ADDITIONAL EQUIPMENT FOR KLYSTRON AMPLIFIERS

To overcome beam spreading, an axial magnetic field is used. The action of the magnetic field is to exert a force on the electrons that keeps them focused into a narrow beam. The magnetic field may be de- veloped by a permanent magnet or by one or more electromagnets. A permanent magnet is used in tubes that are physically small or of medium power rating. Unfortunately, the size and the weight of a permanent magnet are excessive for long or high-power tubes, making it necessary to use electromagnets. In some large tubes, several separate electromagnets are used. The current in each coil is individually adjustable to optimize the magnetic field shape. The magnetic field is normally terminated a short distance beyond the output cavity so that the beam can spread before it hits the collector. This tends to spread the electron beam interception over a large surface on the collector, minimizing collector cooling problems that otherwise would result from the beam remaining concentrated at the time of interception. Even with an axial magnetic field, some electrons stray from the main electron beam. These electrons are intercepted by the anode or the klystron drift tubes. In high-power tubes, it is particularly important to minimize the number of stray electrons because of the heat generated when they strike the drift tubes. And in a high-power klystron, this heating maybe a very severe problem because drift tubes are quite dif- ficult to cool. Temperatures may become high enough to melt the drift tubes, thus destroying them. Klystron amplifiers normally have actual metal grid structures across the gaps in the resonant cavities. Many low-power klystrons have wire mesh grids. However, most high-power klystrons do not have actual grids across the gaps. Such grids would inter- cept sizable quantities of electrons. It is very difficult to cool grid structures, and a large amount of beam interception would melt the grids, thus destroying the tube. Fortunately, by proper design, the klystron may be made to operate effi- ciently without actual grid wires across the cavity gaps. The absence of these grids does not change the operating principles, but it does have a secondary effect on klystron performance. If the electron beam has a small diameter compared to the diameter of the drift tube, the beam does not couple energy to the cavities very well. Therefore, the performance of a klystron amplifier, which does not have gridded gaps, may sometimes be improved by permitting the elec- tron beam to be as wide as possible, while keeping the body current down to the maximum specified for the tube. The width of the beam may be somewhat con- trolled by the magnetic field strength. Body current usually increases with RF input level, because it is the RF input that causes the bunches to form. The dense electron concentration in the bunch causes mutual repulsion of electrons, and the diameter of the bunch may become larger than the diameter of the beam with no bunches present. Consequently, some of the electrons in the bunch may be lost to the drift tubes, and the body current may increase. ADDITIONAL EQUIPMENT FOR KLY- STRON AMPLIFIERS.— Additional equipment is required for a complete amplifier system. Various power supplies are necessary to deliver required volt- ages and current. In high-power systems, various RF circuit components are required to control and meas- ure the RF input to the klystron tube and to measure the RF output from the tube. A large collection of meters and protective devices is needed to monitor performance and protect operating personnel and equipment in the event of a malfunction or operator error. In most klystron tubes, the anode and the RF section are connected inside the vacuum envelope. These connected parts are called the tube body and are generally operated at ground potential. It is conven- ient to operate the tube body at ground potential because the input and output connections (either waveguide or coaxial) are then also at ground poten- tial. This makes it easier to connect the klystron into the rest of the system. In addition, the cavity tuners are at ground potential, eliminating any danger to per- sonnel tuning the tube. The beam supply provides the voltage required to accelerate the electrons and form the beam. It must also deliver the required beam current. The crowbar 2-15