Charging Reactors

In the modulator of Fig. 252, a reactor is used to charge the pulse-forming network to nearly double d-c voltage E. The inductance of this reactor is not critical but should not be so great as to resonate with PFN at a frequency f_r less than half the pulse repetition frequency (PRF). That is, the voltage across PFN would not reach 2E if the resonant-charging frequency f_r were less than PRF/2, and the hold-off diode would be useless. It is the function of this diode to prevent discharge of PFN through the d-c source. This makes it possible to use reactors with normal manufacturing tolerances, or with variations in PRF, within the limit set by f_r > PRF/2.

When no hold-off diode is used, both reactor inductance and PRF must be held to close tolerances, to maintain high voltage across PFN, with resonant frequency f_r exactly equal to PRF/2. Linearity of inductance with change in direct current is also necessary for accurate control. Reactors designed by Fig. 71, p. 100, are usually linear enough for the purpose.

If f_r < PRF/2 the hydrogen thyratron fires before voltage across PFN reaches its peak, but after steady-state conditions obtain this voltage nearly equals 2E. If f_r is low enough, the increase of voltage is approximately linear with time. This result is called linear charging. Reactor inductance is large in this case, and charging current flows continuously. Here also a range of PRF's may be used, all greater than 2f_r

Reactor voltages and currents for the three methods of charging are illustrated by Fig. 256.

Fig. 256. Pulser charging reactor voltage and current.

In all cases, Q should be high (more than 10) for efficient pulser operation. Voltages in Fig. 256 are for infinite Q. Voltage is shown for the reactor terminal that connects to PFN. With a hold-off diode, this voltage appears on both terminals during the intervals when current is zero. Without the diode, or if diode and reactor in Fig. 252 are interchanged, the voltage at the left-hand terminal, Fig. 252, is E. Insulation may then be graded accordingly. In all cases, peak voltage across the reactor is E. A-c flux density may be calculated as in a filter reactor, but the effective frequency is 2f_r because the flux excursion is in one direction only at frequency f_r.

In a-c resonant charging, reactor voltage would increase to QE, where E is the a-c input voltage, if charging continued for a sufficient number of cycles. If reactor Q > 10 and the thyratron is fired at the end of one full charging cycle, maximum PFN voltage is πE_pk, where E_pk is the peak value of applied a-c voltage. In a-c charging the supply transformer inductance may be used instead of a separate reactor and results in somewhat less total weight. A high reactance transformer is useful for this purpose, with shunts as in Current-Limiting Transformers. If a hold-off diode is used with a-c charging, an ordinary plate transformer may be used and weight reduced still further.⁽¹⁾

(1)	For a detailed description of a-c charging, see Glasoe and Lebacqz, op. cit., p. 380.