Transmission line transformer for reliable and low-jitter triggering of a railgap switch Rishi Verma, Ekansh Mishra, Karuna Sagar, Manraj Meena, and Anurag Shyam Citation: Review of Scientific Instruments 85, 095117 (2014); doi: 10.1063/1.4896117 View online: http://dx.doi.org/10.1063/1.4896117 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A novel structure of transmission line pulse transformer with mutually coupled windings Rev. Sci. Instrum. 85, 035110 (2014); 10.1063/1.4867250 A Tesla-type repetitive nanosecond pulse generator for solid dielectric breakdown research Rev. Sci. Instrum. 84, 105114 (2013); 10.1063/1.4826295 An accurate online calibration system based on combined clamp-shape coil for high voltage electronic current transformers Rev. Sci. Instrum. 84, 075113 (2013); 10.1063/1.4815831 Novel high-frequency, high-power, pulsed oscillator based on a transmission line transformer Rev. Sci. Instrum. 78, 074703 (2007); 10.1063/1.2753833 Optimal performance for Tesla transformers Rev. Sci. Instrum. 73, 3332 (2002); 10.1063/1.1498905

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 095117 (2014)

Transmission line transformer for reliable and low-jitter triggering of a railgap switch Rishi Verma,a) Ekansh Mishra, Karuna Sagar, Manraj Meena, and Anurag Shyam Energetics & Electromagnetics Division, Bhabha Atomic Research Centre, Autonagar, Visakhapatnam, A.P. 530012, India

(Received 2 March 2014; accepted 8 September 2014; published online 24 September 2014) The performance of railgap switch critically relies upon multichannel breakdown between the extended electrodes (rails) in order to ensure distributed current transfer along electrode length and to minimize the switch inductance. The initiation of several simultaneous arc channels along the switch length depends on the gap triggering technique and on the rate at which the electric field changes within the gap. This paper presents design, construction, and output characteristics of a coaxial cable based three-stage transmission line transformer (TLT) that is capable of initiating multichannel breakdown in a high voltage, low inductance railgap switch. In each stage three identical lengths of URM67 coaxial cables have been used in parallel and they have been wounded in separate cassettes to enhance the isolation of the output of transformer from the input. The cascaded output impedance of TLT is ∼50 . Along with multi-channel formation over the complete length of electrode rails, significant reduction in jitter (≤2 ns) and conduction delay (≤60 ns) has been observed by the realization of large amplitude (∼80 kV), high dV/dt (∼6 kV/ns) pulse produced by the indigenously developed TLT based trigger generator. The superior performance of TLT over conventional pulse transformer for railgap triggering application has been compared and demonstrated experimentally. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896117] INTRODUCTION

Pinch experiments like Magnetized Target Fusion (MTF) require fast rising, high peak power, and high-energy pulses for their operation. MTF can be achieved using fast compression of liners.1 In liner implosion experiments, the driving electromagnetic pressure directly depends on peak current (PB α I2 ) and for obtaining higher implosion velocities it is desirable to have rate of rise of current as high as possible.2 To perform such experiments, a 1.2 MJ Capacitor Bank “RUDRA” capable of delivering 3.6 MA of peak current with ∼7 μs rise time has been designed and commissioned in our laboratory. The total energy stored in the 1.2 MJ bank is segmented in to six modules (of 200 kJ each) and transferred to load through railgap switch connected with each module. Single module consists of four number of 52 μF/44 kV capacitors, connected through parallel plate transmission lines that further couples to a railgap switch. The individual railgap switch is capable of transferring maximum 10 C of charge at a peak current of 600 kA. All of the six modules deliver energy to central collector plate assembly through RG218 coaxial cables of 3 m length to provide transit time isolation of ∼15 ns. Judiciously chosen transit time isolation ensures safe operation of capacitor bank by avoiding cross flow of energy from one module to another within the specified jitter limit. Optimum operating voltage range of a typical railgap switch is governed by: (i) inter-electrode separation, (ii) choice of the filling gas mixture, and (iii) adjustment of the filling gas pressure. For 10 kV to 30 kV operation range, the a) Author to whom correspondence should be addressed. Electronic

addresses: [email protected] and [email protected]

