A high-power synthesized ultrawideband radiation

A high-power synthesized ultrawideband radiation source A. M. Efremov, V. I. Koshelev, V. V. Plisko, and E. A. Sevostyanov

Citation: Review of Scientific Instruments 88, 094705 (2017); doi: 10.1063/1.5003418 View online: http://dx.doi.org/10.1063/1.5003418 View Table of Contents: http://aip.scitation.org/toc/rsi/88/9 Published by the American Institute of Physics

REVIEW OF SCIENTIFIC INSTRUMENTS 88, 094705 (2017)

A high-power synthesized ultrawideband radiation source A. M. Efremov, V. I. Koshelev,a) V. V. Plisko, and E. A. Sevostyanov Institute of High Current Electronics, SB, RAS, 2/3 Akademichesky Ave., Tomsk 634055, Russia

(Received 28 March 2017; accepted 5 September 2017; published online 22 September 2017) A high-power ultrawideband radiation source has been developed which is capable of synthesizing electromagnetic pulses with different frequency bands in free space. To this end, a new circuit design comprising a four-channel former of bipolar pulses of durations 2 and 3 ns has been elaborated and conditions for the stable operation of gas gaps of independent channels without external control pulses have been determined. Each element of the 2 × 2 array of combined antennas is driven from an individual channel of the pulse former. Antennas excited by pulses of the same duration are arranged diagonally. Two radiation synthesis modes have been examined: one aimed to attain ultimate field strength and the other aimed to attain an ultimate width of the radiation spectrum. The modes were changed by changing the time delay between the 2-ns and 3-ns pulses. For the first mode, radiation pulses with a frequency band of 0.2–0.8 GHz and an effective potential of 500 kV have been obtained. The synthesized radiation pulses produced in the second mode had an extended frequency band (0.1–1 GHz) and an effective potential of 220 kV. The pulse repetition frequency was 100 Hz. Published by AIP Publishing. https://doi.org/10.1063/1.5003418

I. INTRODUCTION

Sources of high-power ultrawideband (UWB) radiation are intended for use in testing electronic equipment for stability to threats,1–3 in studying biological effects,4–6 and in radar recognition of objects.7–9 In developing a high-power UWB radiation source, one should solve two main problems: to extend the frequency band of the radiation and to increase its effective potential rE p (peak electric field E p multiplied by far-field distance r). We will estimate the spectral width as the ratio of the high boundary frequency f H to the low boundary frequency f L , both determined, as in Ref. 10, at a level of 10 dB (boundary frequency ratio). There are two ways of solving these problems. First, both problems are solved successfully by creating high-power UWB radiation sources based on Impulse Radiating Antennas (IRAs).11 The use of a reflector of large diameter makes it possible to reduce the low boundary frequency f L of the radiation and to simultaneously increase its directivity and, hence, effective potential. Thus, with a reflector of diameter 4 m, radiation pulses having an effective potential of 1.3 MV were produced.12 The boundary frequency ratio, according to our estimates, reached 60. The pulse repetition frequency was 10 Hz in continuous operation and increased to 200 Hz when the pulse train was reduced to 500 pulses. A significant drawback of this type of UWB source is its low energy efficiency (less than 1%, according to our estimates). This is due to the use of a voltage pulse with short rise time (100 ps) and long fall time (over 20 ns). As the radiated field is proportional to the time derivative of the pulsed voltage at the antenna input, only a small amount of energy was radiated efficiently in a time following immediately after the

