Investigation of the characteristics of atmospheric laser induced plasmas

Summary form only given. A laser fired through the atmosphere can generate a plasma by freeing charge carriers from the air molecules in the path of the beam. The characteristics of this plasma, especially the delay time between the propagation of the laser and initiation of the arc, were investigated. These plasmas were generated by focusing the output of a KrF excimer laser (248 nm wavelength, 20 nsec (FWHM) pulsewidth) operating between 200 and 525 mJ per pulse with a cylindrical lens generating a line focus across an air gap at the end of a ten foot section of four inch co-axial transmission line. The gaps tested were 4 mm, 8 mm or 11 mm in length, with the center conductor of the line connected to a variable high voltage DC power supply. The arc was detected by measuring current with a B-dot probe mounted near the gap inside the transmission line. To obtain accurate timing results with respect to the laser pulse a UV photodetector was utilized to trigger the oscilloscope. This probe measured delays that ranged between 72 and 412 ns. The delay time increased with increasing gap length and decreased with increasing laser pulse energy and applied voltage.