0034-6748/2014/85(9)/095117/6/$30.00

switch is pressurized with gas mixture of Argon (90%) and Oxygen (10%). Oxygen is electronegative in nature and therefore free electrons are easily captured by neutral oxygen forming negative ions. Operation with [Ar+O2 ] gas mixture provides wider working voltage range, shorter breakdown time delay, and small jitter. This is contributed due to smaller effective collision ionization coefficient of mixed gas. Its smaller value facilitates railgap switch discharge process and results in easier breakdown.3, 4 For obtaining synchronized operation of the parallel connected railgap switches, the characteristic parameters of major concern are (i) breakdown time delay tbd (i.e., the time lag between initiation of trigger pulse and the start of main gap conduction) and (ii) jitter σ bd (i.e., the measure of standard deviation in the breakdown time delay). In addition to operating parameters like main gap voltage, operating gas pressure, and the type of gas mixture, the main factor that prominently affects the tbd and σ bd is the amplitude and dV/dt of applied trigger pulse. In railgap switch, multichanneling along the rails leads to very low switch inductance, attainment of faster rise times, and lessening of electrode erosion. However, multichannels are formed only when the main gap is over-volted uniformly and if many channels are formed before voltage across the main gap drops significantly. The jitter between closing of successive channels must be in the order of one-tenth the fall time of voltage across the main gap.5 The rapid application of high voltage pulse to the edge electrode produces large temporal and spatial electric field gradient that results in the initiation of multiple channels.6, 7 In the present experimental investigation, switching performance of the railgap has been evaluated by iterating two similar amplitude (∼80 kV) trigger

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FIG. 1. Test bench for the characterization of a railgap switch.

pulses of different slew rates produced by pulse transformer (dV/dt ∼0.07 kV/ns) and transmission line transformer (dV/dt ∼6 kV/ns) technique.8 EXPERIMENTAL SETUP AND DIAGNOSTICS ARRANGEMENT

A demountable test bench has been developed to investigate the breakdown characteristics of a railgap switch in correlation with inter-electrode gap, operating voltage, filling gas pressure, and the applied trigger pulse characteristics (refer the set-up shown in Fig. 1). For minimizing the connection inductance, railgap switch under test is directly mounted over the energy storage capacitor (0.25 μF/100 kV) through copper plates of thickness 2 mm. A 50 kV/2 mA battery powered DC-DC switch mode converter has been used for capacitor

charging. A high voltage probe model no. #FLUKE 80K-40 has been used for monitoring the charging voltage in the range of 0–40 kV DC. An indigenously developed Rogowski coil and coaxial capacitive sensor (having bandwidth/sensitivity as 20 MHz (0.5 mV/A) and 50 MHz (0.1 V/kV), respectively) have been used as diagnostic tool for providing time fiducial reference for the measurements of crucial switching parameters like conduction delay and jitter. For high voltage, high bandwidth measurements of fast pulses, a standard 100 kV, 80 MHz probe (PVM-5/North Star) was used. For the precise measurement of filling gas pressure inside the railgap switch, a piezo-resistive transducer (WIKA S-10/Wexlar) has been used. For the time integrated imaging of multi-channel formation along the inter-electrode spacing, a general-purpose digital camera Panasonic Lumix DMC-FS42 (10 megapixel, videography resolution –30 fps) has been used in movie/video recording mode. The data acquisition and storage system consists of Agilent DSO7104B, 4 GS/s, 1 GHz oscilloscope, and a computer. The sensitive electronic equipment and data acquisition systems used in the experiment were installed inside a shielded enclosure and double shielded coaxial cables were used for signal transport, to safeguard them from electromagnetic noise interference generated during experimental runs. For preventing noise injection through ground, oscilloscope and the computer was floated on battery powered inverter supply. Two different pulse generation schemes were indigenously developed and utilized for characterizing switching performance of railgap. The detailed electrical schematic of pulse transformer and transmission line transformer (TLT) based impulse generation schemes have been collectively shown in Fig. 2.

FIG. 2. Electrical schematic of triggering schemes for railgap switch characterization.

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FIG. 3. Waveform of 80 kV output pulse generated by pulse transformer (the rise time tr (10%–90%) is ∼1.2 μs and pulse width (FWHM) is ∼2.2 μs).

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FIG. 4. Waveform of 80 kV output pulse generated by 3-stage TLT (the rise time tr (10%–90%) is ∼13 ns and pulse width (FWHM) is ∼64 ns).

