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We have investigated the guiding and triggering of discharges from a Tesla coil type 280 kHz AC high voltage source using filaments created by a femtosecond Terawatt laser pulse. Without the laser the discharges were maximum 30 cm long. With the laser straight, guided discharges up to 110 cm length were detected. The discharge length was limited by the voltage amplitude of the Tesla coil.
©2012 Optical Society of America
1. Introduction
There have been several reports of the use of filaments created by high peak power femtosecond laser pulses for guiding high voltage (HV) discharges over meter length distances using unipolar pulsed and DC HV sources during recent years [1–6]. The work has mainly been motivated by triggering of lightning from storm clouds for e.g. airport protection.
In our experiment we use a Tesla coil that is a low cost and compact source for pulsed AC HV of a few hundred kV. We do this to evaluate if triggering and guiding discharges from AC HV sources over distances of several meters is possible. The discharge occurs during a time that is much shorter than the oscillation period of the AC voltage and the charge transfer is probably unidirectional. Possible applications exist in e.g. testing of discharge sensitivity of large electrical installations where it could be of interest to precisely direct multiple discharges to the same point.
A laser beam with peak power higher than the critical power, a few GW for 800 nm wavelength in air, will self-focus by the non-linear Kerr effect. When the intensity gets high enough a weak plasma is created by multi-photon ionization. A dynamic balance between the self-focusing and the defocusing from the plasma causes self-guiding of the laser beam over distances that may be order of magnitudes longer than the Rayleigh distance. This long thin structure is often called a filament. Detailed discussion of the filamentation process, its consequences and applications is available in e.g [7–9]. Often external focusing by lenses or mirror telescopes is used to control the position of the filament [10].
The rather weak plasma (of the order of 1016 cm−3 [11]) that is generated by the laser beam will reduce the breakdown voltage of the air and guide HV discharges. The free electrons have a short lifetime, almost independently of the initial concentration the free electron density will be reduced to below 1011 cm−3 in less than 1 µs [12]. The removal of free electrons is to a large part caused by attachment of electrons to oxygen molecules, creating O2--ions. The electrons are less tightly bound to these ions than to the neutral molecules, reducing the necessary electrical field for impact ionization, and the effect of lowering the breakdown voltage may thus last longer than the lifetime of the free electrons. The exact process of the laser guided discharge is not yet well known and will not be clarified here even though we hope that the experimental data we provide can help in more detailed investigations. The mechanism behind triggering and guiding of discharges of DC voltage using ultrashort laser pulses is discussed in e.g [13–15].
2. Laser guiding setup
2.1 Tesla coil high voltage source
The Tesla coil HV source is an air core transformer developed by Nikola Tesla in 1891. There are today a number of design variations, we used a static spark gap variety. The off-the-shelf Tesla coil (Information Unlimited, BTC40, http://www.amazing1.com/tesla.htm#BTC40) was modified to allow synchronization with the femtosecond laser.
The actual Tesla coil consists of two inductively resonantly coupled LC-circuits. The electrical setup is illustrated in Fig. 1 . The primary circuit contains a large capacitance (CP) and a small inductance (LP), while the secondary circuit is a large inductance (LS) and a top load toroid with a small capacitance to the ground (CS). The primary circuit also includes a current switch in the form of a spark gap to induce a rapid change in the current. During the charging phase the spark gap is non-conducting and the primary capacitance is charged to 4 kV from iron core transformers coupled to standard single phase 120 V, 60 Hz power. The power was provided by a petrol generator to avoid disturbances on the power grid of our facility. The charging circuit was modified to include a rectifier diode to keep the voltage over the capacitance constant once the peak voltage has been reached. The spark gap was set wide enough that no spontaneous discharge was possible from the charging voltage. To trigger the spark gap at a well-defined moment in time a nanosecond laser pulse from a Q-switched Nd:YAG laser was focused on one of the electrodes inside the spark gap. The laser pulse created a small plasma that initiated a full breakdown of the spark gap. When the primary LC-circuit is closed by the conducting plasma in the spark gap a rapidly (280 kHz in our case) oscillating current will transfer the stored energy back and forth between the capacitance and the inductance. By inductive coupling this current will generate an oscillating current in the secondary circuit where HV is created on the top load because the smaller capacitance needs higher voltage to store the same energy. The output HV waveform could not be directly measured, but from discharge measurements we infer that we generated a 280 kHz oscillating voltage pulse with around 10 µs envelope. Two decoupling coils (LC) are used to protect the charging circuit from the rapidly oscillating current. The primary capacitance was nominally 22 nF, giving a stored energy of 0.7 J per discharge. A metal screw was fixed to the Tesla coil top load to provide a break-out point for the discharges and concentrate them to the point where the laser beam passed.
