High-voltage Subnanosecond Pulsed Power Source ... - IEEE Xplore

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C. Yao et al.: High-voltage Subnanosecond Pulsed Power Source with Repetitive Frequency based on Marx Structure

High-voltage Subnanosecond Pulsed Power Source with Repetitive Frequency based on Marx Structure Chenguo Yao, Zhongyong Zhao, Shoulong Dong and Zhou Zuo State Key Laboratory of Power Transmission Equipment & System Security and New Technology School of Electrical Engineering Chongqing University Chongqing 400030, China ABSTRACT The electromagnetic radiation effect of high-voltage, subnanosecond pulsed electric fields attracts strong interest from researchers because of its significant development potential in biological treatments, particularly in noninvasive diagnosis and treatment. But up to now, high-voltage subnanosecond pulse generators are widely used for national defense and military, and they are difficult to implement successfully in the civil medical field. A compact, self-contained, repetitive frequency, high-voltage subnanosecond pulsed power source is proposed in this paper. It was designed, built, and tested successfully. Based on a four-stage, low-inductance Marx generator, the pulsed source produces subnanosecond rise-time pulses. A chopping switch was designed to cut off unformed signals and generate subnanosecond pulses. A measurement device, based on the principle of a capacitive voltage divider, was also constructed to determine both the amplitude and the rise-time of the pulse delivered by the source. Preliminary tests show that the source can produce repetitive frequency pulses with a peak value that exceeds 30 kV, as well as rise-time and pulse width (full wave at half maximum) within 1 ns under atmospheric pressure conditions. The pulse amplitude may be extended to hundreds of kilovolts by filling the switch system with an inert gas. The rise-time would be shortened as well. The pulsed power source shows optimistic prospects in the biological fields. Index Terms - Subnanosecond, pulsed power source, Marx structure, coupling capacitance, capacitive voltage divider.

1 INTRODUCTION THE generation of high-intensity electromagnetic pulses has recently become an important branch of pulsed power technology. Pulsed power sources are usually composed of primary energy storage, the load, and an intermediate pulseforming device. When high-voltage subnanosecond pulses are produced by a pulsed power source operating into loads of ultra-wideband (UWB) antennas, it is possible to transfer the pulsed energy to specific targets. In this case, the technique is of interest for a wide variety of civil applications, especially for noninvasive diagnosis and treatments in the biological field [1–3]. Many international studies have analyzed subnanosecond pulse generators extensively. Camp [4] used self-breakdown spark gap switches to steepen and to truncate nanosecond pulses, thus producing subnanosecond pulses with an amplitude of 50 kV and a pulse width of 150 ps. Kentech [5] developed a pulsed power source based on an avalanche Manuscript received on 29 September 2014, in final form 12 January 2014, accepted 12 January 2015.

transistor. This source produces short pulses with an amplitude greater than 45 kV, a pulse rise-time of 150 ps, a pulse width of 3 ns, a pulse repetition frequency of 500 Hz, and a pulse-to-pulse time jitter of less than 20 ps. Cadihon et al. [6–8] successively developed multiple high-voltage, subnanosecond pulsed power sources mainly for military applications. The maximum output voltage amplitude of these devices can reach 100 kV, and the rise-time and pulse width are less than 350 and 850 ps, respectively. These important and fruitful works have contributed significantly to the development of pulsed power technology. However, the noted existing high-voltage, subnanosecond pulsed power sources are unsuitable for biological applications because these devices generally deliver more energy than required. Moreover, the bulky devices complicate field application. In the present paper, a compact, repetitive frequency, highvoltage, subnanosecond pulsed power source is proposed, designed, built, and tested. The basic operating principle of the device is briefly introduced in section 2. The construction of the pulsed power source based on Marx structure is elaborated upon in section 3, along with the establishment of the measurement system. The simulation studies and preliminary

DOI 10.1109/TDEI.2015.004966

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 22, No. 4; August 2015

experimental tests are presented in section 4. Finally, the conclusion and recommendations for future work are provided in section 5.

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that fills the switch system. This adjustment is consistent with the right-hand side of Paschen’s curve.