RESULTS AND DISCUSSION

In the pulse transformer scheme, by discharging the 4 μF/1 kV capacitor into the air core step-up transformer of 1:80 gain, an output pulse of about 80 kV with ∼1.2 μs rise time was generated (i.e., dV/dt ∼0.07 kV/ns). The generated output pulse has been shown in Fig. 3. A coaxial cable based three-stage TLT was designed and developed for generating high voltage pulse with dV/dt exceeding >5 kV/ns.9 The utilized URM 67 coaxial cable has an impedance of 50 , an outside diameter of 10.3 mm, and rated for maximum DC operating voltage of 40 kV. In each stage three identical lengths of cables (each of which is 10 m long) have been used in parallel and they have been wounded in separate cassettes to enhance the isolation of the output of the transformer from the input. The input and output impedance of the transformer is ∼5.5  and 50 , respectively. The primary capacitance of ∼8 nF has been used to minimize the droop while the transmission lines are being charged. An indigenously developed trigatron spark gap switch was used to discharge the parallely connected bunch of transmission lines. At ∼20 kV of primary charging, the TLT produces ∼80 kV output pulse of ∼64 ns duration (FWHM) with rise time of about ∼13 ns (10%–90%). This corresponds to voltage gain efficiency of >65% and dV/dt of >6 kV/ns. The uniquely defined 50  output impedance of the TLT based trigger generator facilitates transport of trigger pulse to railgap switch without any distortion in its temporal characteristics. The waveform of TLT generated output pulse has been shown in Fig. 4. This result of implemented TLT configuration evidences polarity inversion at the output. The photograph shown in Fig. 5 illustrates view of integrated assembly and cascaded construction of the TLT based trigger generator. The fabrication methodology ensures that the inductive isolation to ground, from the output of the transformer to the input, increases from the bottom stage upwards, also the potential difference between the top and bottom stage of the transformer is uniformly divided between three stages of the transformer.

For the purpose of investigating switching performance of railgap with two similar amplitude trigger pulses of different temporal characteristics, the first task was to finalize the optimum operating region (i.e., working voltage range) and the corresponding filling gas pressure regime. This finding required determination of “static-breakdown voltage” (SBV) for corresponding filling gas pressures at fixed inter-electrode spacing of 10 mm.10 SBV is defined as the point at which the gap itself breakdowns without being any trigger pulse applied (at a particular pressure and inter-electrode gap). Fig. 6 shows the static-breakdown curve obtained for switch voltage up to 30 kV. The filling gas mixture [Ar (90%) + O2 (10%)] pressure was varied in the range of 0 to 1.4 bars (i.e., relative pressure above atmospheric level). It may be noted that each data point shown in the relevant graphs is an average of 5 consecutive shots taken at an interval of 10 min (as SBV is strongly influenced by the time between discharges). Flushing was necessarily done after the completion

FIG. 5. Integrated assembly of 3-stage TLT based pulser.

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FIG. 6. Operating and static breakdown voltage curves obtained for experimented railgap switch.

and each set of discharge and then fresh gas mixture was filled at required pressure. Below the SBV curve, “optimum operating region” of the railgap switch has been indicated. For the most reliable operation with minimum conduction delay and jitter, triggered gas discharge switches are recommended to be operated in the range of 60% to 80% of SBV. Operation in the range of 50% to 70% of SBV gives longer useable lifetime of switch but it is suggested to be used only for the cases where conduction delay and jitter is not crucial (e.g., in systems with single discharge switches). Operating railgap switch at voltages 0.8 × SBV, results in large increase in the probability of mis-fire and pre-fire, respectively.11 Considering aforesaid issues, railgap characterization experiments have been performed in the range of 14 kV to 22 kV at their respective filling gas mixture pressures. In order to further investigate the role of rate of rise of applied trigger pulse on breakdown/conduction time delay and jitter, output pulses generated by indigenously developed pulse transformer and transmission line transformer were independently fed to the trigger electrode. It may be recalled that both the schemes generate trigger pulses of about −80 kV amplitude, however, the dV/dt generated by the respective schemes were ∼0.07 kV/ns and ∼6 kV/ns, respectively. The breakdown time delay of the switch is characterized as the difference in timing of two signals, i.e., t2 –t1 . Here, t1 is the time instant at which trigger pulse reaches to its peak value and t2 is the time instant at which the railgap switch starts conducting. Jitter is basically defined as the statistical measure of shot-to-shot variation in breakdown time delay. Experimentally, it is calculated as standard deviation of variation in breakdown time delay at fixed operating parameters. Sample waveforms shown in Figs. 7(a) and 7(b) indicate the temporal variation in breakdown time delay obtained by application of low and high dV/dt trigger pulses generated by pulse transformer and TLT, respectively (for same operating voltage and filling gas pressure, i.e., ∼20 kV/1 bar). It may be noted that with the application of similar amplitude

FIG. 7. Sample waveforms indicating variation in breakdown time delay obtained by application of ∼80 kV trigger pulse generated by (a) pulse transformer (dV/dt ∼0.07 kV/ns) and (b) TLT (dV/dt ∼6 kV/ns).

