Pulse Engineering, High-Voltage

an area of electrical engineering that deals with the generation, measurement, and use of high-voltage pulses with amplitudes of 10² to 10⁷ volts (V) and current pulses with amplitudes of 10² to 10⁷ amperes (A). Pulse duration varies from 10⁻¹ to 10⁻¹⁰ sec. Such pulses may be nonrecurrent pulses, or they may be recurring, with a high off-on time ratio.

High-voltage pulses are used to test electrical equipment and to simulate system-generated overvoltages and lightning surges in power distribution systems and lightning-protection equipment. In experimental physics, high-voltage pulses are used to generate strong pulsed electric fields, to study processes of electric breakdown, to produce short bursts of X radiation (10⁻⁷ to 10⁻⁶ sec), to generate pulsed electron and ion beams, and to provide power supply for spark chambers, image converters, Kerr cells, and charged-particle accelerators.

Voltage pulses with amplitudes up to 10⁷ V are produced by pulse voltage generators. Such generators contain a group of capacitors C (Figure 1), which are connected in parallel through resistors R during charging by the DC voltage source S. When the voltage on the capacitors reaches the required value, they are switched in series by means of spark gaps G (the Arkad’ev-Marks circuit). The pulse rise and decay times can be adjusted by the damping resistors R_d, the discharge resistor R_dis, the capacitor C_f, and the load capacitance O.

Figure 1. Circuit of a pulse voltage generator (PVG, or Arkad’ev-Marks circuit): (S) DC voltage source, (C) capacitors, (R)charging resistors, (R_d) damping resistors, (R_dis) discharge resistor, (G) spark gaps, (O) object being tested

Equipment other than pulse voltage generators—for example, high-voltage test transformers, whose primary windings are fed by capacitor banks—is used to produce pulses with an amplitude of 10⁶ V, rise time of ∼ 10−4 sec, and a decay of ∼ 10⁻³ sec. A special capacitor charged by a pulse voltage generator and discharged through an additional “sharpening” spark gap is used to produce pulses with a sharper front edge.

Pulses with a rise time of ∼10⁻⁹ sec, a total duration of ∼10⁻⁸’10⁻⁷ sec, and an amplitude of 10⁴–10⁶ V can be produced using nanosecond pulse generators. The circuit of such a generator differs from that shown in Figure 1 in that the capacitors are replaced by sections of coaxial cable, which has distributed capacitance, and resistors R_dR_f are omitted. Nanosecond pulses can also be produced using sections of coaxial cable connected according to the circuit shown in Figure 2, sections of a three-part strip line (Bliimlein circuit; Figure 3), or sections of a spiral strip line (spiral generators; Figure 4). In the last two generators voltage doubling (Figure 3) or multiplication (Figure 4) occurs upon breakdown of the spark gap G and reflection

Figure 2. Circuit of a cable generator of high-voltage nanosecond pulses: (C) sections of coaxial cable, (G) spark gap, (O) load

Figure 3. Circuit of a Blumlein generator: (S) DC voltage source or pulse voltage generator, (t) three-part strip line

of the wave from the terminal of the line. If there are no special requirements for the shape of the voltage pulse, pulses with an amplitude of the order of 10⁴–10⁵ V can be produced using pulse transformers (Ruhmkorff coil or Tesla transformer).

Figure 4. Spiral generator

Pulse amplitudes are measured with special capacitive, resistance, or resistance-capacitance voltage dividers.

High current pulses are used (1) in the generation of pulsed magnetic fields in thermonuclear equipment and charged-particle accelerators, in acceleration of plasma and metal bodies, in magnetic-pulse metalworking, in quick-opening electromagnetic valves, and in pulsed electric drives; (2) for rapid heating of gases and conductors (the heating of gas in aerodynamic and thermonuclear research, the generation of powerful shock waves and diverging fluid streams for echo sounding and seismic prospecting, the deformation and destruction of materials, electric explosion of conductors, power supply for pulsed light sources, spark-erosion machining of metals, and pulse welding); and (3) for testing electrical devices and switching apparatus and the simulation of destructive effects of lightning current.

Devices used as sources of current pulses include shock-excited oscillators, which are capable of storing energies up to 10₈ joules as the kinetic energy of a massive rotor; accumulators; capacitor banks (capacitive storage devices) charged by a DC voltage supply (for example, a Gorev circuit); inductive storage devices, in which the energy is stored in induction coils; and explosion generators, in which the current-carrying circuit or coil decreases in volume as a result of an explosion or the action of a magnetic field (Figure 5).

Figure 5. Amplitudes and durations of currents produced by various pulsed current sources: (I) explosion generators, (II) capacitive energy storage devices, (III) inductive storage devices, (IV) pulse accumulators, (V) Gorev circuit, (VI) shock-excited oscillators

Devices used to connect the load to the source of high current pulses include thyratrons (for currents up to 10³–10⁴ A and voltages up to ⋃20–30 kV), spark gaps, which may be pressurized or at atmospheric pressure (for currents up to 10⁶ A and voltages up to 10⁵ V); continuously pumped vacuum spark gaps (for currents up to 10⁶A and voltages up to 10–20 kV); and sealed vacuum spark gaps (for currents up to 10³ A and voltages up to 10⁵ V). Spark gaps with a solid dielectric are also used (for currents of ∼ 10⁶ A and voltages of ∼ 10⁴ V); the dielectric is replaced after each discharge. Pulse transformers are used to match the load to the capacitive or inductive storage devices. Pulse currents are measured by means of shunts or instrument transformers (Rogovskii loops) with integrating circuits. Another type of apparatus used for the same purpose takes advantage of the phenomenon of rotation of the plane of polarization (the angle of rotation of the polarization plane is proportional to the intensity of the magnetic field generated by the current being measured).