Repetitive Nanosecond-Pulse Breakdown in Tip

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 5, OCTOBER 2006

Repetitive Nanosecond-Pulse Breakdown in Tip–Plane Gaps of Air Tao Shao, Guangsheng Sun, Ping Yan, Member, IEEE, and Shichang Zhang

Abstract—Repetitive pulsed power is becoming an important area of high-power technology. Dielectric failure data concerning electrical insulation play a basic role, but breakdown has been inadequately studied for the repetitive nanosecond-pulse conditions. This paper is concerned with the breakdown characteristics of tip–plane gas gaps under repetitive burst conditions at variant repetition rates (rep-rates) and diverse gap distances. The relationship among applied voltage, breakdown time lag, number of applied pulses to breakdown, repetitive pulse stress time, and rep-rates is presented. The experimental results presented show that breakdown polarity dependence is not distinct. The data also indicate that significant concentrations of excited particles and residual charges would be formed during the consecutive nanosecond pulses and would present a memory effect that affects the development of gas breakdown. In addition, the detachment of negative ions, cathode collision of positive ions, and deexcitation of metastable species can provide the source of avalanche-initiating electrons. Index Terms—Gas breakdown, memory effect, nanosecond breakdown, number of applied pulses, repetitive pulse stress time, tip–plane geometry.

I. I NTRODUCTION

T

HE APPLICATION of repetitive pulsed power technology is rapidly expanding not only in the laser high-energy physics realm but also in the practical industrial regime, such as materials treatment, waste processing, and thick-film deposition. Most of the basic short-pulse electrical breakdown data are related to a single nanosecond pulse [1]–[6], but data concerning electrical insulation under repetitive pulse conditions are not available. Therefore, information about dielectric characteristics and fundamental breakdown processes for repetitive pulse conditions is needed. Compared with the conventional gaseous breakdown investigations with dc, ac, or microsecond-pulse applied voltage, the information on breakdown with nanosecond pulse is very limited. In past studies, most of these limited experimental results were based on uniform fields using plane–plane or sphere–sphere electrodes at different gaseous pressures [1]–[5]. Nesterikhin et al. investigated pulse breakdown in extremely Manuscript received September 29, 2005; revised March 31, 2006. This work was supported in part by the National Nature Science Foundation of China under Contracts 50207011 and 50437020. T. Shao is with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100080, China, and also with the Graduate University of the Chinese Academy of Sciences, Beijing 100039, China (e-mail: [email protected]). G. Sun, P. Yan, and S. Zhang are with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100080, China (e-mail: [email protected]; [email protected]; [email protected]). Digital Object Identifier 10.1109/TPS.2006.877213

nonuniform field created by point–plane electrodes at atmospheric pressure. In this paper, the distribution of breakdown time lag was discrete even if the gap was illuminated [1]. Mankowski et al. reported breakdown results of point–plane electrodes in air at higher pressures, in which the breakdown strength of the point–cathode increased by 30%–80% when compared with point–anode dielectric strength for air [5]. Krompholz et al. studied point–plane breakdown of 1 mm gap distance in the subnanosecond regime below 15 kV and found a similar breakdown polarity dependence. The authors concluded that the breakdown mechanism was dominated by electron field emission [6]. These experimental investigations were about the single-pulse breakdown phenomenon of nanosecond or subnanosecond regime. For the sake of practical application, most experimental investigations about the breakdown characteristics under repetitive pulsed conditions are of liquid and solid dielectrics [7]–[9]. In these cases, the pulse breakdown field of transformer oil is reduced by a factor of 2 compared with that under the single-pulse condition in the experimental study made by Lehr et al. [7]. Other studies on pulsed corona discharge or glow discharge for repeated conditions [10]–[14], such as Allen et al.’s measured variations of positive and negative ion densities at a repetition rate (rep-rate) of one per 20 s. In this paper, a corona discharge results in an increase in the population of ions of the same polarity as the corona electrode, whereas the density of ions of opposite polarity is decreased [10]. To simulate corona discharges in ultrawideband radiating systems, Mankowski et al. also investigated subnanosecond corona inception in a coaxial chamber at a rep-rate up to 6 kHz. This paper indicated that the transmitted peak power decreased as the rep-rate increased [12]. Repeated glow discharges were described under low pressures in [13] and [14], and a memory effect of charged and metastable species plays a dominant role. We investigated the breakdown characteristics of an atmospheric air for parallel-plane gaps by repetitive burst condition and found that the electric breakdown strength is close to the dc value when rep-rate is up to 1 kHz [15]. The work presented in this paper focuses on the pulse breakdown characteristics of a nonuniform field using tip–plane gaps due to repetitive pulses at different pressures. II. E XPERIMENTAL S ETUP AND M EASUREMENT A schematic diagram of the experimental setup arrangement is given in Fig. 1. In this experiment, repetitive nanosecond pulses were produced by a semiconductor opening switch (SOS)-based pulse generator called SPG200 [16]. The SPG200 consists of three units, namely: 1) primary charging unit;