(∼80 kV) trigger pulse having slow rate of rise of voltage (dV/dt ∼0.07 kV/ns), the breakdown time delay and jitter, both the performance parameters of railgap switch are largely affected. Application of slow rate of rise of trigger pulse resulted in unstable breakdown time delay and consequently increased jitter.11 The trends obtained from sampled data in the operating voltage range of 14 kV to 22 kV for breakdown/conduction time delay and jitter measurement, with the use of TLT and pulse transformer as trigger generators, have been shown in Figs. 8 and 9, respectively. The downward inclinations following with increasing main gap voltage, shown in Figs. 8 and 9, explicitly indicate that breakdown/conduction time delay and jitter is consistently lower for the entire range of operating voltage, when the railgap switch was triggered by high rate of rise of pulse (dV/dt ∼6 kV/ns) that is generated by indigenously developed TLT. Another prominent observation in the respective graphs shown is that breakdown time delay and jitter is higher when the railgap switch was operated at its lower threshold of operating region (i.e., ∼0.5 × SBV). Whereas on the

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FIG. 8. Measured breakdown time delay as a function of main gap voltage.

contrary, there is significant reduction in the respective parameters (tbd and σ bd ) when working voltage has been at the higher threshold of operating region (i.e., ∼0.8 × SBV). While using TLT as trigger generator, the breakdown/conduction time delay and jitter reduced up to the limit of ∼60 ns and ∼2 ns, respectively, at ∼22 kV operating voltage. At lower threshold of operating region (∼14 kV), breakdown time delay and jitter remained ∼1100 ns and ∼150 ns, respectively. These findings mainly indicate the influence of the electric field across the main gap and filling gas pressure. A simple empirical description of the behavior of breakdown time delay in pressurized gas discharge switches has been given as12 tbd (ns) = 9.78 × 1013

ρ 2.44 . E 3.44

Here, ρ is the gas density in g/cm3 , tbd is the breakdown time in nanoseconds, and E is the magnitude of mean electric field across the main gap in kV/cm. It may be realized from the above relation that the breakdown time delay is a strong func-

FIG. 10. Sample time-integrated photograph of channel formation recorded during the discharge of railgap switch when it is being triggered with ∼80 kV pulse generated by (a) pulse transformer (dV/dt ∼0.07 kV/ns) and (b) TLT (dV/dt ∼6kV/ns).

tion of electric field across the gap ∝ E−3.44 and thus depends on voltage across main gap electrodes. At lower electric fields statistical processes become more dominant and it increases breakdown time delay as well as jitter.5, 11 Sample time-integrated photographs of channel formations recorded with a general purpose digital camera (Panasonic Lumix DMC-FS42) during the discharge of railgap switch while it is being triggered with ∼80 kV pulse having low (∼0.07 kV/ns) and high (∼6 kV/ns) rate of rise of voltage generated by pulse transformer and transmission line transformer, respectively, have been collectively shown in Fig. 10. No grey filters have been used in between. It may be noted that in the respective photographs, vertical white dribbles are artefacts due to over-saturation of some CCD pixels. For extracting every single frame of captured video, video-to-jpg converter software available from Free Studio, M/s dvdvideosoft has been used. It may be seen that, when the switch is being triggered with pulse having low rate of rise of voltage, the channel formation is localized and breakdown is not uniform across the length of the electrodes. This effect contributes outsized inductance and reduced shot life of switch. Obtained photograph of multi-channel formation while the switch is being triggered with pulse generated by TLT unambiguously evidences distributed charge transfer along the rail electrodes, resulting in low inductance (e.g., for the same operating parameters and experimental conditions, residual system inductances were obtained and as ∼210 nH and ∼170 nH, with pulse transformer TLT, respectively, as trigger generators) and longer shot life of switch. CONCLUSION

FIG. 9. Estimated jitter as a function of main gap voltage.

