PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. Kirov

No. 251 1970

Spark Generator of Rectangular Ignition Pulses

V. F. Panin, L. K. Parfenov

(Presented by a scientific seminar of the Research Institute of High Voltages at the Tomsk Polytechnic Institute).

According to [1], in studies connected with optimizing the form of spark-discharge ignition pulses, with measuring the minimum ignition energies of explosive vapor-gas systems, and with studying the stability of their concentration ignition limits under the action of ignition pulses with a broad spectrum of energies and durations, it is expedient to use rectangular ignition pulses, i.e., pulses whose instantaneous power during initiation remains constant or close to constant.

To implement the rectangular model of the optimum pulse, the authors developed, built, and adjusted an electrical circuit whose main elements, Fig. 1, are: the spark generator proper, a trigger generator, a pulse-delay unit, a system for cutting off voltage pulses (a powerful pulse thyratron), a recording unit, a power-supply unit, and a high-voltage charging device.

The electrical circuit of the generator is shown in Fig. 2. The ignition-pulse generator proper is a discharge circuit based on a storage capacitance \(C_1\), a thyratron \(Tr\), a discharge resistance \(R_T\), and a working resistance \(R_4\); by varying the magnitude of the latter, the energy of the pulse released in the working spark gap \(I_2\) is regulated. The rectangularity of the ignition pulse is achieved by cutting off the “tail” of the pulse, which is close to exponential, at the moment when the instantaneous power of the pulse

\[ P(t) = u(t)\cdot i(t) \tag{1} \]

decreases by no more than a factor of 2 in comparison with the power of the pulse at its front.

Cutoff is carried out by means of a cutoff unit that feeds the cutoff pulse to the main element of the cutoff system—the thyratron TGI-400/16. The cutoff unit, triggered by a firing generator, is assembled on three thyratrons; its parameters are chosen so as to make it possible to regulate the duration of the rectangular ignition pulse within the range \(10^{-6}\)–\(10^{-3}\) sec.

The current and voltage in the working spark gap \(P_2\) are measured by means of a current shunt \(R_{\mathrm{sh}}\) and a capacitive voltage divider \(D_{\mathrm{N}1}\) in the unit with a DESO-1 oscilloscope.

The entire circuit is started by a special firing generator assembled according to the same circuit as the actual generator of rectangular ignition pulses.

The actual rectangular-pulse generator is powered from a rectifier unit with a voltage of up to \(\pm 15\) kV. The circuit of the generator firing is powered from another rectifier unit with a voltage of up to \(\pm 5\) kV.

The anode, grid, and filament circuits of the thyratrons are powered from a universal power supply SIP-1. The filament circuit of the main thyratron TGI-400/16 is powered from a separate filament transformer. Variation of the value of the working resistance \(R_4\) of the generator proper is carried out by means of a series of nichrome-wire rheostats wound bifilarly on insulating cylindrical frames.

The operation of the entire circuit of the rectangular ignition-pulse generator consists of the following sequence of stages. By closing switch \(K_1\), a voltage of \(-5 \div 15\) kV is applied to the output of the bridge rectifier circuit; through the circuit \(R_{\mathrm{lim}} - C_1 - B_1\) it charges the capacitor bank \(C_1\).

Before the actual ignition-pulse generator is started, the cutoff unit is in its initial state: capacitors \(C_2\), \(C_3\), and \(C_4\) in the anode circuits of thyratrons TG3-0.1/1.3 (two units) and TG3-0.1/0.3 are charged to a voltage of \(+300\) V.

![Block diagram with labeled blocks: pulse generator, spark generator, object, registration unit, power-supply unit, delay unit, t.s.v. system, charging device.]

Fig. 1. Block diagrams of the spark generator of rectangular ignition pulses.

![Electrical schematic of the spark generator.]

Fig. 2. Basic electrical circuit of the spark generator (an explanation of the designations and a description of the generator operation are given in the text).

Pressing button $K_2$ applies the line voltage to transformer TR3 and then to the circuit with rectifier $D_5$. The time constant of the charging circuit of capacitor $C_5$ is chosen to be sufficiently small, so that breakdown of the spark gap $P_1$ of the ignition generator occurs practically at once. The voltage pulse from resistor $R_3$ is applied to the thyratron Tg and ignites it.