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SHAO et al.: REPETITIVE NANOSECOND-PULSE BREAKDOWN IN TIP–PLANE GAPS OF AIR

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Fig. 1. Schematic diagram for the repetitively pulsed experiment. The areas with dashed lines show that these are immersed in the transformer oil.

2) magnetic compression unit; and 3) SOS amplifying unit. The SPG200 has an output voltage range of 0 to −200 kV with a rise time of approximately 10 ns and a full-width at half-maximum of 20–30 ns. The rep-rates vary from single shot to 2000 Hz. A trigger generator was used to control the rep-rate. The output voltage was adjusted using a cycled saltwater solution through a parallel connection with the discharge circuit. To avoid damage to the SPG200 and the resistive shunt when tip–plane gap was short-circuited, a 200-Ω resistor was used in series with the gap. The limiting resistor was connected in series with the test chamber, and the charging time of the discharge circuit is below 1 ns. The columned test chamber was made of polymethyl methacrylate (PMMA), and the tip–plane brass electrodes were mounted along the axial direction. The 2-cm-diameter 4-cmlong tip electrode was asymmetrical with a taper geometry and a tip diameter of 2 mm. The diameter of the plane electrode was approximately 6 cm. The point–plane gap voltage was measured using a capacitive voltage divider with a voltage ratio of approximately 4780, which is located at the output end of SPG200 close to the SOS. The breakdown current was detected by a wideband shunt made of an inductance-free tubular resistor of metal foil with a resistance of 0.206 Ω. When breakdown occurred, the signal detected by the resistive shunt, via 20 dB attenuation, was used to trigger the oscilloscope TDS684A (with a bandwidth of 1 GHz and a time resolution of 5 GS/s) to record both voltage and current signals simultaneously. In repetitive nanosecond-pulse breakdown experiments, breakdown takes place after a series of nanosecond pulses, therefore, the time over which the voltage is applied and the number of applied pulses are important parameters to analyze breakdown characteristics. In our experiments, the TDS684A recorder cannot obtain the pulse-by-pulse train of nanosecond width and millisecond interval. We provided a convenient method to measure the time duration from the first applied pulse to the breakdown occurrence. In addition, the pulse number could be calculated by the time duration. The measuring arrangement is also shown in Fig. 1. The repetitive pulse stress time tRST was measured by TDS3054B with a bandwidth of 0.5 GHz and a time resolution of 5 GS/s. The measurement method is outlined as follows: The sweep start signal from the trigger generator was recorded by the one channel of TDS3054B, as shown in the upper waveform in

Fig. 2. Typical waveforms of applied voltage, discharge current, and repetitive pulse stress time. (a) Applied voltage and breakdown current. (b) Sketch of the repetitive pulse stress time.