2 BASIC PRINCIPLE The basic principle of the subnanosecond pulsed power source is illustrated in Figure 1. A high-voltage direct-current (DC) power supply is first used to charge the Marx generator. This power supply has a maximum amplitude of 30 kV. The use of specific capacitors, resistors, and spark gap switches can generate nanosecond pulses with a rapid rise-time. Peakingchopping switches are added at the last stage of Marx generator construction. These two switches function as a pulse-forming device that produces high-voltage pulses with a rise-time and pulse width (full wave at half maximum) within 1 ns. The subnanosecond pulses are then transferred to the load by the coaxial cable. Concurrently, the pulses are recorded by a measurement system that is installed prior to the load.

Figure 2. Electric field distribution of switches before breakdown.

The repetition frequency of the output pulse in a lowrepetitive frequency Marx generator is not determined according to the recovery time of gas after each discharge but rather by the capability of the power source to load the Marx capacitor after each discharge [9]. The charging time constant [10] of the last-stage capacitor satisfies equation (1).

  RS CN 2 Figure 1. Basic principle of the pulsed power source.

3 DESIGN 3.1 MARX GENERATOR The initial nanosecond pulses are produced by the Marx generator; therefore, the performance of the Marx circuit is critical. A low-inductance Marx structure should be considered to produce nanosecond pulses with a fast rise- time. Hence, the Marx generator is designed as a four-stage, compact coaxial structure that significantly improves the reliability of the generator and reduces the stray parameters of the space. The values of these parameters affect the output waveform. The switches are designed as self-breakdown spark gap switches to improve the amplitude of output voltage in consideration of the withstand voltage of components. This design is vital because it is difficult to generate pulses with fast front and high amplitude simultaneously when the semiconductor switches are installed. The semiconductor device has flaws that differ from those of the spark gap switches, although it has developed significantly in the past two decades. The gap distance among the spark gap switches is set to 1 mm at the first stage and to 1.2 mm at the latter three stages. The switches are composed of a pair of brass electrodes at each stage. The structure of the spark gap switches and the electric field distribution of the switches before breakdown are depicted in Figure 2. This structure ensures that the intensity of the electric field is maximized at the center of the electrodes and that the discharge of the electrodes is synchronized and stable. In practice, the switches are surrounded by a cylindrical Plexiglas layer for support and insulation. The amplitude of the output voltage in the Marx generator can be adjusted by tuning the amount of inert gas

(1)

where  is usually equal to 0.9. Thus, the value of the charging resistor is approximately equal to that obtained with

RS 

1 5fCN 2

(2)

where C represents the value of each stage storage capacitor; N is the stage number of the generator; and f represents the repetition frequency. The proposed subnanosecond pulsed power source is mainly used in biology; thus, the designed repetition frequency need not be too high. In the present work, the maximum repetition frequency is set to 17 Hz. The Marx bank is composed of four stages, each of which principally consists of a pair of brass electrodes and four parallel 1 nF/50 kV ceramic capacitors. The equivalent capacity of the Marx generator is equal to 1 nF. The value of the charging resistor is calculated as 200 kΩ.

Figure 3. Simplified equivalent circuit of the Marx generator.

The simplified equivalent circuit of the Marx generator is shown in Figure 3. R is the load, and its value is equal to the input impedance of the UWB antenna. C is the storage capacitor of each stage, the capacity of which is 4000 pF. CS

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and LS represent the stray capacitance and inductance of each brass electrode, respectively. LW indicates the inductance of the stainless torus of each stage. This torus functions as the tie plate of capacitors and brass electrodes. In accordance with the specific structure of each part, L0 and C0 are calculated to be approximately 211.73 nH and 1.09 nF, respectively. The output pulse of the Marx generator is measured and presented in Figure 4 when charged with 5.5 kV DC. The voltage amplitude is approximately 18 kV, and the 10-90% fall-time is roughly 11 ns.

stray inductance of the Marx generator; LP corresponds to the equivalent stray inductance of the peaking switch; and R represents load resistance. S1 is “off”, S2 is “on”, and CP is charged by C0 when the Marx generator is erected. When S2 is “off” as a result of breakdowns, both C0 and CP charge the load. According to Kirchhoff’s law, CP, LP, and R first produce a resonance with an amplitude higher than that generated by the Marx generator because CP is much less than C0. Then, the lagging edge of the pulse on the load is mainly determined by C0, L0, and R. The chopping switch actually consists of a pair of electrodes. One end of the electrodes corresponds to one side of the peaking switch, and the other end is a 5 mm thick stainless steel disc that is connected to the metal shell. The trailing edge of nanosecond pulses can be cut off, and subnanosecond pulses can be produced appropriately.