2.2 Laser
The Terawatt & Terahertz portable laser facility used for these experiments is a 4 TW Ti:Sapphire laser system delivering 160 mJ per pulse with a pulse duration of 40 fs. The central wavelength is 805 nm and the pulse repetition rate is 10 Hz. The compact laser system sits on a 1.25 m by 2.5 m optical table. The laser system has been integrated into a standard sea container into which a modular clean room of class 100,000 was inserted. The clean room occupies about half of the container, the remaining part being used as a control room. The control room is filled with the air conditioning unit, the power supplies and chillers for the laser systems as well as the control rack. The user is able to fully control the laser either from the clean room, the control room or from the exterior of the container via an Ethernet link. The control room also acts as an airlock for the clean room.
2.3 Experimental method
The freight container with the mobile laser system was inside a garage next to the test area to protect it from possible electromagnetic interference. The distance from the laser to the Tesla coil was around 15 m. The laser beam was sent out through a pellicle-covered hole in the garage door. The beam was focused by a 5 m focal length lens (laser beam diameter of 2.5 cm FWHM) to control the position of the filaments. The FWHM length of the filament fluorescence was measured to 0.8 m, but there was clearly measurable fluorescence over 1.8 m. The measurement of the UV fluorescence was performed by scanning a PMT covered by a UG11 filter along the laser beam path. The filament was positioned so that it started slightly before the HV electrode and the strongest part was approximately 1 m further away.
The laser passed right next to the two electrodes, with the metal tip on the Tesla coil closest to the laser and a metal target rod connected to ground that was used as second electrode further away, as shown in Fig. 2 . The ground of the Tesla coil secondary circuit and the ground of the target rod were both connected to the same buried ground electrode. During all measurements care was taken that the filaments did not impact the electrodes and no plasma was generated on them. The filaments were however close enough to the electrodes that the weak outer part of the background photon bath hit the electrodes and the reflected laser light could be detected. The distance between the electrodes was varied from 10 to 150 cm.
Fig. 2 The experiment setup with a 80 cm long guided discharge as imaged by video camera. The Tesla coil is seen on the left and the target electrode on the right. Image has been modified for enhanced viewing.
The femtosecond laser was used as master clock and a delay generator was used to adjust the timing of the Q-switched Nd:YAG laser that triggered the Tesla coil. The timing jitter of the spark gap firing compared to the femtosecond laser pulses was around ten nanoseconds pulse to pulse, but there was a thermal drift of a few microseconds per hour in the Nd:YAG laser. The timing was in some cases measured to allow compensation for this drift in the analysis. The trigger signals were sent using fiber optics to avoid disturbances from the electromagnetic fields.
We used a gated ICCD camera with a UG11 filter to image the discharges during daytime. The ICCD camera was used with 0.2 to 10 µs gate widths to try to capture the dynamics of the discharges and with longer gates (100 µs) to measure the discharge probability. The UG11 filter has its main transmission between 250 and 400 nm, but also a secondary transmission peak around 700 to 800 nm. During evening and night-time measurements images of the discharges were also captured with a standard video camera.
A Hall effect Ampere meter was used to measure the current in the wire from the target rod to the ground. The Ampere meter had only 35 kHz bandwidth and could thus not be used to measure the actual current, but only to confirm discharges. The signal often saturated, indicating peak currents larger than 600 A. A photo-multiplier tube (PMT) with a lens telescope was used to image the discharges from a distance to measure the plasma emission with high time resolution. Also the PMT was covered with an UG11 filter to reduce the background.
