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Cathode Pulsers: Hard-Tube Modulators (10) 159 and so on up the stack. The radii of curvature that are required for corona-free operation continually increase, but the capacitor can and bushing geometries do not, because most practical banks are built up of identical capacitors. Capacitors, therefore, are usually mounted on “rafts” that are surrounded by oblong, donutshaped corona shields whose cross-sectional diameters increase as the rafts ascend in height and voltage. An example of such construction is shown in Fig. 10- 5. Please note that if spacings between conductors are adequate, very little shielding is required to prevent voltage breakdown. This is because corona under these conditions is self-healing; the exterior of the ionization sheath tends to expand so as to have the same effect as a large radius-of-curvature surface. However, in today’s sophisticated systems, which involve ever-more-sensitive and susceptible low-level circuitry, the electrical noise resulting from the corona dis- 2.0 1.8 1.6 1.4 from power SUPPV 1.2 i * g 1.0 ~ r!! 0.8 Energy dissipated 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 12 Per-unit capacitor discharge ~ Figure 10-6. Capacitorchargingenergyrelationshipsas afimction of amountof capacitor discharge.
160 High-Power Microwave-Tube Transmitters charge is increasingly intolerable. (Remember what we used to pick up on the AM car radio when we drove near high-tension wires, especially on a foggy day?) We have discussed the fact that the removal of charge from a capacitor bank, no matter how large, will result in its terminal voltage being smaller than it was before the charge-removal interval, or pulse. In the interpulse interval, therefore, it will be necessary to replace the charge removed during the pulse. In the discussion of line-type modulators, much was made of the issue of recharging without incurring energy loss if the current was limited by resistance. Is there a similar concern when it comes to replacing partial discharge? The answer is not nearly as much. The reason for this is shown graphically in Fig. 10-6, which plots energy removed from the dc power supply, energy transferred to the capacitor bank, and energy dissipated in series resistance versus the per-unit amount that the capacitor is discharged each pulse. (This graph assumes that the resistance is small enough so that the RC time-constant is less than approximately 1/3 of the minimum interpulse interval, assuring that the capacitor will recharge to approximately the power-supply terminal voltage between pulses.) If we look at full discharge, or per-unit discharge of unity, the results are familiar. The perunit capacitor energy transfer and resistor energy dissipation are both unity, and the per-unit energy removed from the power supply is 2. Notice, however, that only the energy removed from the power supply is a linear function of per-unit discharge voltage. For less-than-unity per-unit discharge, energy transferred to the capacitor falls off less rapidly than dissipation in the resistor. Therefore, the efficiency of energy transfer increases as AV/Vdecreases. The reason for this can be seen if we note the change in charge during a pulse of current, AQ = CAV, and the change in capacitor energy, AW = AQ, times average capacitor voltage during discharge, which can be expressed as ~Avx V+ V–AV 2 or ~vAv _ C(AV)2 The energy removed from the power supply during recharge is AQ times the power supply voltage, which can be expressed as CAV x V. Recharging efficiency is the ratio of energy transferred to the capacitor to the energy removed from the power supply, or 2“ +Av-a=, ——— l[AV) CVAV 2V”
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3.6M north10.pdf - Dean-O's Toy Box

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