Arcing in Magnetrons

During initial operation high-powered arcing in magnetrons are from cathode to plate and must be properly BROKEN IN or BAKED IN. Actually, arcing in magnetrons is very common. It occurs with a new tube or following long periods of idleness.

One of the prime causes of arcing is the release of gas from tube elements during idle periods. This arcing may also be caused by the presence of sharp surfaces within the tube, mode shifting, and by drawing excessive current. While the cathode can withstand considerable arcing for short periods of time, continued arcing will shorten the life of the magnetron and may destroy it entirely. Therefore, each time excessive arcing occurs, the tube must be baked in again until the arcing ceases and the tube is stabilized.

The baking-in procedure is relatively simple. Magnetron voltage is raised from a low value until arcing occurs several times a second. The voltage is left at that value until the arcing in magnetrons dies out. Then the voltage is raised further until arcing again occurs and is left at that value until the arcing again ceases. Whenever the arcing in magnetrons becomes very violent and resembles a continuous arc, the applied voltage is excessive and should be reduced to permit the magnetron to recover. When normal rated voltage is reached and the magnetron remains stable at the rated current, the baking-in is complete. A good maintenance practice is to bake-in magnetrons left idle in the equipment or those used as spares when long periods of nonoperating time have accumulated.

The preceding information is general in nature. The recommended times and procedures in the technical manuals for the equipment should be followed when baking-in a specific type magnetron in order to eliminate or reduce the arcing in magnetrons.

The Crossed-Field Amplifier (Amplitron)

The CROSSED-FIELD AMPLIFIER (cfa), commonly known as an AMPLITRON and sometimes referred to as a PLATINOTRON, is a broadband microwave amplifier that can also be used as an oscillator. The cfa is similar in operation to the magnetron and is capable of providing relatively large amounts of power with high efficiency. The bandwidth of the cfa, at any given instant, is approximately plus or minus 5 percent of the rated center frequency. Any incoming signals within this bandwidth are amplified. Peak power levels of many megawatts and average power levels of tens of kilowatts average are, with efficiency ratings in excess of 70 percent, possible with crossed-field amplifiers.

Because of the desirable characteristics of wide bandwidth, high efficiency, and the ability to handle large amounts of power, the cfa is used in many applications in microwave electronic systems. This high efficiency has made the cfa useful for space-telemetry applications, and the high power and stability have made it useful in high-energy, linear atomic accelerators. When used as the intermediate or final stage in high-power radar systems, all of the advantages of the cfa are used.

Since the cfa operates in a manner so similar to the magnetron, the detailed theory is not presented in this tutorial. Detailed information of cfa operation is available in the Navy's manual "NAVSHIPS 0967-443-2230, Handling, Installation and Operation of Crossed-Field Amplifiers".

As mentioned earlier, crossed-field amplifiers are commonly called Amplitrons. You should note, however, that Amplitron is a trademark of the Raytheon Manufacturing Company for the Raytheon line of crossed-field amplifiers. An illustration of a crossed- field amplifier is shown in the illustration below.

Crossed field amplifier Amplitron.

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SOLID-STATE MICROWAVE DEVICES

As with vacuum tubes, the special electronics effects encountered at microwave frequencies severely limit the usefulness of transistors in most circuit applications. The need for small-sized microwave devices has caused extensive research in this area. This research has produced solid-state devices with higher and higher frequency ranges.

The new solid-state microwave devices are predominantly active, two-terminal diodes, such as tunnel diodes, varactors, transferred-electron devices, and avalanche transit- time diodes. This section will describe the basic theory of operation and some of the applications of these relatively new solid-state devices.

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