3. Results
Without any filament bridging the electrode gap the discharges from the Tesla coil follow an erratic path as shown in Fig. 3 . The maximum natural discharge length measured was 30 cm. We were unable to probe the voltage with the available equipment without perturbing the HV circuit of the Tesla coil, and the peak voltage is thus unknown. The electric field necessary for breakdown is around 3 × 106 V/m for DC fields with electrodes that are large compared to the distance between them. The small radius electrodes used in our setup should reduce the necessary field strength. It is probable that the voltage from the Tesla coil was a few hundred kV, while the theoretical value provided by the manufacturer for optimum tuning was 500 kV. The voltage may have varied during the experiments because of variation in temperature and moisture content of the air, which will affect the capacitance of the Tesla coil top electrode and thus the energy transfer efficiency from the primary to the secondary circuit.
When the gap between the electrodes was bridged by the laser-generated filament longer straight discharges occurred, as for the 100 cm gap in Fig. 4 . At gaps longer than 80 cm initiation of a discharge did not always succeed, but most of the time a streamer that followed the laser path was generated even though it did not reach the target electrode.
We have tried to capture the dynamics of the streamer propagation across the gap and the discharges using shorter gate widths for the ICCD camera. The position of the fluorescence produced by the streamers moves in rapid leaps that are separated by approximately 2 µs, between these leaps the fluorescence fades out. When the gap is bridged the discharge appears as a stronger flash covering the whole distance at once. The peak voltages of the 280 kHz AC waveform are separated by 1.8 µs, which is consistent with the temporal separation in the data. We never saw any UV emission close to the ground electrode until the actual discharge. The propagation of streamers is thus unidirectional in our experiment, and starts from the toroid of the Tesla coil.
The timing between the nanosecond laser pulse that triggers the HV source and the femtosecond laser pulse that generates the filament with free charges is critical. In Fig. 5 it is evident that there is an approximately 5 µs long time window where the probability for guided discharges is high. Discharge statistics were derived from Ampere meter data and the presence of discharges was also confirmed by the visible light emission. The measurement uncertainty does not allow us to conclude if the variation within this time region is random or if there are some deterministic factors arising from the AC nature of the voltage.
Fig. 5 Percentage of guided discharges (Extracted from 30 pulses per delay) measured at different gap distances for varying timing between Tesla coil and laser pulses. The exact timing of the femtosecond laser pulse is not known, but the offset should be constant. Longer delays means that the femtosecond laser pulse comes before the peak voltage, shorter delays that the femtosecond laser pulse comes after the peak voltage.
Figure 5 also shows that we go from very high probability of guided discharges over 80 cm for the right timing to just a few discharges (around 20%) over 110 cm and no discharges at all recorded for 120 cm gap between the electrodes.
The PMT measurements of discharges in Fig. 6 show the timing of the discharges compared to the fs laser pulse. The discharges appear between 3 and 12 µs after the filament was created for 70 cm discharge length. There is never more than one spike per AC HV pulse, showing that we extract a large enough portion of the energy stored in the Tesla coil with the discharge that there is no current transfer in later oscillation of the AC waveform. This has been illustrated by highlighting six of the traces in Fig. 6 in different colors. The approximately 1.8 µs delay between bunches of discharges fits very well to half the period of the 280 kHz AC frequency of the HV and is the same as seen with the ICCD. The amplitude of the PMT signal should correspond to the UV emission from the discharge and probably be proportional to the current in the discharge. The variation of signal strength with delay seen in Fig. 6 indicates that the amplitude, and thus the strength of the discharges, depends on the voltage at the instant when the discharge occurs.
Fig. 6 Traces of 129 individual 70 cm long discharges as measured with PMT. 6 traces were highlighted in different colors to show the individual measurements with a single pulse per discharge. The time zero corresponds to the fs-laser pulse.
As the time resolution of the PMT and of the oscilloscope is higher than the FWHM of the measured signal from the PMT it is probable that the 52 ns FWHM width with 9 ns standard deviation of the fluorescence signals in Fig. 6 is related to the temporal width of the current pulse in the discharge. The mean FWHM values of the temporal width τ for the fluorescence signal show a linear dependence on the discharge length d with a fit τ = 14 ns + 54 ns/m × d, where d was varied from 40 to 100 cm. There seems to be no correlation between amplitude and width of the PMT signal.
