Magnetic Pulse Generators

In Magnetic Amplifiers it was seen that thyratron operation can be approximated by self-saturating magnetic amplifiers. This fact points to a saturable reactor to replace the hydrogen thyratron in the pulser of Fig. 252. Several factors militate against the direct substitution of saturable reactors for thyratrons:

1.  Departure of core material from sharp rectangularity interferes with steep pulse voltage rise.

2.  Saturated value of inductance interferes with large current flow needed during pulse.

3.  Reactors are a-c devices; hence a-c charging must be used. This limits the choice of PRF.

Despite these difficulties, saturable reactors have been used successfully in pulsers. Low power pulses may be formed by use of the circuit of Fig. 202. Reactor L₁ is linear and resonates with C₁ at the supply frequency. Resonance therefore tends to maintain current i sinusoidal in wave form. Current i is large enough to saturate reactor L₂, which has rectangular B-H loop core material. Twice each cycle current i passes through zero, and near these current zeros L₂ inductance becomes large. This large inductance forces most of current i instantaneously into C₂ and R_L, and builds up a comparatively large peak voltage across R_L. Such pulses are peaked in wave shape and alternate in polarity twice each cycle. Pulse durations of less than 0.1 microsecond have been obtained with this circuit. Owing to the large interval of time during which i is large, and not producing pulses, the power output is limited to small values.

Fig. 262. Magnetic pulse generator.

In Fig. 262 the voltage across L₂ at a given line frequency f is nearly proportional to the saturation flux density B_s of the core for rectangular loop material. If the core area is A_c and turns N in L₂, then this voltage is, neglecting losses,

[145]

where Θ_s/2 is the angle, starting from zero, at which saturation is reached, as in Fig. 193 (p. 247). If Θ_s is very small, and 2μ/ω >> R_LC₂ >> Θ_s/ω, substantially all of e_s appears across R_L.

Higher power may be obtained from cascaded stages as in Fig. 263.

Fig. 263. Cascaded stages in magnetic pulse generator.

Reactor L₁ is linear and resonates with C₁ at the supply frequency. Reactors L₂, L₃ and L₄ are saturable; bias windings are used, but not shown. Suppose that L₃ and L₄ are initially unsaturated, and L₃ is saturated. C₁ charges in series with L₁ and L₃. As the voltage across L₂ reaches maximum, L₂ saturates and discharges C₁. Discharge current causes L₃ to become unsaturated and L₄ saturated; then C₂ charges until L₃ saturates again. As the wave proceeds towards R_L both charge and discharge times become successively shorter. Pulse duration in each stage is determined by the saturated value of inductance and associated C. In the last stage, the pulse is shaped by PFN to the desired duration and flatness at the top. As this sequence is repeated once each cycle, the line current is not sinusoidal, so a line capacitor is useful for supplying the current harmonics. One modification of this circuit is the use of saturating transformers instead of reactors in order to provide the stepped-up voltage necessary for magnetron operation. With a magnetron, R_L is replaced by a pulse transformer primary winding. In another modification saturable reactors are the series elements and capacitors the shunt.⁽¹⁾

(1)	See "The Use of Saturable Reactors as Discharge Devices for Pulse Generators," by W. S. Melville, J.I.E.E. (London), 98, Part III, p. 185.