Paul Mortensen
For the first time in an outdoor experiment, lightning has been triggered by a laser beam. A group of researchers from Kansai Electric Power Co., the Institute for Laser Technology, and Osaka University (all Osaka, Japan) believes its laser system will be useful in reducing the number of power failures due to lightning hits on transmission lines. Japan records more than 500 such hits annually, and they cause widespread power failures, especially along that country`s West coast.
The outdoor experiment—conducted on the summit of Mount Dakeyama near the town of Mihama facing Wakasa Bay (Fukui Prefecture)—succeeded in conducting natural lightning discharges to ground of 35,000 A. The experiment involved exposing the tip of a 50-m conducting tower to 20-µs pulses of light condensed on concave mirrors and discharged from two carbon dioxide (CO2) lasers producing a peak output of 20,000 MW.
The laser system consisted of the electron-beam-controlled CO2 lasers, two large-aperture focusing telescopes with focusing mirrors of 500 and 1000 mm, and a lightning tower (see figure). Output from the system created plasma channels at the top of the 50-m lightning tower. 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 superheated air.
In their experiments, the re searchers noted two essential re quirements for a lightning strike to be triggered: accurate monitoring of the motion of thunderclouds to choose the right timing for a laser shot and production of intense, uniform laser-plasma channels. With precise shot timing, a continuous plasma channel of 1 m is sufficient to trigger lightning.
For the first requirement, the researchers set u¥a thunderstorm-monitoring network around the field site. Each station in the network measured electric fields under clouds and sent real-time data to the laser system. The microsecond response of the laser trigger produced plasma channels at just the time when thunderclouds were ready to discharge. The precise timing was achieved by detecting preliminary breakdown pulses—precursor ultrahigh-frequency pulses generated in thunderclouds when the electrical field becomes critical.
Experimental difficulties
The researchers faced several difficulties creating the plasma channels during this field test. Most thunderstorms, for example, came with sweeping snow falls. Snow particles degrade the line density of the laser plasma by attenuating the laser energy near the focal point and depleting the aerosol cores of plasma—hence, the line intensity of the plasma channel in snowy weather is less dense than in fine weather. Furthermore, when the electric fields were more than 1000 kV/m, a plasma channel could not be produced within 30 cm of the top of the antenna. This was attributed to corona space charge generated by high electric fields at the top of the tower.
To address these problems, the researchers built beam ducts 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. They also sprayed alumina powder 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 group, which started its research in 1990, began outdoor experiments in 1994 after succeeding with indoor experiments. However, work on lightning conduction by laser was originally started by NASA in the USA in 1978. Meanwhile, in Japan, Kyushu Electric Power Co. and Kyushu University have also attempted outdoor tests but without success.
In future field tests the researchers plan to use a laser with higher output and ultrashort pulses.