A compact and inexpensive TLT based trigger generator that is capable of initiating multi-channel breakdown in

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a high voltage, low inductance railgap switch has been conceptualized, designed, constructed, and tested. At ∼20 kV of primary charging, the 3-stage TLT produces ∼80 kV output pulse of about ∼60 ns duration (FWHM) with rise time of ∼13 ns (10%–90%). This corresponds to voltage gain efficiency of >65% and dV/dt of ∼6 kV/ns. The input and output impedance of the transformer is ∼5.5  and 50 , respectively. The uniquely defined 50  output impedance of the TLT based trigger generator facilitates transport of trigger pulse to railgap switch without any distortion in its temporal characteristics. Aforementioned performance parameters of newly developed TLT ensure its utilization for obtaining reliable and synchronized triggering of parallely connected railgap switches in a high-energy capacitor bank. The comprehensive summary of important conclusions drawn by characterizing multi-channel switching performance of railgap switch while iterating over two similar amplitude trigger pulses of different temporal characteristics are: (i) For initiating multi-channel breakdown, rate of rise of trigger voltage (dV/dt) must be considerably high (typically exceeding >5 kV/ns) along with amplitude that largely exceeds the main gap voltage (typically >3× ). Multichanneling was noted to be more dominant when working voltage was at higher threshold of operating region (i.e., ∼0.8 × SBV). (ii) Irrespective of rate of rise of trigger pulse voltage, characteristic breakdown time delay and jitter of the railgap switch reduces with increase in the voltage applied across railgap electrodes (i.e., working voltage). (iii) Triggering of railgap switches by high voltage pulses of low slew rate results in localized breakdown rather being distributed over the electrode length.

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ACKNOWLEDGMENTS

The authors are grateful to P. Lahiri, Regional Director, BARC, Visakhapatnam, for the consistent encouragement, guidance, and support. 1 M.

M. Basko, A. J. Kemp, and J. Meyer-ter-Vehn, “Ignition conditions for magnetized target fusion in cylindrical geometry,” Nucl. Fusion 40(1), 59–68 (1999). 2 M. G. Haines, “A review of the dense Z-pinch,” Plasma Phys. Controlled Fusion 53(9), 093001 (2011). 3 K. Masugata, H. Tsuchida, H. Saitou, and K. Yatsui, “Studies and performance of decreased railgap switch inductance by enhancing multichanneling via gas mixture,” IEEE Trans. Plasma Sci. 25(1), 97–99 (1997). 4 Z. Liu, J. Zeng, F. Sun, and A. Qiu, “Breakdown properties on gas rail gap switch with operating voltage of ±100kV,” in Proceedings of the International Conference on Electrical Machine and Systems (ICEMS) (IEEE, 2008), pp. 897–900. 5 K. R. LeChien and J. M. Gahl, “Investigation of a multichanneling, multigap Marx bank switch,” Rev. Sci. Intrum. 75(1), 174–178 (2004). 6 A. A. Kim, B. M. Kovalchuk, V. V. Kremnev, E. V. Krumpjak, A. A. Novikov, B. Etlicher, L. Frescaline, J. F. Leon, B. Roques, F. Lassalle, R. Lample, G. Avrillaud, and F. Kovacs, “Multi gap, multi channel spark switches,” in Proceedings Of the 11th IEEE International Pulsed Power Conference (IEEE, 1997), pp. 862–867. 7 B. M. Kovalchuk, “Multi gap spark switches,” in Proceedings Of the 11th IEEE International Pulsed Power Conference (IEEE, 1997), pp. 59–67. 8 J. C. Martin, “Nanosecond pulse techniques,” Proc. IEEE 80(6), 934–945 (1992). 9 G. R. Neil and R. S. Post, “Multichannel, high energy railgap switch,” Rev. Sci. Instrum. 49(3), 401–403 (1978). 10 J. Niedbalski, “High voltage multichannel rail gap switch triggered by corona discharges,” Rev. Sci. Instrum. 74(7), 3520–3523 (2003). 11 A. K. Saxena, T. C. Kaushik, M. P. Goswami, and S. C. Gupta, “Printed circuit board based electrically triggered compact railgap switch,” Rev. Sci. Instrum. 81, 056106 (2010). 12 T. H. Martin, “An empirical law for gas switch breakdown delay,” in Proceedings of the 7th IEEE International Pulsed Power Conference (IEEE, 1989), pp. 73–79.

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Transmission line transformer for reliable and low-jitter triggering of a railgap switch.

The performance of railgap switch critically relies upon multichannel breakdown between the extended electrodes (rails) in order to ensure distributed...
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