Figure 4. Output pulse of the Marx generator when the DC power supply is 5.5 kV.

3.2 PEAKING-CHOPPING SWITCHES Peaking–chopping switches are incorporated in the last stage of Marx generator construction. The structure is exhibited in Figure 5. The peaking switch is actually a resonant circuit that produces a high-amplitude resonance. The peaking capacitor CP is integrated in this stage as well, and it is composed of an annular polyvinyl chloride (PVC) layer. This layer is located between the last electrode and the metal shell. The PVC torus functions as the dielectric medium of the peaking capacitor between the last tie plate and the metal shell. The size and structure of the PVC torus are important because they determine CP completely. The capacitance of the designed peaking capacitor is approximated at 33.5 pF in this work.

Figure 6. Equivalent circuit of the peaking switch connected to the Marx generator.

The entire pulsed power source was designed as a coaxial structure. The switch system, the storage capacitor, and the peaking–chopping switches are located in the center of this structure. They are arranged around the central axis. All inner components are surrounded by a cylindrical stainless steel shell; the shell does not only act as the ground but also supports the structure of the pulsed power source. The initiated subnanosecond pulses are then transferred to the load by a coaxial cable. The 3D structure of the pulsed power source (except for the DC power supply, the coaxial cable, and the load) is shown in Figure 7.

Figure 5. Structure of the peaking–chopping switches.

The connection of the equivalent circuit of the peaking switch to the Marx generator is depicted in Figure 6. Switches S1 and S2 represent the equivalent switch of the Marx circuit and the peaking switch, respectively; C0 denotes the equivalent capacitance of the Marx generator, which has an initial voltage V0; L0 indicates the equivalent

Figure 7. 3D structure of the pulsed power source (part).

3.3 MEASUREMENT SYSTEM A high-bandwidth measurement system should be developed to record the transient behavior of high-voltage subnanosecond pulses. However, fast rise-time and strong



amplitude signal are difficult to measure simultaneously. This system should be designed carefully because a type of subnanosecond waveform is the expected output. In this study, a homemade capacitive voltage divider was constructed as the measurement system. Parts of the jacket and the shield of a coaxial cable were removed, and a thin layer of copper was first placed on the surface of the cable insulating layer. Then, a thin, similarly sized polyethylene (PE) layer was positioned just above the thin copper. Finally, a special designed copper clip with a Bayonet Neill—Concelman (BNC) connector served as the ground of the capacitive voltage divider and led the signal from the thin copper to the BNC connector. In this divider, the thin copper, the inner conductor, and the cable insulating layer constitute the capacitor CH, whereas the thin copper, the copper clip, and the thin layer of PE comprise the capacitor CL. Thus, the primary capacitive voltage divider is composed of capacitors CH and CL. The structure of the capacitive voltage divider is displayed in Figure 8.

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Recording high-voltage subnanosecond pulses directly with a primary capacitive voltage divider and an oscilloscope is impossible because the divider ratio of this divider is not high enough. Hence, the circuit of the secondary voltage divider was installed along with an attenuator to render the waveform suitable for inspection. The transient response of the entire capacitive voltage divider is depicted in Figure 9. The test result shows that the divider ratio of the entire measurement system is 16400:1.

Figure 9. Transient response of the capacitive voltage divider.

4 SIMULATION AND EXPERIMENTAL TESTS 4.1 SIMULATION RESULTS Figure 8. Structure of the capacitive voltage divider.

According to [11], the values of CH and CL can be calculated using equations (3) and (4).