a)Author

to whom correspondence should be addressed: koshelev@ lhfe.hcei.tsc.ru

0034-6748/2017/88(9)/094705/7/$30.00

voltage rise time. Therefore, the duration of the radiated pulse was short (about 100 ps at half maximum). When the amplitude of the voltage pulse at the input of a half-IRA (HIRA) system13 was increased to 1000 kV, the effective potential of the radiation reached 5.4 MV and the boundary frequency ratio was 40. In this case, the peak field efficiency k E , defined as the ratio of the radiation effective potential to the voltage pulse amplitude at the antenna input, was less than 6. The second approach, being elaborated at the Institute of High Current Electronics, is the development of high-power UWB radiation sources operating due to the excitation of combined antenna KA arrays by bipolar voltage pulses.14 A combined antenna KA, having an extended frequency band, is a combination of radiating elements of electric type (short TEM horn) and magnetic type (loop) that are excited from a common input. A series of high-power UWB radiation sources was fabricated based on 2 × 2,15 3 × 3,16 4 × 4,17–20 and 8 × 8 KA arrays21 excited by bipolar voltage pulses of amplitude 100–200 kV and duration 0.2–3 ns. In these UWB radiation sources, open-line single-channel bipolar pulse formers were used.22 The pulse former circuit design, proposed by Kremnev, is different from the well-known Blumlein circuit by using an additional pulse-forming line and a peaking spark gap. To provide synchronous excitation of the array elements by bipolar pulses, a power divider was used. The energy efficiency of an isolated KA array excited by a bipolar pulse reached 90% with the boundary frequency ratio equal to 4–5. The efficiency of a UWB radiation source, defined as the ratio of the radiated pulse energy to the energy stored in a high-voltage pulse-forming line (PFL) charged from a Tesla transformer at a frequency of 100 Hz, increased with bipolar pulse duration and reached 30%. In the experiment,21 radiation pulses with an effective potential of up to 4.3 MV were produced. A KA array with 1.4 × 1.4 m cross dimensions was excited by a bipolar pulse of duration 1 ns and

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amplitude 200 kV. In this case, the peak field efficiency k E was 21, which is considerably greater than that of IRA-based and HIRA-based UWB radiation sources. It had been clear at the beginning of our research23 that an increase in KA energy efficiency due to an increase in the antenna-feeder matching band could not extend significantly the radiation spectral width, as it is limited by the spectral width of the antenna-exciting bipolar voltage pulse. Therefore, Koshelev24 proposed to extend the radiation pulse spectrum by synthesizing pulses of different spectral widths, produced by KA arrays excited by bipolar voltage pulses of different durations, in the far-field region. The research24 included a theoretical study of the performance of an 8-element array excited by bipolar current pulses of durations 1, 3, and 5 ns. Pulses of the same amplitude were locked to the time of current passage through zero. The spectral width of the synthesized radiation pulse was increased 3–4 times compared with that of the identical pulses. Subsequent calculations have shown that the spectrum of the synthesized radiation pulse can be extended considerably, even at a small number of array elements, by setting optimum time delays between the pulses. Thus, for a 3-element KA array excited by bipolar pulses of durations 1, 2, and 3 ns with optimum time delays, the spectral width of the synthesized radiation pulse can be increased by a factor of 3.7.25 For a 4-element KA array excited by bipolar pulses of durations 0.5, 1, 2, and 3 ns with optimum time delays, the spectral width of the synthesized radiation pulse can be increased about fivefold.26 Thus, the proposed line of research makes it possible to create sources of high-power UWB radiation with increased spectral width and pulse duration and, hence, with increased energy. This opens up new opportunities for the applications discussed above. To create a source of high-power UWB radiation with increased spectral width, it is necessary to develop multichannel formers of bipolar pulses of different durations. The formerly developed 4-channel pulse former produces bipolar pulses of the same duration (3 ns).27 The aims of the present work were to develop a 4-channel pulse former which would be capable of producing bipolar pulses of durations 2 and 3 ns with the spark gaps of individual channels showing low-jitter operation with no control pulses and to create a source of highpower UWB radiation with increased spectral width based on a 4-element KA array. II. DESIGN OF THE SYNTHESIZED RADIATION SOURCE

The appearance of the UWB radiation source is shown in Fig. 1. The source comprises the SINUS-160 monopolar

