Editing Cavity magnetron (section)

==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.<ref>{{cite web|url=http://zipcon.net/~swhite/docs/physics/electronics/Valves.html|title=Electric Valves: Diodes, Triodes, and Transistors|first=Steve|last=White|website=zipcon.net|access-date=5 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170825070438/http://www.zipcon.net/~swhite/docs/physics/electronics/Valves.html|archive-date=25 August 2017}}</ref>

===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]]<ref name=mag>{{cite web |url=http://electriciantraining.tpub.com/14183/css/14183_103.htm |title=The Magnetron |website=electriciantraining.tpub.com |access-date=5 May 2018 |url-status=live |archive-url=https://web.archive.org/web/20160303222027/http://electriciantraining.tpub.com/14183/css/14183_103.htm |archive-date=3 March 2016}}</ref>{{Better source needed|date=October 2019}} 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===
[[File:Split-anode magnetron.jpg|thumb|upright=1.3|Split-anode magnetron (c. 1935). ''(left)'' The bare tube, about 11&nbsp;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 [[plate electrode|plates]], the electrons' trajectory could be modified so that they would naturally travel towards the lower voltage side. The plates were connected to an oscillator that reversed the relative voltage of the two plates at a given frequency.<ref name=mag/>

At any given instant, the electron will naturally be pushed towards the lower-voltage side of the tube. The electron will then oscillate back and forth as the voltage changes. At the same time, a strong magnetic field is applied, stronger than the critical value in the original design. This would normally cause the electron to circle back to the cathode, but due to the oscillating electrical field, the electron instead follows a looping path that continues toward the anodes.<ref name=mag/>

Since all of the electrons in the flow experienced this looping motion, the amount of RF energy being radiated was greatly improved. And as the motion occurred at any field level beyond the critical value, it was no longer necessary to carefully tune the fields and voltages, and the overall stability of the device was greatly improved. Unfortunately, the higher field also meant that electrons often circled back to the cathode, depositing their energy on it and causing it to heat up. As this normally causes more electrons to be released, it could sometimes lead to a runaway effect, damaging the device.<ref name=mag/>

===Cavity magnetron===
The great advance in magnetron design was the '''[[resonator|resonant cavity]] magnetron''' or '''electron-resonance magnetron''', which works on entirely different principles. In this design the oscillation is created by the physical shape of the anode, rather than external circuits or fields.

{{clear}}
[[Image:Resonant Cavity Magnetron Diagram.svg|thumb|right|upright=1.5|A cross-sectional diagram of a [[resonator|resonant cavity]] magnetron. Magnetic lines of force are parallel to the geometric axis of this structure.]]

Mechanically, the cavity magnetron consists of a large, solid cylinder of metal with a hole drilled through the centre of the circular face. A wire acting as the cathode is run down the center of this hole, and the metal block itself forms the anode. Around this hole, known as the "interaction space", are a number of similar holes ("resonators") drilled parallel to the interaction space, connected to the interaction space by a short channel. The resulting block looks something like the cylinder on a [[revolver]], with a somewhat larger central hole. Early models were cut using [[Colt's Manufacturing Company|Colt]] pistol jigs.<ref name=Brittain>{{cite journal
|title=The Magnetron and the Beginnings of the Microwave Age
|author1=J. Brittain
|journal=Physics Today
|volume=38
|issue=7
|pages=60–67
|year=1985
|doi=10.1063/1.880982
|bibcode=1985PhT....38g..60B
}}</ref> Remembering that in an AC circuit the electrons [[skin effect|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.<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/waves/magnetron.html|title=Magnetron Operation|website=hyperphysics.phy-astr.gsu.edu|access-date=5 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170911224636/http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/magnetron.html|archive-date=11 September 2017}}</ref>

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 radar]]s, nor for microwave ovens.