CH 

 H  0l



 H  0l



W ln(r2 r1 ) r2

CL 

W ln(r3 r2 ) r3

where

 H represents

(3)

(4)

the relative dielectric constant of

l and W denote the length and width of the thin copper, respectively. r1

the insulating layer of the coaxial cable.

corresponds to the radius of the inner conductor of the cable; r2 represents the distance between the axis center of the cable and the thin copper; and r3 indicates the distance between the axis center of the cable and the thin PE layer. CH can be determined with Eq. (3), but the value of r3 is difficult to measure because the surfaces of the thin copper and of the PE layer are closed. Thus, a broadband LCR meter was used to compute CL instead.

Figure 10. Simulation circuit of the entire subnanosecond pulsed power source.

The simulation circuit model of the entire subnanosecond pulsed power source is presented in Figure 10. The stray inductance and capacitance of each part are integrated. The simulated output waveforms at the load without and with a chopping switch are exhibited in Figure 11 and 12, respectively, when the DC power supply is set to 5.5 kV. In both simulations, the peaking switch is “off” and operates efficiently. In the latter simulation, the chopping switch cooperates with the peaking switch at the appropriate point. Figure 11 indicates that the amplitude of the output pulse is

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37 kV and that the corresponding fall-time is 670 ps. Figure 12 shows that the amplitude of the output pulse is 32 kV and that the corresponding fall-time is 570 ps. 0V

-10KV

-20KV

-30KV

-40KV 0s

20ns V(T2:A+)

40ns

60ns

80ns

100ns

120ns

140ns

160ns

180ns

Time

Figure 11. Typical simulated output voltage at the load without a chopping switch. 20KV

results. When the Marx bank was charged to 5.5 kV, all of the switches broke down and the Marx generator was erected. A typical output waveform at the load is illustrated in Figure 14. This waveform was measured by a homemade capacitive voltage divider. The amplitude of the output pulse is 36.1 kV and the corresponding fall-time is 700 ps based on the divider ratio of the measurement system. This amplitude is much greater than that of the output pulse of the Marx generator, which is presented in Figure 4. Second, tests were also performed with chopping switches when the DC power supply was similar to the initial one. A typical output waveform at the load is shown in Figure 15. The amplitude of the output pulse is 31.2 kV, the corresponding fall-time is 600 ps, and the pulse width is approximately 600 ps in accordance with the divider ratio of the measurement system. The experimental and simulation results are highly similar, thus confirming the feasibility of the designed subnanosecond pulsed power source.

10KV

0V

-10KV

-20KV

-30KV

-40KV 46ns 47ns V(T2:A+)

48ns

49ns

50ns

51ns

52ns

Time

Figure 12. Typical simulated output voltage at the load with a chopping switch.

4.2 EXPERIMENTAL RESULTS

Figure 14. Typical output waveform without a chopping switch.

Figure 15. Typical output waveform with a chopping switch. Figure 13. Image of the coaxial Marx generator with a peaking network of resonant output voltage.

A series of tests were conducted under atmospheric pressure conditions to verify the accuracy of the simulation model and the performance of the complete device. The main components of the pulsed power source, namely, the Marx generator and the peaking–chopping switches, are depicted in Figure 13. The main part is 490 mm high, and the metal shell bottom is 450 mm in diameter. First, tests were conducted without chopping switches in a high voltage lab for comparison with the simulated

5 CONCLUSION AND FUTURE WORK A compact, self-contained, adjustable output voltage, subnanosecond pulsed power source is designed and tested successfully in this work. The measured amplitude of the typical pulse is approximately 31.2 kV, the corresponding fall-time is 600 ps, and the pulse width is roughly 600 ps. Tests are conducted under the atmospheric pressure condition. Subnanosecond pulses with repetition frequency and an amplitude of hundreds of kilovolts can be generated when the entire switch system is filled with inert gas, such as nitrogen. Future works should conduct further experiments to



improve the performance of the pulsed power source and to increase the amplitude of output pulses. This pulsed power source can be applied to the field of noninvasive treatments in combination with UWB antennas.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51377175, 51321063).