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pulse generator (1), a four-channel former of bipolar pulses of different durations (2), and a 4-element KA array (4). Pulses of different durations are transferred from each pulse former channel to the array elements via coaxial lines (3) and PK50-17-51 cordel-insulated cables (not shown in Fig. 1). The (series-connected) coaxial lines and cables were filled with SF6 gas to a pressure of 5 atm to enhance their electric strength. The radiation characteristics were measured in an anechoic chamber. III. THE FOUR-CHANNEL BIPOLAR PULSE GENERATOR

The bipolar pulse generator consists of a monopolar pulse generator and a four-channel former of bipolar pulses of different durations. In the circuit diagram of the bipolar voltage pulse generator shown in Fig. 2, the SINUS-160 monopolar pulse generator is represented by a PFL and a spark gap switch (S0 ). Line PFL was charged to 400 kV with a pulse repetition frequency of 100 Hz from the secondary winding of a Tesla transformer and was switched by the switch S0 , via line FL1 and the limit resistor R0 , to line FL2 . The separating line FL1 , consisting of coaxial line sections with the wave impedance varying from 45 to 88 Ω, is an equivalent coaxial adapter between switch S0 and line FL2 . The use of resistor R0 partially damps voltage oscillations in the discharge circuit after the formation of bipolar pulses and reduces erosion of the spark gap electrodes. The former of bipolar pulses of different durations consists of lines FL2 –FL3 , peaking spark gap S1 , and four monopolar-to-bipolar pulse transformation channels F1 –F4 . Channels F1 –F4 contain lines FL41 –FL44 , FL51 –FL54 , FL61 – FL64 , FL71 –FL74 , FL83 , and FL84 , chopping spark gaps S21 –S24 , and loads R11 –R14 . The parameters of the lines and loads are given in Fig. 2. When the peaking spark gap S1 operates at a near-maximum charge voltage, line FL2 discharges into line FL3 and further, via a matched four-channel adapter, into channels F1 –F4 . Chopping spark gaps S21 –S24 operate in a traveling wave mode with delays equal to round-trip propagation times for lines FL51 –FL54 , respectively. As spark gaps S23 and S24 operate earlier than spark gaps S21 and S22 , they may distort the waveforms of voltage pulses across loads R11 –R12 . To eliminate the mutual influence of the spark gaps, separating lines FL41 –FL44 are connected in the circuit. When the spark gaps operate perfectly, the duration of bipolar pulses in each of channels F1 –F4 is determined by the total round-trip time for lines FL51 and FL61 , FL52 and FL62 , FL53 and FL63 , and FL54 and FL64 , respectively, and the amplitudes of the output pulses across the loads are equal to half the charge voltage of line FL2 . In experimental conditions, the durations of the output bipolar pulses increase and their amplitudes decrease

FIG. 1. Appearance of the radiation source: 1—monopolar pulse generator, 2—four-channel former of bipolar pulses of different durations, 3—transmission lines, and 4—antenna array.

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FIG. 2. Circuit diagram of the fourchannel bipolar pulse generator.

because of the finite switching time and of the energy loss in the spark gaps. Bipolar pulses of duration 3 ns are formed in channels F1 and F2 and those of duration 2 ns are formed in channels F3 and F4 . The circuit of the bipolar pulse generator (Fig. 2) was simulated using the PSpice computer program. The simulation took into account the transfer capacitances of spark gaps S1 and S21 through S24 . The transient time TTRAN for spark gaps S1 and S21 through S24 was set 1.2 and 1.15 ns with their resistance decreased from 10 kΩ to 3 Ω and from 10 kΩ to 0.5 Ω, respectively. The tapping points for the simulated output voltage pulses V 1 , V 2 , and V 3 are shown in Fig. 2. The simulated waveform of the charge voltage V 1 across line FL2 is given in Fig. 3 (curve 1). The simulated waveforms of the bipolar voltage pulses V 2 across channels F1 and F2 and V 3 across channels F3 and F4 are given in Fig. 4 (curves 1). Figure 5 presents the time variation of the energy loss W 1 /W 0 in spark gap S1 (curve 1), total energy loss W 2 /W 0 in

FIG. 3. Waveforms of the charge voltage across line FL2 : the simulated pulse V 1 (curve 1) and the measured pulse from voltage divider D1 (curve 2).

