A Comment on Lightning Control Using a ... - ulaval ULAVAL

Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 2011–2012 Part 1, No. 4A, April 1999 c °1999 Publication Board, Japanese Journal of Applied Physics

Short Note

A Comment on Lightning Control Using a Femtosecond Laser See Leang C HIN ∗ and Kenzo M IYAZAKI Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan (Received December 4, 1998; accepted for publication January 20, 1999)

The current technique of lightning control using nanosecond CO2 laser pulses and its limitations are analysed. The advantages of replacing the CO2 laser pulses by intense femtosecond laser pulses without changing the basic design of the lightning control tower are analysed and discussed. KEYWORDS: lightning control, CO2 laser, intense femtosecond laser, multiphoton and tunneling ionization, self-focusing, filamentation

Recently, the Osaka University group1) has made a breakthrough by demonstrating the first successful CO2 laserinduced lightning in a real field test. This was achieved by generating a one-meter-long laser-induced plasma column above a 50-m-high lightning tower so as to initiate a leader from the top of the tower. This is indeed an excellent step forward in the difficult task of producing laser controlled lightning. From the previously published work of Yamanaka et al.,1) one can see that one challenge in the future is to make the experiment more reliable in terms of the success rate. It should be noted that in this pioneering work, of the 22 shots in 9 thunderstorms, only two shots were successful. To improve the success rate, it seems to us that it would be more advantageous to use femtosecond intense laser pulses. We first analyse the current technique: 1. The CO2 laser-induced plasma column consists of plasma beads, i.e., it is a non-continuous plasma column. This means that one has to rely on a strong external field to connect these beads and trigger lightning. Currently, the external field is a high static field in air immediately before the arrival of a natural thunderstorm. 2. The CO2 laser technique requires plasma seeding for generating the plasma column to trigger lightning because the concentration of dust particles in the corona zone around the tower tip is insufficient. 3. The laser pilot leader (plasma column) is short, only one meter long. The consequence is that the lightning (discharge) leader is practically noncontrollable, i.e., it follows not the desired path but a zigzag path dictated by the irregular external static field distribution in the atmosphere. The consequence is that one cannot find a good timing to trigger the laser at an appropriate moment, hence the inconsistency. It would be desirable if the laser pilot leader could reach the cloud. The use of ultrafast intense laser pulses, such as those from a Ti:sapphire laser, would provide an improvement over the current technique without changing the lightning tower design. Such laser pulses can self-focus into a long filament during their propagation in the atmosphere. This filamentation phenomenon can be explained by the moving focus model as shown by recent experiments.2–5) Essentially, depending on the laser peak power, each slice of the laser pulse selffocuses at different positions along the propagation axis in neutral air medium due to the Kerr nonlinearity. As the slice self-focuses, the intensity increases rapidly, resulting in local ∗ On

leave (July–October, 1998) from Center for Optics, Photonics and Laser, and Department of Physics, Laval University, Quebec City, Quebec, Canada G1K 7P4.

multiphoton ionization (MPI) and/or tunnel ionization (TI). The electron density in the focal region is sufficiently high to produce a defocusing effect which balances the self-focusing. This results in a continuous string of foci, or the perception of a filament. In the following, we discuss the possible advantages of using such femtosecond laser pulses as compared to using the CO2 laser pulse. 1. There are no plasma beads because the moving selffocus results in a continuous string of foci, hence a filament is formed.2–6) Inside this filament, ionization takes place, hence, a conducting column is formed. Furthermore, it is not necessary to use a focusing lens to create the filament. 2. Ionization in the filament is independent of dust particles. MPI and TI are in themselves sufficient to ionize air molecules7, 8) and are the dominant processes responsible for the generation of the plasma. This is because the pulse rise time is very short so that laser-induced breakdown (a phenomenon that takes time to develop during the cascade ionization process) cannot occur before the threshold of selffocusing is reached. Once self-focusing starts, the moving focus2–6) limited by MPI and/or TI dictates the propagation resulting in the filamentation. Thus no plasma seeding is needed. 3. The filament is long, of the order of 10 km, and hence could touch the cloud. This latter phenomenon has been demonstrated recently by the Jena group9) in which the filamentation of a 2 terawatts Ti:sapphire laser pulse could reach up to 10 km into the sky. This would make the discharge more controllable, and hence more consistent. 4. An added advantage of using femtosecond laser pulses is the possibility of varying the length and the position of the filament by changing some parameters of the laser. In fact, according to the theory of moving focus,10) the length of the filament as well as the beginning of the filament depends on the diffraction length, as has recently been verified experimentally.2) The experimental results show that for a given beam diameter, the position of the end of the filament is practically the same for all peak powers and is equal to the diffraction length of the slice immediately above the self-focusing threshold.2) The beginning of the filament depends on the chirp of the laser. Up-chirping or down-chirping of the pulse will move the beginning of the filament further away from the compressor.11) The beginning of the filament also depends on the peak power of the laser. The higher the peak power is, the earlier the filament starts and the longer the filament is. This is because the most intense slice of the pulse self-focuses earlier as the peak power increases. It is also ex-

