Producing microwaves in a controlled fashion was quite difficult until Albert Wallace Hull's invention of the magnetron in 1921. The magnetron uses crossed electric and magnetic fields in a device that resembles a hollow cylinder with a rod down its center and small cavities spaced around its outside. A magnetron can quite efficiently convert electrical energy into microwaves; however, is has certain limitations. To overcome some of these problems, Pyotr Leonidovich Kapitsa invented a related device that was able to produce microwaves at even higher power and precision. Kapitsa described the device as being much like a magnetron that had been cut open and spread out in a plane. Due to the planar nature of the device, Kapitsa called it a planotron.

The planotron consists of a metal plate kept at a voltage above that of the rest of the apparatus. This is the planotron's cathode. At a fixed distance from the cathode are a series of identical and uniformly spaced anodes (conducting pieces kept at a lower voltage than the cathode). The space between the cathode and the anodes is the working space for the planotron, in which the microwaves are produced. The entire planotron must be operated in a vacuum. When the anodes are heated, electrons will try to flow across the space between the anodes to the cathodes. However, in operation, a magnetic field is established perpendicular to the electric field (hence, parallel to the cathode). Electrons moving in a magnetic field move in a circular path. However, electrons moving in the presence of both electric and magnetic fields move in a trochoidal path (the path drawn by a point at the edge of a rolling circle). An alternating electric field is then established between the anodes, with adjacent anodes being of opposite phase. This causes the electrons to move in a path that resembles an epitrochoid.

As the electrons move in their cyclic motion, they gradually lose energy into electromagnetic radiation. The frequency of radiation is determined by the spacing of the anodes and by the alternating field between them. Typical spacings yield frequencies that are those of microwaves. Like the magnetron, the planotron is extremely efficient at converting electrical energy into microwaves, but it is able to operate at far higher power.

Not only can a planotron efficiently convert electrical energy into microwaves, it can also work in reverse, converting microwaves into electrical energy with similar efficiency. Kapitsa wrote about the possibility of using planotrons for wireless transmission of electrical energy. For terrestrial applications, though, this concept proved not to be commercially practical. However, since Kapitsa's death, there have been proposals to perhaps place giant solar panels into Earth orbit to produce electrical energy. This energy could be beamed to Earth in the form of microwaves, with devices similar to planotrons at each end of the microwave link.

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