spark gaps S21 through S24 (curve 2), and energy W /W 0 in the load (curve 3) related to the energy W 0 stored in the pulseforming line of the SINUS-160 generator. It can be seen that the total energy in the loads makes 14.3% of the stored energy. Most of the energy loss (27%) occurs in spark gap S1 . Structurally, the four-channel bipolar pulse former consists of six gas chambers. Figure 6 shows two of the four channels. The first chamber, filled with nitrogen at 40 atm, contains lines FL1 –FL3 , spark gap S1 , and capacitive charge voltage divider D1 . In four isolated identical chambers bounded by insulators 1 and 2 and filled with nitrogen at 40–65 atm, lines FL41 –FL44 , FL51 –FL54 , FL61 –FL64 , FL83 , and FL84 are located together with spark gaps S21 –S24 . Lines FL61 – FL64 are insulated with kaprolon. The internal diameters of lines FL2 , FL3 , FL41 –FL44 , FL51 –FL54 , FL61 –FL64 , FL71 – FL74 , FL83 , and FL84 are 70, 70, 16.4, 16.4, 7, 7, 11, and 11 mm, respectively. The ends of the inner conductors of lines FL2 and FL3 serve as the electrodes of peaking ringshaped spark gap S1 . The electrode gap spacing in spark gap S1 is 1.4 mm. The electrodes of chopping spark gaps S21 –S24 are 2-mm thick disks 3 and cylindrical inserts placed 0.5 mm apart on the outer conductors of lines FL51 –FL54 . The spark gap electrodes are made of copper. The axes of the four channels are arranged at diametrically opposite points of a 53-mm diameter circle. Four transmission lines FL71 – FL74 , identical in design and containing built-in coupled-line voltage dividers D21 –D24 , and loads R11 –R14 (not shown in Fig. 6) were insulated with SF6 gas while the generator was set up. The charge voltage pulse produced by the SINUS-160 generator arrived, via line FL1 , at the bipolar pulse former. The output bipolar pulses, on operation of spark gaps S1 and S21 –S24 , were transferred, via 50-Ω transmission lines FL71 –FL74 , to matched resistive loads or transmitting antennas. To record the output bipolar pulses of voltage dividers D21 –D24 and the charge voltage pulses across line FL2 , a TDS 6604 oscilloscope with up to 6 GHz bandwidth was used. Voltage dividers D1 and D21 –D24 were calibrated.

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FIG. 4. Simulated (curves 1) and measured bipolar voltage pulses (curves 2).

Spark gap S1 was broken down at a charge voltage of 170 kV with a 4.3-ns delay (see Fig. 3, curve 2). By varying the pressures in spark gaps S21 –S24 , symmetric waveforms of the bipolar voltage pulses of dividers D21 –D24 were attained

FIG. 5. Time variation of the energy loss W 1 /W 0 in spark gap S1 (curve 1), total energy loss W 2 /W 0 in spark gaps S21 through S24 (curve 2), and energy W /W 0 in the load (curve 3) related to the energy W 0 stored in the pulse-forming line of the SINUS-160 generator.

and zero crossing times of bipolar pulses of durations 2 and 3 ns were synchronized. The output bipolar pulses of voltage dividers D21 –D24 are shown in Fig. 4 (curves 2). The pulses have amplitudes of up to 80 kV and durations of 2 and 3 ns at a repetition frequency of 100 Hz. The pulse durations were estimated by extending the leading and trailing edges to their zero crossing points. The energy delivered to the loads reached 12% of the energy stored in the pulse-forming line of the SINUS160 generator, which is close to the value predicted by the simulation. The zero crossing points of the 3-ns bipolar pulses in channels F1 and F2 are delayed by 260–270 ps relative to those of the 2-ns bipolar pulses in channels F3 and F4 . The timing

FIG. 6. Schematic diagram of the four-channel bipolar pulse generator: FL— pulse-forming lines, S—spark gaps, D—voltage dividers, 1 and 2—insulators, and 3—disks.

