SETUP FOR STUDYING THE EXCITATION OF FAST CHEMICAL PROCESSES IN ELECTRIC FIELDS

Yu. N. Sukhushin, V. P. Rotshtein, B. E. Velednitskii, Yu. A. Zakharov

(Presented by the scientific seminar of the Solid-State Chemistry Laboratory)

Recently, research has expanded considerably on chemical processes that arise at extreme values of the exciting parameters, forming a new branch of chemistry—high-energy chemistry [1]. In it, both for theory and for practice, an important place is occupied by the study of reactions excited by the action of an electric field in gases (plasma chemistry, gas electrochemistry [1, 2]) and in condensed media (for example [3, 4, 5]). These processes are also of definite interest for studying a number of phenomena accompanied by electrical effects, such as the interaction of laser radiation [6] and shock waves [7] with matter, the detonation of condensed explosives [8], and certain others.

In studying electrical methods of exciting chemical processes, the possibility of operation under a short-term, controlled voltage action—that is, in a pulsed regime—is of great importance. The methodological basis for such studies should be the achievements of modern pulsed electrical engineering and electronics [9, 10, 11, 12]. However, the field of application of modern pulsed technology (high-voltage engineering, radio engineering, quantum radio electronics, instruments for nuclear research, controlled thermonuclear fusion, etc.) rarely concerns the range of energies and voltages convenient for exciting processes in gases and solids.

We have developed a setup intended for studying the excitation of fast processes in solids, which makes it possible to study processes under rectangular and oblique voltage pulses from 0.01 to 10 kV over an action-time interval from 5 ns to several hours.

The schematic of the setup is presented in Fig. 1. The setup consists of three interconnected units. Unit A is intended for operation with low energies and short pulses; unit B, for operation with long pulses and high energies; unit V, for operation at constant or slowly varying voltage.

Unit A. The generating capacitance $C_{r1} = 10$ pF–$0.5$ μF, through a charging resistance limiting the charging current of the capacitance $R_{31} = 10$ MΩ, is charged from the high-voltage source UPU-1M. By means of a shock switch—a spark gap (design in Fig. 2a)—$C_{r1}$ is discharged into line $l_1$, made of RK-2 coaxial cable 50 m long. The use of this switch design provides a short leading edge of the im-

pulse (down to 1.5 nsec) and high operational safety. Switching the variable inductance \(L_{\phi}\) in front of \(l_1\) makes it possible to increase the duration up to 1 \(\mu\)sec. For oscillographic recording of the voltage in circuit A, an active cable divider is convenient: the resistance \(R_{\mathrm{d}1}\) soldered into the cable shield forms a divider with the characteristic impedance of the cable. During assembly it is desirable to avoid parasitic inductance of the soldering. With a parasitic inductance of 2 \(\mu\)H, calculation shows that, without distortion, a front with a duration of 1 nsec and greater is transmitted.

Fig. 1. Schematic of the setup for studying the excitation of chemical processes by an electric field

For oscillographic recording of the current, a low-inductance current shunt with a resistance of 1 ohm is used, made up of 10 coaxially soldered cable resistors of 10 ohms each, connected in parallel.

Calibration of the voltage divider is carried out with \(L_{\phi}\) disconnected, and calibration of the current shunt by feeding a voltage pulse from circuit A directly to the shunt. For oscillographic recording, a two-beam OK-19 oscilloscope or two single-beam high-speed oscilloscopes S1-11 were used.

The problem of synchronizing the current and voltage oscillograms is solved automatically, since the S1-11 oscilloscopes have a spread in triggering time no worse than 0.1 nsec, and the cables feeding the current and voltage signals \(l_2, l_3\) have the same length. Block A provides rigid synchronization of the signals with the triggering of the oscilloscopes and the possibility of operation over a wide range of voltages and pulse-front durations. A further increase in the front duration is associated with increasing the discharge resistance or the generating capacitance. In circuit A the first possibility is not feasible, since \(R_{\mathrm{pc}1}\) is chosen equal to the characteristic impedance of the cable, in order to eliminate pulse distortion, while the second is limited by the power capabilities of commutator II.

Block B. Allows operation with pulses of duration from 1 \(\mu\)sec up to 1000 \(\mu\)sec and more, as well as with high-power pulses. Ge

Fig. 2. Design of the commutators: \(1a\)—impact switch–disconnector, \(b\)—four-electrode spark gap.

a) 1, 2—contacts, 4—switch body, 5—sliding contact, 6—switch holder, 7—locking plate, 8—retainer, 9—handle;

b) 1—trigger-pulse inputs, 2—gap regulator between the triggering and main gaps, 3—spark-gap body, 5, 7—molybdenum points, 6, 8—regulators of the spark-gap spacings, 9—busbars, 10—fixing nut.

the non-shunting capacitance is switched to the discharge resistance—the active divider \(R_{\mathrm{d}2}\) and \(R_{\mathrm{d}3\mathrm{s}}\)—by the powerful IR discharger (Fig. 2b), which is triggered by a spark discharge from circuit A. The presence of the front resistance \(R_{\mathrm{f}}\) and capacitance \(C_{\mathrm{f}}\) makes it possible to regulate the pulse front over very wide limits. Current and voltage are recorded with a C1-7 two-beam oscilloscope. No special requirements are imposed on the design of the active divider.

Block B. Along with the investigation of pulsed excitation of rapidly proceeding processes, phenomena arising under prolonged application of voltage are of great interest, since in this case low-mobility charge carriers (for example, heavy ions) are drawn into the process, imparting a distinctive character to the process or even completely changing its mechanism. For these investigations we used a setup of the UPU-1M type, in which, by means of a reducer and a synchronous motor, an automatic rise of voltage was provided; this voltage is recorded on an oscilloscope by means of a high-resistance divider. Since the rate of voltage change is rigidly controlled, it is possible to record current phenomena and voltage on a single-beam oscilloscope, applying the voltage from the divider to the horizontal deflection plates and the current from the current shunt to the vertical plates. For this purpose it is necessary to feed the current and voltage signals directly to the plates of any single-beam oscilloscope.

Thus, the circuit described makes it possible to carry out investigations over a wide range of times and energies of the acting voltages. The article is of interest to those beginning work in the field of excitation of chemical processes in an electric field.

REFERENCES

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