Fig. 2(b). When breakdown occurred, a low-level signal generated by one transistor–transistor logic (TTL) port of TDS684A was also recorded by the other channel of TDS3054B simultaneously. As shown in the lower waveform in Fig. 2(b), time interval from the “T” sign to the drop point was repetitive pulse stress time. The number of applied pulses can be obtained by taking the integral part of the formula f · tRST + 1, where f is the applied rep-rate. Considering that the signal time delay of the measurement circuit is several hundred nanoseconds, the applied maximal rep-rate is 1 kHz, and the measured repetitive pulse stress time is on the microsecond scale or longer, hence, the time delay of the measurement circuit can be ignored. Typical waveforms are shown in Fig. 2(a) and (b). III. E XPERIMENTAL R ESULTS Due to the random nature of the breakdown, the measured data are scattered. Some experimental results are summarized by the several graphs shown in Figs. 3–5. The error bars in all figures indicate one standard deviation above and below


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Fig. 3. Breakdown time lag versus repetition rate. (a) Negatively charged plane. (b) Negatively charged tip.

the mean value of the measured data. Although the output of SPG200 is negative pulse and we exchanged the relative positions of the tip and plane electrodes to change the polarity of the tip or plane electrodes, the experimental results with the negatively charged tip or plane electrodes were obtained as shown in Figs. 3–5, respectively. The experiments with an applied voltage peak of 80 kV at pressure p = 760 or 1520 torr and gap distance d = 5 or 1 mm were designed, and approximately ten times of breakdown measurements were made for each data point. From the waveform of the breakdown current shown in Fig. 2(a), the observed breakdown time lag is defined as the time interval between the initial spike current resulting from the displacement current across the gap and the subsequent large current rise. Fig. 3 shows that with the increase of applied reprates, breakdown time lags decrease regardless of negatively charged plane or tip electrodes. Moreover, when rep-rates are up to 0.5 or 1 kHz, the difference of the time lags is minor. Fig. 4, similar to Fig. 3, indicates that as the pulse rep-rate is increased, the repetitive stress time decreases regardless of the polarity of electrodes. Comparing Fig. 4 with Fig. 5, it is found that the values of repetitive stress time below 0.5 or 1 kHz are approximately the same or much smaller than those under lower rep-rates, but the number of pulses required for

Fig. 4. Repetitive pulse stress time versus repetition rate. (a) Negatively charged plane. (b) Negatively charged tip.

breakdown under high pulse rates (0.5 or 1 kHz) is larger than those required at lower pulse rates. The possible reasons on the scattered measured data are given as follows: First, the output high voltage of SPG200 is not stable and rises with the increase of load resistance. Even for the stable resistive load, the jitter of the output voltage amplitude is approximately 3%. In our experimental conditions, the resistance of the cycled saltwater solution load was not very constant. Second, the pulse rate was controlled by the trigger generator, but the measurement of pulse rate was subject to a small error. Third, the formula f · tRST + 1 was used to calculate pulse numbers, measuring error of stress time magnified f times. IV. D ISCUSSIONS Using nonlinear fitting, the measured data comply with the law of exponential decay curve of the first order. Figs. 6 and 7 describe the fitting curves of breakdown time lag and repetitive stress pulse time with applied rep-rates in the range of 10–1000 Hz, respectively. The difference of time-lag values for the negatively charged plane and tip electrodes is small. In conventional nonuniform breakdown experiments with dc, millisecond, or microsecond-pulse applied voltage, breakdown strength dependence on voltage polarity is generally accepted.


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Fig. 6. Nonlinear fitting curves of breakdown time lag versus repetition rate.

Fig. 5. Number of applied pulses versus repetition rate. (a) Negatively charged plane. (b) Negatively charged tip.