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jitter between the output pulses of the channels was estimated by the spread in the hold-off times. The hold-off time of a channel, t 1 , was measured between the negative leading edge of the channel output signal and the zero crossing point of the output pulse of another channel. For all channels, the rms deviation of t 1 was not over 100 ps. The generator reached these conditions within 5 min of its operation at a frequency of 100 Hz. With 1% rms deviation of the SINUS-160 charge voltage amplitude relative to the mean, the rms deviation of the bipolar pulse amplitude relative to the mean was no more than 4%–5% for all channels F1 –F4 . In contrast to the previously used openline pulse formers, in this pulse former design, the peaking spark gap and the chopping spark gaps are in different chambers so that the chopping spark gaps cannot be backlighted from the peaking one. Nevertheless, the jitter of the bipolar pulses turned out to be close to that of the previously recorded pulses produced by open-line pulse formers. This indicates that there is no backlighting at subnanosecond switching times. As the pressure was increased to above 40 atm, the holdoff voltage of spark gap S1 increased, but its operating jitter increased as well. Therefore, at nitrogen pressures higher than 40 atm, bipolar pulses with low-jitter amplitudes could not be formed. IV. CHARACTERISTICS OF COMBINED ANTENNA ARRAYS

The UWB pulse radiated by a KA array was simulated by the time derivative of the sum of two Gaussian functions,25

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! ! 2 2 4t 8 4t 8 4t −2) −( 4t τ − −2 e − 4 e−( τ −4) , E(t, τ) = τ τ τ τ

(1)

where τ is the duration of the KA-exciting bipolar voltage pulse at a 10%-amplitude level. The pulse waveforms calculated by formula (1) are given in Fig. 7(a) and their spectra S( f ) are given in Fig. 7(b). When radiation pulses are combined in the far field region, the result is determined both by the characteristics of the pulses and by their time delays relative to each other. In the simulation, the synthesized pulse was optimized with respect to two parameters: the maximum amplitude of the field and the maximum width of the spectrum. The peak field strength is a maximum at a delay ∆t = 0.76 ns, which corresponds to zero jitter between the positive maxima of the radiated pulses. In this case, the pulse field amplitudes are combined, but the high-to-low boundary frequency ratio increases insignificantly compared with the pulses radiated by the antennas excited by 2-ns and 3-ns bipolar pulses. The width of the radiation spectrum reached a maximum at the delays between the pulses equal to 0.66 and 2.16 ns. Compared with a single pulse, the high-to-low frequency ratio of the combined pulse increased by a factor of 1.85 and was over three octaves, whereas the amplitude of the combined pulse field decreased insignificantly. The simulation has shown that for the pulse jitter equal to 100 ps, the spectral width of the synthesized pulse should decrease insignificantly. For KA arrays and 2 × 2 arrays radiating UWB pulses, the peak power pattern E p 2 was used as before.15–20 When

FIG. 7. Radiation pulse waveforms calculated by formula (1) for the excitation of antennas by bipolar voltage pulses of durations 2 (curve 1) and 3 ns (curve 2) (a) and their respective spectra (b).

FIG. 8. Waveforms of the pulses radiated by antennas excited by bipolar pulses of durations 2 (curve 1) and 3 ns (curve 2) (a) and their respective spectra (b).

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radiation pulses of different durations were synchronized with respect to the maximum radiated field (∆t = 0.76 ns), the pattern of the array was identical to that of an array excited synchronously (∆t = 0 ns) by pulses of equal duration. However, for the synthesized pulse with maximum spectral width (∆t = 0.66 ns), the peak power direction of the pattern depended on the array configuration. The simulation has shown that a KA array with diagonally arranged elements is optimal (see Fig. 1). In this case, the peak power direction of the pattern is normal to the array plane for both modes of radiation pulse synthesis. The simulation and the low-voltage test measurements similar to those presented in Refs. 16–20 have shown that the far-field boundary for the 4-element array excited by bipolar pulses of durations 2 and 3 ns corresponds to r = 8 m. As the high-voltage measurements of rE p were carried out in an anechoic chamber at a distance 4.4 m from the array, the relation rE p (r) was extrapolated from the array to the far-field boundary. The data obtained indicate that the effective radiation potential determined in the far-field region is greater than rE p (r = 4.4 m) by 3.2%.