REFERENCES R. K. Amineh, M. Ravan, A. Trehan and N. K. Ravan, “Near-Field Microwave Imaging Based on Aperture Raster Scanning With TEM Horn Antennas,” IEEE Trans. Antennas Propag., Vol. 59, No. 3, pp. 928-940, 2011. [2] J. F. Kolb, S. Xiao, J. T. Camp, M. Migliaccio, C. Bajracharya, and K. H. Schoenbach, “Sub-Nanosecond Electrical Pulses for Medical Therapies and Imaging,” Fourth European Conf. Antennas and Propagation (EuCAP), Barcelona, Spain, pp. 1-5, 2010. [3] K. H. Schoenbach, S. Xiao, and J. T. Camp, “Subnanosecond Electrical Pulses for Medical Therapies and Medical Imaging,” IEEE Int’l. Power Modulators and High Voltage Conf., Las Vegas, Nevada, USA, p. 60, 2008. [4] J. T. Camp, S. Xiao and K. H. Schoenbach, “Development of A High Voltage, 150ps Pulse Generator for Biological Applications,” IEEE Int’l. Power Modulators and High Voltage Conf., Las Vegas, Nevada, USA, pp. 338-341, 2008. [5] Kentech Instruments Ltd, “High Voltage Avalanche Pulser Summary,” http://www.kentech.co.uk/index.html?/&2, 2011. [6] B. Cadilhon, L. Pécastaing, T. Reess, A. S. Ferron, P. Pignolet, S. Vauchamp, J. Andrieu, and M. Lalande, “High Pulsed Power Sources for Broadband Radiation,” IEEE Trans. Plasma Sci., Vol. 38, No. 10, pp. 25932603, 2010. [7] R. K. Amineh, M. Ravan, A. Trehan and N. K. Ravan, “Near-Field Microwave Imaging Based on Aperture Raster Scanning With TEM Horn Antennas,” IEEE Trans. Antennas Propag., Vol. 59, No. 3, pp. 928-940, 2011. [8] L. Pécastaing, J. Paillol, T. Reess, A. Gibert, and P. Domens, “Very Fast Rise-Time Short-Pulse High-Voltage Generator,” IEEE Trans. Plasma Sci., Vol. 34, No. 5, pp. 1822-1831, 2006. [9] B. Cadilhon, L. Pécastaing, T. Reess, and A. Gibert, “Low-stray inductance structure to improve the rise-time of a Marx generator,” IET Electr. Power Appl., Vol. 2, No. 4, pp. 248-255, 2008. [10] T. Heeren, J. T. Camp, J. F. Kolb, K. H. Schoenbach, S. Katsuki, and H. Akiyama, “250 kV Sub-nanosecond Pulse Generator with Adjustable Pulsewidth,” IEEE Trans. Dielectr. Electr. Insul., Vol. 14, pp. 884-888, 2007. [11] Y. H. Zhang, A. B. Chang, Y. Q. Gan, and T. Y. Miu, “Design of A High Voltage Coaxial Capacitive Voltage Divider,” High Voltage Engineering ., Vol. 29, No. 1, pp. 37-41, 2003.(in Chinese)

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Chenguo Yao (M’08) was born in Nanchong, Sichuan, China, on February 1, 1975. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 1997, 2000, and 2003, respectively. He became a Professor with the College of Electrical Engineering, Chongqing University, in 2007. His current works include pulse power technology and its application in biomedical engineering, online monitoring of insulation condition and insulation fault diagnosis for HV apparatus.

[1]

Zhongyong Zhao was born in Guangyuan, Sichuan, China, on 1 October 1988. He received the B.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 2011, where he is currently working toward the Ph.D. degree with the combined Master–Ph.D. Program in electrical engineering. His areas of research include pulse power technology, online monitoring of insulation condition and insulation fault diagnosis for HV apparatus.

Shoulong Dong was born in Taian, Shandong, China, on 2 December 1989. He received the B.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 2011, where he is currently working toward the Ph. D. Degree in electrical engineering. His areas of research include pulse power technology, new technology of electrical engineering in biomedicine and its treatment apparatus.

Zhou Zuo was born in Sichuan, China, in 1987. He received a Bachelor’s degree in electrical engineering in 2011 from Chongqing University, China, where currently he is a Ph.D. candidate at the State Key Laboratory of Power Transmission Equipment & System Security and New Technology. His research interests include pulse power technology, electrical and thermal ageing of insulation dielectrics and online monitoring of insulation condition.