SOS generators

SOS diodes have been employed in nanosecond rep-rate generators [16, 21, 22, 27 and 30] serving experimental purposes in various domains of electrophysics. The circuit design of these generators has been based on the aforementioned approach, according to which power, which is necessary to generate a pulse, first is accumulated in a thyristor charger (TCU) and then is time-compressed in a magnetic compressor (MC). The opening switch employing SOS-diodes functions as the final power amplifier and produces a nanosecond pulse at the generator output. The generators do not employ gas-discharge switches and therefore the fundamental limitation on the pulse repetition rate has been removed. In the continuous mode the pulse repetition rate is limited by the thermal load on the generator elements, primarily cores of magnetic switches. When the generator runs intermittently in the burst mode, the pulse repetition rate is limited by the rep-rate performance of the TCU, i.e. the thyristor recovery time and the charging time of the primary storage. The burst mode, when the generator operates for tens of seconds to several minutes at the rep-rate and the output power, which are several-fold higher than the nominal, is significant both in certain technical applications and for optimization and modeling of new technologies under laboratory conditions. With this in mind, to utilize most the rep-rate potentials of this approach in the generators, the TCU has been designed allowing for the minimum energy accumulation time, whereas the generator elements have been chosen proceeding from their calculated adiabatic heating in the burst mode. The available generators provide a 5- to 10-fold increase in the nominal pulse repetition rate and the output average power with the burst length of 30 to 60 seconds.

SM-1N Generator

Fig.8

Figure 8 shows the external appearance and Table II gives parameters of the SM-1N generator. This generator produces pulses about 24 ns long with the amplitude of up to 250 kV at the resistive load of 200-300 Ohm. It runs continuously at the pulse repetition rate of 100 Hz and of 1000 Hz in a 30-sec burst. The output average power of the burst attains 8 kW. The opening switch consists of two parallel branches with three series-connected SOS-diodes each. The current cutoff time is nearly 3 ns. The rise time of the output voltage pulse is determined by the time needed to charge the output stray capacitance with the current of the inductive storage and is equal to 10-12 ns.

Fig. 9

Figure 9 shows elements of the high-voltage module installed in a tank with transformer oil.

Table II. Specifications of the SM-1N generator.

SM-2N Generator

Fig. 10.

The SM-2N generator (see Fig. 10 and Table III) was initially designed to study characteristics of SOS-diodes at the pulse repetition rate of up to 5 kHz. Later generators of this type were used in experiments concerned with the study of the streamer corona discharge and breakdown of liquid dielectrics. This generator has two series-connected SOS-diodes with the working voltage of 100 kV each. Thanks to the inductive storage and the opening switch, it allows the output pulse amplitude to be adjusted over broad limits by changing the value of the impedance, which by-passes the generator output. The output voltage of the SM-2N generator can be adjusted from 20 to 200 kV. The maximum and minimum values of the power put to the load within this adjustment interval differ by a factor of 6. If a capacitive storage with a similar voltage adjustment interval is used, the values of the pulse energy would differ by 2 orders of magnitude.

Table III. Specifications of the SM-2N generator.

SM-3N Generator

Fig. 11.

The SM-3N generator is water-cooled and operates at the rep-rate of 300 Hz continuously with the output average power of up to 3 kW, and the rep-rate of 2 kHz in a 30-sec burst with the output average power of up to 16 kW. The external appearance of this generator is shown in Fig. 11. For basic technical characteristics of the SM-3N generator see Table IV.

Fig. 12.

Figure 12 shows a typical oscillogram of the reverse current passing through the SOS-diodes of the opening switch. The amplitude of the cutoff current is 2.2 kA. The cutoff time is 4 ns.

Table IV. Specifications of the SM-3N generator.

S-5N Generator

We also have developed a nanosecond pulse generator type S-5N. The S-5N generator represents the most powerful generator based on the SOS approach (see Table V) [30]. Given an external load impedance of 100 Ohm to 1 kOhm, it provides a voltage from 400 kV to 1 MV at a current from 1 to 3 kA. When the pulse power ranges between 1 and 1.6 GW and the FWHM is 40 to 60 ns, the output pulse energy equals 40 to 65 J. The generator operates continuously at the pulse repetition frequency of 300 Hz. The generator runs at a frequency of 500 Hz in a burst mode 3 minutes long. The average output power is 30 to 35 kW at a frequency of 500 Hz. The flow rate of water in the cooling system of the generator components is 15 liters per minute. The generator is 3.5 meters long. The mass of the generator with transformer oil is nearly 2.5 tons. The generator has been designed for carrying out experiments on removal of toxic impurities from the air with the use of an electron beam or streamer corona discharge.

Visual appearance of the generator is shown in Fig. 13.

Fig. 13.

Fig. 14 shows one unit of the semiconductor opening switch. This unit has the operating voltage of 1.2 MV and interrupts currents up to 4 kA. The unit is 400 mm long and its mass is about 5 kg. It contains 20 SOS-diodes: 10 diodes in series and 2 diodes in parallel. The generator is outfitted with 1 unit or 2 such units in parallel depending on the storage inductance. A SOS-diode in the unit consists of 128 series-connected semiconductor structures having the surface area of 0.25 cm2 each. The structures have sinks to remove heat to the oil.

Fig. 14.

The generator was employed in experiments concerned with ignition of a large-volume streamer corona discharge, which can be used in new technologies designed to remove harmful and toxic impurities from air. As an example of these experiments a photograph in Fig. 15 shows a streamer corona discharge in the gap between a metal plane and a semi-sphere. The semi-sphere is 30 cm in diameter. The sharp edge of the semi-sphere (the region of the most intense glow) and the opposite metal plane are spaced 70 cm. The discharge has the following parameters: the voltage amplitude of 760 kV, the total discharge current of 1.4 kA, the pulse power of 1 GW, the FWHM of 56 ns, energy deposited into the discharge in one pulse of 45 J, and the pulse repetition frequency of 500 Hz.

Fig. 15.

Table V. Specifications of the S-5N generator.