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 4A

pected that the diffraction length would be proportional to the square of the beam diameter.10) Thus, by changing the diameter and the peak power of the beam, one expects to be able to change the length and position of the filament in space as desired. 5. Although so far the absolute value of the plasma density distribution inside the filament has not been measured, an estimation may be made. MPI or TI gives rise to an electron density whose defocusing effect is sufficient to balance the Kerr self-focusing effect. Since the laser pulse is very short, a high intensity, of the order of 1014 W/cm2 , is reached quickly (see refs. 2–5). At such intensities, the ionization of the major constituents of air, namely, nitrogen and oxygen molecules, is significant.12) According to the estimation of Bloembergen,13) at an electron density of the order of 1018 cm−3 , the defocusing effect due to the plasma is sufficient to counterbalance the focusing effect. This value is reasonable since it is smaller than that given by the saturation of MPI or TI for the generation of singly charged ions. The density at ionization saturation would be of the order of 1019 cm−3 , the density of air at one atm. Thus, it can be expected that the plasma density in the filament is of the order of 1018 cm−3 . 6. The natural imperfection of the laser wavefront would give rise to multiple filaments. This multiple filamentation phenomenon could be put to good use in the present application. The multiple filaments are normally randomly distributed at different positions along and around the principal propagation axis. The combination of these filaments would increase the total number of charges (hence, the conductivity) in the narrow column around the propagation axis, as well as increase the length of this column. In fact, we suspect that the very long filament together with the very strong supercontinuum emission (or white light laser14) as identified by us recently) observed by the Jena group9) is the result of the combination of many filaments distributed randomly along and around the propagation axis. This latter phenomenon has been observed in water with short filaments.15) 7. There are other interaction phenomena that would contribute to ionization of the air molecules inside the self-focus and to nonlinear index changes. They are the third- and higher order harmonic generation, the white light supercontinuum (laser)14) generation and the Coulomb explosion. High-order harmonics emitted from above the ionization limit of a molecule16, 17) will mostly be reabsorbed by the molecules situated in the forward direction. Such reabsorption would cause mostly ionization. Furthermore, the third harmonic might be strong enough to cause MPI of the molecules either by themselves or together with the main pulse. In the latter case, the ionization could be enhanced or suppressed depending on the relative phase of the two pulses. All these would contribute to the generation of extra charges in the laser-induced conducting column. Harmonics emitted from below the ionization limit would contribute to the direct excitation of molecules in the forward direction as well as the coherent excitation of these molecules together with the main laser pulse. These processes would principally create a nonlinear index different from that of the ground state molecules. In particular, recent experiments show that an electron can be trapped in some highly excited or some Rydberg states during the process of excitation of that electron into the ionization continuum.18, 19) These highly ex-

