ADDITIONAL EQUIPMENT FOR KLYSTRON AMPLIFIERS

system quickly discharges the beam supply in the event of an internal klystron arc or other high-voltage fault condition. For high-power systems, it is normal to have some value of series resistance between the beam supply and the klystron cathode. This limits tube current to a finite value if the tube should arc from cathode to ground. Some klystrons have a grid or a modulating anode to control the number of electrons in the electron beam. Such grids are often used in pulse systems to turn the tube full-on or full-off. A few systems use grid modulation to transmit intelligence. In most gridded klystron tubes, the grid is never allowed to go positive with respect to the cathode, as it might cause undue grid interception of the beam and result in the burnout of the grid element. A grid power supply is required in those tubes that have grids. These power supplies and pulsers may take many forms, depending on the system application, and, therefore, are not discussed in this chapter. Some klystrons are made with the electromagnets physically part of the tube itself. However, in most systems, the electromagnets are separate from the tube, and the klystron is inserted into the electro- magnet structure. Many modern klystrons have only one electro- magnet, and, therefore, require only one power sup- ply. Others may have as many as six separate coils, requiring one power supply for each coil. Voltage and current metering is usually supplied for each of the electromagnet power supplies. If an electromagnet power supply should fail, the electron beam would almost certainly spread, and most of the beam current would be intercepted on a small section of the drift tube. In most cases, this would cause the drift tube to melt and permanently destroy the tube. Therefore, klystron amplifier equip- ment normally has undercurrent protection in each of the electromagnet circuits. When the electromagnet current falls below a pre- determined level, the beam supply is automatically turned off to prevent damage to the klystron. Redun- dant protection is provided by the body-current over- load circuits, which also turn off the beam supply in the event of magnet current failure or misadjustment. Figure 2-9 (on page 17) is a simplified diagram showing some of the power supplies, monitoring de- vices, and protective devices used in a typical power klystron amplifier. It also shows three electromagnets wrapped around the body of the klystron. In addition, figure 2-9 shows a method used to monitor body cur- rent, collector current, and beam current separately. (In many systems, separate monitoring of collector current is not done, since the collector current and the total beam current are almost equal.) It is quite unusual in a relatively high-power kly- stron amplifier system to allow the body current to exceed 10 percent of the beam current. High body current usually means low efficiency, and it increases the danger of burning out drift tubes. In very-high- power klystrons, the body current is often limited from 1 to 2 percent of the total beam current. If a klystron arcs internally, the arc will always occur between the cathode and the anode. When this occurs, the body current immediately becomes exces- sive, tripping out the body-current overload relay. An arc also causes beam current to be much higher than normal, and the beam-current overload will also trip out. In fact, almost any high-voltage system fault (such as an insulation breakdown) will cause exce- ssive current through the body-current meter and the overload relay. Because of the possibility of extremely high cur- rents flowing under fault conditions, the protection of body-current and beam-current meters presents a somewhat difficult problem. This problem is usually solved by using very-high-current, solid-state recti- fiers, back to back, across the meters. In some cases, it is necessary to add a small resistance or inductance in series with the meter. Surge capacitors are normally placed across the combination. It is necessary to connect the rectifiers back to back because fault conditions often cause oscillating currents to flow through the meters. 2-16