4. Discussion
Using TW peak power ultrashort laser pulses to create a line of free charges in air through filamentation increases the maximum discharge distance from 30 to 110 cm in our experiment using a Tesla coil pulsed AC HV source. This is a larger effect from the laser guiding than what has been measured with pulsed DC HV sources where the reduction of the breakdown voltage for long discharges has been only around 30-40% [1,3].
The maximum guided discharge length achieved in these trials was 1.1 m. The filament was strong past the target electrode and movement of the focusing lens by decimeters did not seem to impact the discharge probability, so the limiting factor is not the length and plasma distribution of the filaments formed. In pulsed DC HV discharge experiments using 2.2 MV voltage discharge over up to 4.5 m gaps has been measured, but with a maximum guiding distance of 2.4 m and the remainder as unguided discharges [2]. There it was determined that the 1 µs lifetime of the laser initiation was the limiting factor for the guided length. As there is no evidence of unguided prolongation of the discharges in our case it is probable that the voltage is our main limitation.
With a pulsed DC HV-source it was concluded that the maximum guided distance is 2.4 m and that this distance is limited by temporal effects in the relation between the lifetime of the filament and the propagation speed of the discharge channel [2]. We are not able to use our data to determine if a similar limitation is present when using a pulsed AC HV-source and further measurements with higher voltage are needed to explore any temporal limitation.
It is also possible that the frequency of the AC HV may have an impact on the maximum guiding distance. In our experiments we were not able to vary this parameter. Our observations were that the streamer propagation across the gap occurred as jumps when the instantaneous voltage was high. Further experiments are needed to determine if a high frequency with many short voltage peaks or a lower frequency with longer voltage peaks will produce the longest discharges.
In conclusion we have shown triggering and guiding of electrical discharges over up to 1.1 m from a 280 kHz AC high voltage Tesla coil using a filament created by self-focusing of a TW power femtosecond laser pulse. This is a factor 3.7 longer than the 30 cm discharges observed without the triggering laser. The streamers grew in leaps separated by half the oscillation period and the current-carrying discharge occurred as a sub-100 ns pulse when the leader bridged the gap. The guiding distance was limited by the voltage amplitude and further experiments with a higher voltage source is necessary to find the maximum guiding distance for discharges initiated by AC HV.
Acknowledgments
We acknowledge the support from the Defence Research & Development Canada Applied Research Program. M. Henriksson acknowledges strategic research funding from the Swedish Armed Forces. The authors acknowledge technical support from Marcellin Jean and Alain Fernet.
References and links
1. M. Rodriguez, R. Sauerbrey, H. Wille, L. Wöste, T. Fujii, Y.-B. André, A. Mysyrowicz, L. Klingbeil, K. Rethmeier, W. Kalkner, J. Kasparian, E. Salmon, J. Yu, and J.-P. Wolf, “Triggering and guiding megavolt discharges by use of laser-induced ionized filaments,” Opt. Lett. 27(9), 772–774 (2002). [CrossRef] [PubMed]