Even in single nanosecond-pulse breakdown, polarity dependence also exists [5], [6], i.e., the breakdown time lag of the positively charged tip should be lower than that of the negatively charged tip under the same conditions; but in our experimental conditions, it is not notable. This is likely due to lots of ions and excited metastable particles, which have survived from the previous pulses, and the action of those space charges by which the contribution on polarity dependence in the tip–plane geometry becomes weaker than the foregoing experiments. Actually, the detailed reasons have not been cleared up to now and need further studies. Nanosecond-pulse breakdown theory is far from being distinct. Some known models have been used to explain the breakdown mechanism [3]. Generally, the conventional avalanche-to-streamer theory is used, but its explanation of the streamer development has some deficiencies, especially in the formative breakdown time lag of several nanosecond or subnanosecond orders. In addition, the models concerning runaway electrons are promising in explaining breakdown under the nanosecond-pulse conditions. In our repetitive pulsed experimental investigations, breakdown mechanism must be associated with memory effect, which has been studied in low pressures of several torr orders [14]. Memory effect would provide some effective initial electrons of breakdown required and affect the development of the streamer. In low pressures,

Fig. 7. Nonlinear fitting curves of repetitive pulse stress time versus repetition rate.

secondary electrons released from the cathode and created by the residual charges and metastable species play an important role in the initial avalanche formation. In our experiments, in addition to the cathode collision of positive ions and deexcitation of metastable species, collisional detachment of negative ions would be an important fashion for providing effective initial electrons, such as A− + B →

A+B+e . AB + e

(1)

A lot of investigations have found that negative ions can strongly influence the discharge inception through collisional detachment [11], [17]–[21]. The removal from insulating gas of any residual ionizations by recombination, attachment, and deexcitation is approximately several tens of microseconds after switching, but the complete recovery of the insulating capability of the gaps would take up to a few seconds [17]. The memory effect of positive ions would last for several hundred microseconds, and the effect of metastable species would last for hours in some repeated pulse glow discharge [14]. Although the densities of ions and metastable species were not measured,


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it is reasonable to assume that large concentrations would exist in the gap after applying consecutive pulses, and this would affect the discharge inception. As shown in Figs. 6 and 7, when the rep-rate is below 400 Hz, memory effect is dominated by the excited metastable species, such as some vibrational and electronic particles. In addition, when the rep-rate exceeds several hundred hertz, space residual ions result in memory effect. Especially while the tip electrode is negatively charged, breakdown time lag and repetitive stress time are less than those under the negatively charged plane electrode. We suppose the reason is that some electrons released by the detachment of negative ions would result in the initiation of electrical breakdown. For the negatively charged tip electrodes, lots of electrons would attach to neutral particles and form negative ions. The energy for the detachment needed is much easier than that for ionization, i.e., the energy for the detachment of O− 2 is approximately 0.5 eV, whereas the energy for the ionization electrons is approximately 12.2 eV [21]. Moreover, it had been confirmed experimentally that ion detachment would provide “seed” electrons for initiating breakdown in some atmospheric discharge [11], [17]–[21]. In addition, the reduction for breakdown time lag is primarily the result of a reduction in the statistical and formative time lags due to memory effect, which provides large density of initial electrons. Furthermore, the remarkable applied number under high rep-rates would result in some gas heating, which would also contribute to the decrease in time lag. If (2) is satisfied, most of the heat generated during a pulse will remain in the interelectrode during interpulse interval [8], where tp is the interpulse interval, Cv is the heat capacity, d is the gap distance, and k is the thermal conductivity. In an atmospheric air of ambient temperature, Cv is 1202 J/m3 · K, and k is 2.6 × 10−3 W/m · K [8]; for 1 cm gap distance, when tp < 4.62 s, heat will accumulate. This is easily satisfied in our experimental condition because all interpulse intervals are below 0.1 s; hence, interelectrode temperature will rise after a large number of pulses. However, gas heating is not very significant, and the influence is secondary because the corona discharge is an incomplete breakdown, i.e., tp