Laser Beam Triggers Lightning Strike
During Japanese Experiment

based on an article by Paul Mortensen as reported in the September, 1997 issue of Laser Focus World

Lightning was triggered by a laser beam for the first time in an outdoor experiment conducted by a group of researchers in Osaka, Japan. As reported in the September, 1997 issue of Laser Focus World, researchers from Kansai Electric Power Company, the Institute for Laser Technology, and Osaka University conducted the experiment.

The impetus for the research is the more than 500 lightning strikes per year on electric transmission lines that result in widespread power outages. The researchers hope to reduce that number through use of the laser system.

The experiment was successful in conducting natural lightning discharges to ground of 35,000 A. By exposing the tip of a 50-m conducting tower to 20-m s pulses of light condensed on concave mirrors and discharged from two carbon dioxide (CO₂) lasers, a peak output of 20,000 MW was produced.

A lightning tower, electron-beam controlled CO₂lasers, and two large-aperture focusing telescopes equipped with focusing mirrors of 500 and 1000 mm made up the laser system. Plasma channels at the top of the 50-m lightning tower were created from the system's output. The two laser heads delivered two 1-kJ/50-ns laser pulses that were directed by the two telescopes and opened a conducting path for lightning by creating a plasma state - ionized positive and negative electric charges in super heated air.

In order for a lightning strike to be triggered, the motion of thunderclouds had to be monitored to choose the precise time for a laser shot to take place. Intense uniform laser-plasma channels of at least 1m also had to be produced

A thunderstorm-monitoring network was set up around the field site. Each station in the network measured electric fields under clouds and sent real-time data to the laser system. Plasma channels were produced when thunderclouds were ready to discharge because of the microsecond response of the laser trigger. Precursor ultra-high-frequency pulses that are generated in thunderclouds when the electrical field becomes critical allowed the precise timing to be achieved by detection of preliminary breakdown pulses.

Creation of the plasma channels during the field test was not without difficulties. Snow typically accompanied thunderstorms. The snow particles tend to degrade the line density of the laser plasma by attenuating the laser energy near the focal point. Aerosol cores of plasma were depleted and caused the line intensity of the plasma channel to be less dense in snowy weather as opposed to fine weather. Additionally, a plasma channel could not be produced within 30 cm of the top of the antenna when the electric fields were more than 1000 kV/m. This was attributed to the corona space charge generated by high electric fields at the top of the tower.

Beam ducts were built to protect the laser beam and irradiated a dielectric target attached to the top of the tower to generate continuous plasma channels on its surface. Alumina powder was sprayed above the target to generate a long plasma channel in air. The plasma channel consists of plasma beads made from the alumina powder as a core.

The research group began outdoor experiments in 1994 after having successfully completed indoor experiments beginning in 1990. NASA originally began work on lightning conduction by laser in the USA in 1978. Other groups have attempted outdoor tests but have been unsuccessful. In the future, researchers plan to use lasers with higher output and ultrashort pulses.

Update!!!!! The ESD Journal recently recieved an e-mail from Dr. Vernon L. Mangold adding new information to our article. His e-mail reads as follows:

"NASA was not the organization that conducted the laser triggering project, it was an Air Force project conducted at NASA KSC during the project Thunderstorm in 1978. It was my program and was not successful because during the short time we had after getting set-up and running, we did not get any lightning cloud formations directly overhead. We fired the laser a number of times, but only had less than a week before we had to tear it down and return to Wright-Patterson Air Force Base in Dayton Ohio. The laser was CO laser, not a CO2 laser."

The ESD Journal thanks Dr. Mangold for his comments.