2. R. Ackermann, G. Méchain, G. Méjean, R. Bourayou, M. Rodriguez, K. Stelmaszczyk, J. Kasparian, J. Yu, E. Salmon, S. Tzortzakis, Y.-B. André, J.-F. Bourrillon, L. Tamin, J.-P. Cascelli, C. Campo, C. Davoise, A. Mysyrowicz, R. Sauerbrey, L. Wöste, and J.-P. Wolf, “Influence of negative leader propagation on the triggering and guiding of high voltage discharges by laser filaments,” Appl. Phys. B 82(4), 561–566 (2006). [CrossRef]
3. G. Méjean, R. Ackermann, J. Kasparian, E. Salmon, J. Yu, J.-P. Wolf, K. Rethmeier, W. Kalkner, P. Rohwetter, K. Stelmaszczyk, and L. Wöste, “Improved laser triggering and guiding of megavolt discharges with dual fs-ns pulses,” Appl. Phys. Lett. 88(2), 021101 (2006). [CrossRef]
4. T. Fujii, M. Miki, N. Goto, A. Zhidkov, T. Fukuchi, Y. Oishi, and K. Nemoto, “Leader effects on femtosecond-laser-filament-triggered discharges,” Phys. Plasmas 15(1), 013107 (2008). [CrossRef]
5. H. Pépin, D. Comtois, F. Vidal, C. Y. Chien, A. Desparois, T. W. Johnston, J. C. Kieffer, B. La Fontaine, F. Martin, F. A. M. Rizk, C. Potvin, P. Couture, H. P. Mercure, A. Bondiou-Clergerie, P. Lalande, and I. Gallimberti, “Triggering and guiding high-voltage large-scale leader discharges with sub-joule ultrashort laser pulses,” Phys. Plasmas 8(5), 2532–2539 (2001). [CrossRef]
6. B. La Fontaine, D. Comtois, C.-Y. Chien, A. Desparois, F. Génin, G. Jarry, T. Johnston, J.-C. Kieffer, F. Martin, R. Mawassi, H. Pépin, F. M. Rizk, F. Vidal, C. Potvin, P. Couture, and H. P. Mercure, “Guiding large-scale spark discharges with ultrashort pulse laser filaments,” J. Appl. Phys. 88(2), 610–615 (2000). [CrossRef]
7. S. L. Chin, T.-J. Wang, C. Marceau, J. Wu, J. S. Liu, O. Kosareva, N. Panov, Y. P. Chen, J.-F. Daigle, S. Yuan, A. Azarm, W. Liu, T. Seideman, H. P. Zeng, M. Richardson, R. Li, and Z. Z. Xu, “Advances in intense femtosecond laser filamentation in air,” Laser Phys. 22(1), 1–53 (2012). [CrossRef]
8. J. Kasparian and J.-P. Wolf, “Physics and applications of atmospheric nonlinear optics and filamentation,” Opt. Express 16(1), 466–493 (2008). [CrossRef] [PubMed]
9. A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 (2007). [CrossRef]
10. W. Liu, F. Théberge, J.-F. Daigle, P. T. Simard, S. M. Sarifi, Y. Kamali, H. L. Xu, and S. L. Chin, “An efficient control of ultrashort laser filament location in air for the purpose of remote sensing,” Appl. Phys. B 85(1), 55–58 (2006). [CrossRef]
11. F. Théberge, W. Liu, P. T. Simard, A. Becker, and S. L. Chin, “Plasma density inside a femtosecond laser filament in air: strong dependence on external focusing,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 74(3), 036406 (2006). [CrossRef] [PubMed]
12. B. La Fontaine, F. Vidal, D. Comtois, C. Y. Chien, A. Desparois, T. W. Johnston, J. C. Kieffer, H. P. Mercure, H. Pépin, and F. A. M. Rizk, “The influence of electron density on the formation of streamers in electrical discharges triggered with ultrashort laser pulses,” IEEE Trans. Plasma Sci. 27(3), 688–700 (1999). [CrossRef]
13. D. Comtois, H. Pepin, F. Vidal, F. A. M. Rizk, T. Wyatt Johnston, J. Kieffer, B. La Fontaine, F. Martin, C. Potvin, P. Couture, H. P. Mercure, A. Bondiou-Clergerie, P. Lalande, and I. Gallimberti, “Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse-II: modeling,” IEEE Trans. Plasma Sci. 31(3), 387–395 (2003). [CrossRef]
14. M. Lampe, R. F. Fernsler, S. P. Slinker, and D. F. Gordon, “Traveling wave model for laser-guided discharges,” Phys. Plasmas 17(11), 113511 (2010). [CrossRef]
15. F. Vidal, D. Comtois, H. Pépin, T. Johnston, C. Y. Chien, A. Desparois, J. C. Kieffer, B. La Fontaine, F. Martin, F. A. M. Rizk, H. P. Mercure, and C. Potvin, “The control of lightning using lasers: Properties of streamers and leaders in the presence of laser-produced ionization,” C. R. Phys. 3(10), 1361–1374 (2002). [CrossRef]
