Shuzo Eto

Dynamics and Kinetics of Laser-Filament Plasma in Strong External Electric Fields and Applications

Physics of atmospheric laser-induced filaments in strong electric fields provides us with good instruments for the distant study and control of atmospheric and industrial high voltage discharges and accompanying processes. Dynamics and kinetics of laser filaments in strong non-uniform electric fields are studied both theoretically and experimentally with the high temporal and spatial resolution. Among others, a considerable reduction of the breakdown potential was found and was attributed to a filament-induced leader. Two breakdown modes, fast and slow, were found in 0.4 MV positive dc-voltage discharges activated by filaments. In the fast mode with duration order of a few microseconds, the filament may acquire the electrode potential and temporarily maintain it, becoming a leader. The slow mode with its duration order of 1 ms appears with a considerably smaller voltage reduction when the leader decays before the secondary streamer. Long, about a half of microsecond, non-uniform corona UV burst is observed after the laser-filament plasma appears nearby an electrode biased (positively or negatively) slightly higher than the corona discharge threshold. The effects of electron runaway in positive high-voltage-impulse discharges were studied. Strong hard (e ＞ 100 keV) x-rays being observed from impulse atmospheric discharges just before the breakdown were completely stopped with the use of the laser-filament plasma. Runaway electrons, generating such x-rays, disappear if the laser-filament plasma is ignited perpendicularly to the runaway nearby the positive electrode. These results are important to understand the filament physics, and also useful for the applications to laser triggered lightning, electric field measurement, protection of high-voltage facility and others.

Quenching electron runaway in positive high-voltage-impulse discharges in air by laser filaments

Strong hard (ε>100 keV) x rays being observed from impulse atmospheric discharges with maximal voltages from U=0.5 to 0.9 MV just before the breakdown were completely stopped with the use of femtosecond-laser-filament plasma. Runaway electrons generating such x rays and being estimated to achieve their maximal energy, ε~U, near the positive electrode disappear if a laser filament plasma is ignited perpendicularly to the runaway near the positive electrode. A preheating mechanism for formation of the electron runaway in air is proposed.

Stand-off detection and classification of CBRNe using a Lidar system based on a high power femtosecond laser

We propose a stand-off system that enables detection and classification of CBRNe (Chemical, Biological, Radioactive, Nuclear aerosol and explosive solids). The system is an integrated lidar using a high-power (terawatt) femtosecond laser. The detection and classification of various hazardous targets with stand-off distances from several hundred meters to a few kilometers are achieved by means of laser-induced breakdown spectroscopy (LIBS) and two-photon fluorescence (TPF) techniques. In this work, we report on the technical considerations on the system design of the present hybrid lidar system consisting of a nanosecond laser and a femtosecond laser. Also, we describe the current progress in our laboratory experiments that have demonstrated the stand-off detection and classification of various simulants. For the R and N detection scheme, cesium chloride aerosols have successfully been detected by LIBS using a high-power femtosecond laser. For the B detection scheme, TPF signals of organic aerosols such as riboflavin have clearly been recorded. In addition, a compact femtosecond laser has been employed for the LIBS classification of organic plastics employed as e-simulants.