The Beam and the Drift TubeNow that we have a basic understanding of how a klystron tube operates, we can see that it's whole purpose in life, is to shoot a narrow beam of electrons up the middle of the tube, and modulate it with a signal, so as to amplify the signal. In short, it's a big hairy amplifier. Like other transistor and tube amplifiers, it can be operated in Class A, B, AB, or C. It can be used as a basic amplifier, oscillator, or mixer. 
BUT - because of its ability to operate with high frequencies at high power levels, it is almost always used as a Class C UHF amplifier with an antenna as a load. It is not necessary, with an antenna as a load, to operate class A in order to produce a linear output signal. Furthermore, Class C is the most efficient (energy saving) mode of operation, and at these power levels.... ya wanna save all the money ya can! Of course, Class C means that it is operating in Saturation for at least 50% of it's duty cycle. 
Now because it's whole purpose in life, is to shoot a narrow beam of electrons up the middle of the tube, and modulate it with a signal, so as to amplify the signal, we should know a little bit about this beam. The beam commonly operates at extremely high voltage and current levels. It is not uncommon to see a klystron with a beam current of 25 THOUSAND VOLTS (that�s 25KV) at 5 Amps. Now if'n I done my math correctly, P=IE, so Power Out = 25,000 multiplied by 5. This tube would have a beam power of 125,000 Watts. 
 What happens when something with that much power does happen to arc? Imagine an arc welder. It operates at 240 Volts, and generates enough heat to melt steel. Quite a concept, eh? Now imagine a big hairy arc welder that operates at 25,000 Volts. Get the picture? The tube can literally destroy itself by arcing over inside. So the beam must be carefully guided up through the drift tube until it reaches it's final resting place. This is usually done with electromagnetic coils. Magnet supply voltages are commonly in the 200 Volt range. Although I have heard of a new and recent development of a special type of klystron using fixed permanent magnets, called a PPM Focused Klystron which was able to obtain power levels on the order of 50 Megawatts. 
The basic principle is that as the beam is being drawn up toward the
collector, the magnets push at the electron beam, deflecting it more
into the center of the tube. Should one of these magnets fail, it could
be disastrous. For this cause, many safety precautions have been
taken.To begin with, arc detectors are often within the tube. These are simple devices which detect the presence of light. Since the tube is a sealed metal container, it is very black inside. Whenever the beam happens to arc over, it creates light, which trips the sensor, which in turn controls a circuit which turns off the beam current. Another good safety measure is to keep close tabs on the body current of the tube. Because the beam carries itself all the way to the collector, it never contacts the body of the tube. Even though a high potential exists between the two, there is no current (or relatively no current) flowing between them. Should the beam come close enough to the body, a current will be developed within the body. If you see the body current of a klystron rising, it's a bad sign, and you need to pay attention to what is causing it. Typically, it is a voltage problem in either the beam or magnet power supplies. 
A special form of klystron, known as the Multiple Stage Depressed Cavity, was developed for the military in the 1970's. It has since found use in commercial and broadcast applications.
The object of this advanced klystron tube, was to recover more energy from the alreadly used electron beam. The collector was constructed in sections, stacked on atop another, each being at a lower (less positive) potential than the one preceding it. The electrons are sorted out according the their perspective impact velocity. The electrons with less velocity strike the closest (and most positive) collector stage first. Only the electrons with the highest velocity strike into the farthest (and least positive) collector. This had an added benifit of better dissipating heat, allowing for higher power levels to be used. By spreading the landing zone of the electrons evenly over a larger surface area, the amount of energy applied can be greater, as the tube is capable of dissipating more heat over the larger surface area. Or to quote an automobile commercial, "Wider is better". Of course in both types, heat radiating fins on the outside metal surface will exchange more heat, and with liquid passing across the fins, even more heat can be dissipated. |
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