Editing Cavity magnetron (section)

===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.

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[[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.