Laser spark obtained by using a conical optics is much more appropriate to form conducting channels in atmosphere. Only two types of lasers are actively considered to be used in forming high-conductivity channels in atmosphere, controlled by laser spark: pulsed sub-microsecond gas and chemical lasers (CO2, DF) and short pulse solid-state and UV lasers. Main advantage of short pulse lasers is their ability in forming of superlong ionised channels with a characteristic diameter of ~ 100 mkm in atmosphere along the beam propagation direction. At estimated electron densities below 1016 cm-3 in these filaments and laser wavelengths in the range of 0.5 - 1.0 mm, the plasma barely absorbs laser radiation. In this case, the length of the track composed of many filaments is determined by the laser intensity and may reach many kilometers at a femtosecond pulse energy of ~ 100 mJ. However, these lasers could not be used to form high-conductivity long channels in atmosphere. The ohmic resistance of this type a conducting channels turned out to be very high, and the gas in the channels could not be strongly heated (< 1 J). An electric breakdown controlled by radiation of femtosecond solid-state laser was implemented in only at a length of 3 m with a voltage of 2 MV across the discharge gap (670 kV/m). Not so long ago scientific group from P.N. Lebedev has improved that result, the discharge gap -1m had been broken under KrF laser irradiation when switching high-voltage (up to 390 kV/m) electric discharge by 100-ns UV pulses. Our previous result - 16 m long conducting channel controlled by a laser spark at the voltage - 3 MV - was obtained more than 20years ago in Russia and Japan by using pulsed CO2 laser with energy - 0.5 kJ. An average electric field strength was < 190 kV/m. It is still too much for efficient applications.
V. V. Apollonov, V. V. Apollonov,
"Energy transmission by laser", Proc. SPIE 9255, XX International Symposium on High-Power Laser Systems and Applications 2014, 92554K (3 February 2015); doi: 10.1117/12.2071198; https://doi.org/10.1117/12.2071198