Capacitance $C_1$ is discharged through resistor $R_1$. The positive voltage pulse from resistor $R_1$, through the operating resistor $R_4$, is applied to the working spark gap $P_2$ and to the current shunt $R_{\mathrm{sh}}$. The voltage at the spark gap, previously “divided” by the voltage divider $DN_1$, and the current through it are recorded by the two beams of the DESO-I oscilloscope.

Simultaneously with the ignition of the thyratron, pulses are applied from divider $DN_2$: to start the DESO-I oscilloscope and to start the blanking unit.

The positive pulse arriving at the grid of the first thyratron TG3-0.1/1.3 opens it, as a result of which capacitance $C_2$ is discharged through resistor $R_7$. The leading edge of the rectangular pulse from resistor $R_7$ is “smoothed” by the $R_8 C_9$ circuit. The duration of the leading edge of this pulse and, correspondingly, the time until the second thyratron TG3-0.1/1.3 fires are regulated by the value of resistor $R_8$. When the voltage on the leading edge of the pulse exceeds the bias level, the second thyratron TG3-0.1/1.3 opens, and capacitor $C_3$ discharges through the primary winding of transformer TR1, as a result of which a positive pulse from the secondary winding of the transformer is applied to the grid of thyratron TG1-0.1/0.3, opening it. Capacitance $C_4$ is discharged through resistors $R_{18}$, $R_{19}$ and the thyratron; the pulse from resistor $R_{19}$ is applied—after a time equal to the required duration of the ignition pulse, set by resistor $R_8$—to the grid

of the main thyratron TGI-400-16. The current through the working spark gap \(P_2\) is cut off; within fractions of a microsecond the current in the gap \(P_2\) decreases to zero.

The energy released in the spark is determined by the difference of two integrals:

\[ E_i=\int_0^{t_i} u(t)i(t)\,dt-\int_0^{t_i} R_{\mathrm{sh}} i^2(t)\,dt \tag{2}, \]

where \(\int_0^{t_i} u(t)i(t)\,dt\) is the integral for determining the total energy released in the spark and in the resistance \(R_{\mathrm{sh}}\) during the time \(t_i\) of action of the ignition pulse in the gap \(P_2\).

\[ \int_0^{t_i} R_{\mathrm{sh}} i^2(t)\,dt \]

is the integral of the energy in the resistance \(R_{\mathrm{sh}}\), \(u(t)\) is the instantaneous value of the voltage acting between the high-voltage electrode of the gap \(P_2\) and ground.

Typical oscillograms of the current and voltage in the ignition pulse are given in Fig. 3.

The energies of the ignition pulse were calculated for several settings of the generator operating resistance \(R_4\), at a charging voltage of 11 kV, capacitance \(C_1=3\ \mu\mathrm{F}\), spark-gap length \(P_2\) of 3 mm, and pulse duration of 150 \(\mu\)sec. From the calculation data the dependence \(E_i=f(R_4)\), shown in Fig. 4, was constructed. Similar dependences are easily constructed for other lengths of the gap \(P_2\).

The dependences \(E_i=f(R_4)\) are convenient in that they make it possible to determine promptly the required order of magnitude of the resistance \(R_4\) for achieving the required or assumed (in the course of experiments) energy level \(E_i\). Of course, when some limiting value is reached (the minimum energy or the concentration limit of ignition of the mixture, for example), oscillograph recording is assumed in each refining experiment.

For the present generator, regulation of the energy in the ignition pulse is carried out within the limits \(10^{-6} \div 10^{-1}\) J, with

Fig. 3. Oscillograms of the current and voltage in the ignition pulse \((R_4 = 2.5\ \Omega,\ C_1 = 3\ \mu\text{F},\) charging voltage \(= 11\ \text{kV},\) energy in the ignition pulse \(0.2\ \text{J})\).

Fig. 4. Dependence of the energy of an ignition pulse of duration \(150\ \mu\text{s}\) on the value of the generator operating resistance for gap \(P_2\) of length \(3\ \text{mm}\).

minimum level of resolving capability (at the lowest pulse energies) in fractions of a microjoule.

REFERENCES

1. V. F. Panin, “On the Principles of Designing Ignition Systems,” present collection.