V. RADIATION OF HIGH-POWER SYNTHESIZED PULSES

In the experiments, KA arrays optimized for the excitation by bipolar voltage pulses of durations 2 ns17 and 3 ns15 were used. The antenna apertures were in the same plane. To estimate the delays for the synthesized pulses produced by an array excited by bipolar high-voltage pulses, the radiation measurements were performed for the array elements excited individually by a pulse of duration 2 ns and by a pulse of duration 3 ns with the transmission lines having the same length. The measurement results are presented in Fig. 8(a). The measured pulses were different in amplitude from the simulated

FIG. 9. Waveform of a synthesized radiation pulse with maximum field amplitude.

pulses by 25%. Figure 8(b) shows the respective pulse spectra. The difference in width between the measured and the simulated spectra [see Fig. 7(b)] made no more than 15%. Based on these measurements, to produce pulses optimized with respect to field amplitude and spectral width, the delays were estimated that occurred in the transmission lines by which the pulses were transferred from the generator to the array elements. To synthesize a pulse with maximum field amplitude, a time delay of 0.82 ns was added to the lines that transmitted bipolar pulses of duration 2 ns. The waveform of the pulse synthesized with this delay is given in Fig. 9. The effective radiation potential was 500 kV. To synthesize a pulse with maximum spectral width, a 0.3-ns delay was induced in the lines that transmitted bipolar pulses of duration 3 ns. Figure 10(a) shows the waveform of the synthesized radiation pulse and Fig. 10(b) shows its spectrum. The effective potential of the synthesized radiation with an extended bandwidth, rE p , was 220 kV, and the

FIG. 10. Waveform (a) and spectrum (b) of a synthesized radiation pulse with maximum spectral width.

TABLE I. Characteristics of high-power UWB radiation pulses. UWB radiation pulse 2 ns 3 ns 2 + 3 ns, maximum field amplitude 2 + 3 ns, maximum spectral width

f L (GHz)

f H (GHz)

f 0 (GHz)

∆f (GHz)

∆f /f 0

b = f H /f L

0.22 0.143 0.18 0.101

0.878 0.69 0.768 1.047

0.549 0.4165 0.474 0.574

0.658 0.547 0.588 0.946

1.199 1.313 1.24 1.648

3.99 4.825 4.27 10.37

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high-to-low boundary frequency ratio was about 10. The measured frequency characteristics of the radiation pulses are given in Table I, where ∆f = f H f L and f 0 = ( f H + f L )/2. The characteristics of the radiation pulses produced by a KA array excited by bipolar pulses of durations 2 and 3 ns are given in the first and second lines, respectively. The characteristics of the synthesized radiation pulses with maximum field amplitude and with maximum spectral width are given in the third and fourth lines, respectively. VI. CONCLUSION

A source of high-power synthesized radiation with an extended bandwidth has been developed and built which is based on a 2 × 2 combined antenna array excited by bipolar voltage pulses of durations 2 and 3 ns with an optimum delay between them. The spectral width of the synthesized pulse has been increased by a factor of 1.4–1.7 (2.1–2.6) with respect to the boundary frequency ratio compared with the pulses produced by UWB sources excited by identical bipolar pulses.15,17 Radiation pulses with an extended bandwidth and an effective potential of 220 kV have been produced. A new circuit design has been proposed and implemented for a four-channel former of bipolar voltage pulses of different durations. This pulse former, using untriggered gas gaps, is capable of producing low-jitter bipolar voltage pulses of durations 2 and 3 ns and amplitudes up to 80 kV at a repetition rate of 100 Hz. ACKNOWLEDGMENTS

This work was supported by the Russian Scientific Foundation (Project No. 16-19-10081). 1 D.

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