes he said make 10 with 6 holes, one with 7 and one with 8; there was no time to amend the drawings. And No 12 with 8 holes was chosen for the Tizard Mission. So Bell Labs chose to copy the sample; and while early British magnetrons had six cavities the American ones had eight cavities.
According to Andy Manning from the RAF Air Defence Radar Museum, Randall and Boot's discovery was "a massive, massive breakthrough" and "deemed by many, even now [2007], to be the most important invention that came out of the Second World War", while professor of military history at the University of Victoria in British Columbia, David Zimmerman, states: