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Creation of extended ionized channels in atmospheric air

Creation of extended ionized channels in atmospheric air by UV radiation pulses for controlling high-voltage electric discharges and for highly directional transportation of MW radiation

Plasma channels generated by laser radiation in atmospheric air or some other gases are of great interest for many fundamental problems and technical applications. Among them are lightning initiation and active lightning-discharge protection, directed transfer of MW radiation to reduce its natural divergence, laser-driven acceleration of electrons. In contrast to earlier experiments with CO2 laser pulses of submicrosecond length, where absorption in dense plasma formed in avalanche ionization limited the length and continuity, new approaches based on the use of long UV pulses or ultrashort femtosecond pulses enable creating extended weakly ionized tracks in gas owing to processes of multiphoton ionization and/or filamentation of the laser beam. As primary electrons rapidly recombine with positive ions and during a time of ~10–50 ns attach themselves to oxygen molecules, it is expedient to use additional visible or UV radiation to maintain electron density for a much longer time. Therefore, a combination of a train of ultrashort highly intense UV laser pulses appears to be sufficiently attractive to produce and further sustain the plasma channel. 
The laboratory path of up to 100 m was used to study the multiple filamentation of a UV laser beam of ultrashort pulses at a peak power exceeding the critical power more than 1000-fold. The conductance of plasma channels formed in atmospheric air at the focusing of combined UV radiation pulses by a spherical mirror with a focus of ~8.0 m was measured. We registered the photocurrent signal determined by the electron component of photoionization plasma, which was created in the region of the drift between two ring electrodes placed at a distance of 20 cm one from the other at applied voltage U = 5–22 kV. The amplitude of the current was in practice close to zero at the front of the free generation pulse and sharply increased at the moment when ultrashort UV pulses emerged. Measurements of electron photocurrent for a 100-ns smooth pulse in the free generation regime (at the shut-off injection of ultrashort UV pulses) gave values two orders of magnitude lower than the values of photocurrent for a modulated laser pulse. This is explainable by the nonlinear character of air photoinization with respect to the intensity of laser radiation. At the displacement of the geometric focus of the mirror within ~1.0 m relative to the inter-electrode gap  the photocurrent signals changed insignificantly, which, evidently, indicates the filamentation of the laser beam of the ultrashort UV pulses and gives an estimate of nonlinear propagation length. Experiments on the initiation of the breakdown of a small-length discharge gap showed that the free generation pulse initiated an electric breakdown of the gap 4.0 cm in length at applied voltage of 50 kV; herewith, the discharge pulse developed with a delay of ~5 µs relative to the laser pulse and the direction of its propagation is not determined by the laser beam. At the same applied voltage the combined pulse of the same energy provides for the propagation of the discharge along the beam to a distance of 7 cm; what is more, with a delay of a minimum by two orders of magnitude lower. For a pulsed discharge of voltage ~400 kV the maximal length of the controlled discharge reached 70 cm in experiments. Formation of an extended conducting channel of a comparatively high conductance, sustained by combined UV plasma pulses for several tens of nanoseconds, enables reckoning on an efficient method of creating extended ionized channels in atmospheric air to control high voltage electric discharges and highly directional transfer of MW radiation, in which the density of electrons up to 1015 cm–3 is  produced and sustained owing to the multiphoton ionization of O2 molecules and the resonance photodetachment of electrons from the electronegative ion of O2– by amplitude modulated UV laser pulses.

  1. V.D. Zvorykin, A.A. Ionin, S.I. Kudryashov, A.O. Levchenko, A.G. Molchanov,L.V. Seleznev, D.V. Sinitsyn, N.N. Ustinovskii, Plasma channels in air produced by UV laser beam: mechanisms of photoionization and possible applications. Neutral Particles Channeling Phenomena Channeling – 2008(S.B. Dabagov and L. Palumbo, eds.), World Scientific Publishing, 2010, pp. 813–823.
  2. V.D. Zvorykin, A.O. Levchenko, A.G. Molchanov, I.V. Smetanin, N.N. Ustinovskii, Channeling of SHF energy in plasma waveguides created in the atmosphere by a high-power UV laser. Kratk. Soobshch. Fiz. FIAN, 2, 49–56 (2010).
  3. V.D. Zvorykin, A.O. Levchenko, N.N. Ustinovskii,I.V. Smetanin, Transportation of SHF radiation in sliding-mode plasma waveguides. Pisma ZhETF, 91(5), 244–248 (2010).
  4. I.V. Smetanin, V.D. Zvorykin, A.O. Levchenko, N.N. Ustinovsky, Transfer of microwave radiation in sliding modes of plasma waveguides. J.  Russian Laser Res., 31(5), 495–508 (2010).
  5. V.D. Zvorykin, A.O. Levchenko, N.N. Ustinovskii, Control of extended high-voltage electric discharges in atmospheric air by UV KrF laser radiation. Quantum Electron.,41(3), 227–233 (2011).
  6. V.D. Zvorykin, A.O. Levchenko, I.V. Smetanin, N.N. Ustinovskii, Long-distance transfer of microwaves in sliding-mode virtual plasma waveguides. Il Nuovo Cimento, 34 C (4), 469–466 (2011).
  7. A.A. Ionin, S.V. Kudryashov, A.O. Levchenko, L.V. Seleznev, A.V. Shutov, D.V. Sinitsyn, I.V. Smetanin, N.N. Ustinovsky, V.D. Zvorykin, Triggering and guiding electric discharge by a train of UV picosecond pulses combined with a long UV pulse. Appl. Phys. Lett., 100,104–105 (2012).
  8. A.A. Ionin, S.I. Kudryashov, A.O. Levchenko, L.V. Seleznev, A.V. Shutov, D.V. Sinitsyn, I.V. Smetanin, N.N. Ustinovsky, V.D. Zvorykin, Triggering and guiding electric discharge by a train of ultrashort UV pulses. AIP Conf. Proc., 1464, 711 (2012).
  9. V.D. Zvorykin, A.O. Levchenko, A.V. Shutov et al., Long-distance directed transfer of microwaves in tubular sliding-mode plasma waveguides produced by KrF laser in atmospheric air. Phys. Plasmas, 19, 033509 (2012).
  10. Yu.P. Voinov, V.S. Gorelik, V.D. Zvorykin, I.G. Lebo, A.O. Levchenko, N.N. Ustinovsky, Laser implantation of KTiPO4 ferroelectric nanoparticles into pores of synthetic opal placed in water. J. Russian Laser Res., 33(1), 1-4 (2012).
  11. V.D. Zvorykin, A.A. Ionin, A.O. Levchenko, et al., Production of extended plasma channels in atmospheric air by amplitude-modulated UV radiation of GARPUN-MTW Ti:sapphire–KrF laser. Part 2. Accumulation of plasma electrons and electric discharge control. Quantum Electron., 43(4), 339-346 (2013).
  12. A.V. Shutov, I.V. Smetanin, A.A. Ionin, A.O. Levchenko, L.V. Seleznev, D.A. Sinitsyn, N.N. Ustinovskii, V.D. Zvorykin, Direct measurement of the characteristic three-body electron attachment time in the atmospheric air in DC electric field. Appl. Phys. Lett., 103, 034106 (2013).