7. 3. 4. 2 THE KLYSTRON A diagrammatic representation of the klystron amplifier is shown in Figure 7-45(a). An electron beam is emitted and formed by a heated cathode and electron gun. The electron beam is then accelerated by a positive potential on the RF cavities. The beam is also velocity modulated by the RF input. The resultant action between the velocity modulation and the tube geometry causes the electrons to ''bunch" or become density modulated. Power is then extracted from the beam by conventional means such as employing a loop or iris coupling. The electrons, after power extraction, are collected by the collector anode. The primary advantage of the klystron over the amplitron or other tubes in producing high power lies in its geometry. The beam formation, RF interaction, and beam collection are separate and independent regions; thus, each region may be designed to perform its own function independent of the others. Since the collector electrode is the primary heat dissipator, its shape and size can be designed for the power under consideration without regard to the other functional regions of the tube. A 50 kW tube has been proposed as an RF generator for the SPS. The gain of the klystron is dependent on the number of cavities and, therefore, its length. Gains in excess of 80 dB are possible. 7. 3. 4. 3 THE AMPLITRON The amplitron is a crossed-field device in which the electron beam is perpendicular to the magnetic and electric fields. The basic structure of the amplitron is shown in Figure 7-45 (b). The electrons are emitted by a cold cathode upon RF signal injection. Amplification is similar to that of the traveling wave tube and is a result of the interaction between a traveling electromagnetic wave and a rotating space-charge wave. The space-charge waves are formed by the interaction between the electron beam and the crossed electric and magnetic fields. The amplitron is a relatively low gain device with typical gains of 7 to 10 dB. The low gain is not a handicap, since the RF input power is not lost but appears as a part of the output. Also, the amplitron acts as a low insertion transmission line when not energized. These characteristics are employed by cascading the amplitrons in the transmitting array. Should a tube fail, the RF power will feed through the nonoperating tube without a serious effect on the SPS. 7. 3. 4. 4 AMPLITRON/KLYSTRON COMPARISON The amplitron was selected for study purposes primarily because of its greater efficiency and potentially greater lifetime. Since both tubes will be operating as dissipation-limited devices, the useful output power is proportional
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