A traveling-wave tube is a form of electron-beam tube that is useful at ultra-high frequencies (UHF) and microwave frequencies. There are several variations on this theme; the two most common are the magnetron and the klystron.

The magnetron
Most magnetrons contain a cathode at the center, and a surrounding plate. The plate is divided into sections, or cavities, by radial barriers. The output is taken from an opening in the plate and passes into a hollow waveguide that serves as a transmission line for the UHF or microwave energy.

The cathode is connected to the negative terminal of a high-voltage source, and the anode is connected to the positive terminal. Therefore, electrons flow radially outward. A magnetic field is applied lengthwise through the cavities. As a result, the electrons move outward in spirals from the cathode to the anode, rather than in straight lines.

The electric field produced by the high voltage, interacting with the longitudinal magnetic field and the effects of the cavities, causes the electrons to bunch up into clouds. The swirling movement of the electron clouds causes a fluctuating current in the anode.

This is the UHF or microwave signal. The frequency depends mainly on the shape and size of the cavities within the magnetron. Small cavities result in the highest oscillation frequencies; larger cavities produce oscillation at relatively lower frequencies.

A magnetron can generate more than 1 kW of RF power at a frequency of 1 GHz. As the frequency increases, the realizable power output decreases. At 10 GHz, a typical magnetron generates about 20 W of RF power output. Magnetrons can produce microwaves for use in cooking. The energy at these frequencies excites the molecules in organic substances like meat, vegetables, and grains. Your microwave oven uses a magnetron rated at about 500 W to 1 kW continuous output power.

The klystron
A klystron is a linear-beam electron tube. It has an electron gun, one or more cavities, and a device that modulates the electron beam. There are several different types of klystron tube. The most common are the multicavity and reflex klystrons.

A reflex klystron has only one cavity. A retarding field causes the electron beam to periodically reverse direction. This produces a phase reversal that allows large amounts of energy to be drawn from the electrons. The reflex klystron produces low-power UHF and microwave signals, on the order of a few watts.

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