COOLING KLYSTRON AMPLIFIERS

Figure 2-9.—Associated klystron amplifier equipment. COOLING KLYSTRON AMPLIFIERS.— Most low-power klystron amplifiers are air-cooled, whereas all high-power klystrons are liquid-cooled. It should be stressed that the technician responsible for main- taining the klystron transmitter must be familiar with the cooling system of the equipment. It should also be stressed that an expensive klystron amplifier system may be destroyed in a matter of seconds if the cooling system fails. A well-designed system uses many pro- tective devices to prevent this from happening. The main source of power and heat in a klystron amplifier package is the beam power supply. Since the power generated by the beam supply must go some- where, part of it is converted to RF power, while the remainder eventually shows up as heat somewhere in the klystron. Klystron cooling is required to be able to dissipate the entire beam power. This is necessary, because if no RF output is being generated (either due to low RF input power or detuning of the klystron tube), all the beam power will be dissipated as heat somewhere within the tube. When most of the elec- trons in the beam eventually strike the collector, their energy is dissipated as heat. Also, the small fraction of the beam lost to the drift tube generates heat. Klystron amplifiers are normally from 30 to 50 percent efficient. Therefore, a tube rated at 10-kilo- watt output must be designed to dissipate between 10 and 23 kilowatts, depending on its efficiency. A tube rated at 100 kilowatts must be capable of dissipating between 100 and 230 kilowatts, depending on eff- iciency. Very advanced cooling techniques are neces- sary. The power levels involved may melt a hole in the drift tube or the collector in a small fraction of a second if the cooling system fails and adequate pro- tective devices are not provided. There are other, smaller sources of heat in a klystron amplifier system. The heat produced by the heater is conducted and radiated to the exterior sur- faces of the electron gun assembly and must be dis- sipated. Large tubes require a blower on the electron gun assembly to dissipate this heat. The power gen- erated by the focus coil power supply is all dissipated in the electromagnets. Large electromagnets are usually liquid cooled. If the electromagnet cooling fails for any reason, the focus coil power supply must be shut off very quickly, or the magnet will burn out. The beam voltage must also be removed (preferably before turning off the focus coil supply) to protect the tube from excessive body current. During operation, the walls of the resonant cavi- ties have oscillating currents flowing in them. Al- though these cavities are made of very high con- ductivity metal, they still present a finite resistance to these oscillating currents. Therefore, heat is generated in the cavity walls. The amount of heat generated may be quite sizable in high-power, high-frequency tubes. 2-17