S. L. C HIN and K. M IYAZAKI

cited molecules would have a higher polarisability or susceptibility and hence a higher nonlinear index. These would contribute to the enhancement of self-focusing of the pulse. The high electric field in the atmosphere just before the arrival of the natural thunder storm would also field ionize these highly excited molecules. This would contribute more charges to the conducting column. The white light supercontinuum (laser) would combine with the main pulse to coherently excite the molecules, resulting in either enhanced or suppressed ionization or in the excitation of molecules in the forward direction. Coulomb explosion is a result of MPI and/or TI of molecules followed by enhanced ionization20–22) which results in the fragmentation of the doubly or multiply charged molecules via Coulomb repulsion. This process would provide more charges in the laser-induced conducting column. It should be noted that some of the physical phenomena pointed out by us above have not yet been studied thoroughly. Some of them are currently being studied in some laboratories. Whatever the outcome, the results could only help the endeavour of lightning control in a positive way. In conclusion, our brief analysis shows that the current technique of lightning control using nanosecond CO2 laser pulses could be improved by using intense femtosecond laser pulses without changing the basic design of the lightning control tower. We appreciate very fruitful discussion with S. Uchida. SLC would like to acknowledge the support of the Institute of Advanced Energy, Kyoto University during his sabbatical visit (July–October, 1998). 1) T. Yamanaka, S. Uchida, Y. Shimada, H. Yasuda, S. Motokoshi, K. Tsubakimoto, Z. Kawasaki, Y. Ishikubo, M. Adachi and C. Yamanaka: Proc. SPIE 3343 (1988) 281. 2) A. Brodeur, C. Y. Chien, F. A. Ilkov, S. L. Chin, O. G. Kosareva and V. P. Kandidov: Opt. Lett. 22 (1997) 304. 3) O. G. Kosareva, V. P. Kandidov, A. Brodeur, C. Y. Chien and S. L. Chin: Opt. Lett. 22 (1997) 1332. 4) O. G. Kosareva, V. P. Kandidov, A. Brodeur and S. L. Chin: J. Nonl. Opt. Phys. & Mater. 6 (1977) 485. 5) V. P. Kandidov, O. Kosareva, A. Brodeur and S. L. Chin: J. Opt. Ocean & Atmos. 10 (1977) 1539 [in Russian]. 6) M. Mlejnek, E. M. Wright and J. V. Moloney: Opt. Lett. 23 (1998) 382. 7) S. F. J. Larochelle, A. Talebpour and S. L. Chin: J. Phys. B 31 (1998) 1. 8) A. Talebpour, S. Larochelle and S. L. Chin: J. Phys. B 31 (1998) L49. 9) L. Worste, C. Wederkind, H. Wille, P. Rairoux, B. Stein, S. Nikolov, C. Werner, S. Niedermeier, F. Ronnerberger, H. Schillinger and R. Sauerbrey: Laser & Optoelekton. 29 (1997) No. 5, 51. 10) J. H. Marburger: Prog. Quantum Electron. 4 (1975) 1. 11) S. L. Chin, A. Brodeur, C. Y. Chien and S. Larochelle: in final report submitted by S. L. Chin to US Army Research Office, Jan. 1999, under grant # DAAG55-97-1-0404. 12) A. Talebpour, J. Yang and S. L. Chin: to be published in Opt. Commun. 13) N. Bloembergen: Opt. Commun. 8 (1973) 285. 14) S. L. Chin, S. Petit, F. Borne and K. Miyazaki: Jpn. J. Appl. Phys. 38 (1999) No. 2A, L128. 15) A. Brodeur, F. A. Ilkov and S. L. Chin: Opt. Commun. 129 (1996) 193. 16) S. L. Chin and P. A. Golovinski: J. Phys. B 28 (1995) 55. 17) S. L. Chin, Y. Liang, S. Augst, P. A. Golovinski, Y. Beaudoin and M. Chaker: J. Nonl. Opt. Phys. Mater. 4 (1995) 667. 18) A. Talebpour, Y. Liang and S. L. Chin: J. Phys. B 29 (1996) 3435. 19) A. Talebpour, C. Y. Chien and S. L. Chin: J. Phys. B 29 (1996) 5727. 20) S. L. Chin, A. Talebpour, T. D. G. Walsh, S. Larochelle, F. A. Ilkov and C. Y. Chien: Multiphoton Processes 1996, eds. P. Lambropoulos and H. Walther (Institute of Physics Publishing, Bristol, 1997) p. 266. 21) T. D. G. Walsh, F. A. Ilkov and S. L. Chin: J. Phys. B 30 (1997) 2167. 22) A. Talebpour, S. Larochelle and S. L. Chin: J. Phys. B 30 (1997) 1927.