Laser-triggered lightning discharges harmlessly

Dr. Frankenstein must have used a massive, high-voltage, heavy-gauge electrical system to harness the lightning bolts he needed to power his biomedical experiments. In real life, power engineers worry more about diverting lightning away from power equipment, rather than harnessing it. Lightning is also the bane of airplanes and rockets -- not to mention golfers. (According to a 1993 National Geographic article, golf is the deadliest sport in the US, due to lightning strikes.)¹

Illustration courtesy of Kansai Electronic Company, Inc.

Therefore, researchers (mostly funded by electric companies) would like to devise a method to trigger lighting and guide the discharge to harmless spots. In the March 1999 issue of Journal of Optical Technology, researcher Jens Schwarz at the University of New Mexico (Albuquerque) explained the process: "The idea is that the laser light creates a conducting path between the cloud and the ground, triggering the lightning discharge."

Groups in Japan, Russia, and the U.S. are using different laser wavelengths and powers to create conductive paths between the ground and thunderclouds. Most of the work is still laboratory based, but one group in Japan reported successfully triggering two lightning strikes using their laser system.² Shigeaki Uchida and colleagues at Osaka University in Japan, appear to have initiated the lightning strikes during field experiments on the shore of the Sea of Japan.

During thunderstorms, a large electrical field between the ground and clouds develops. High conducting points, such as the tops of towers built specifically to draw lightning, are usually surrounded by a corona discharge. When this discharge is quenched (such as when there is an abrupt change in the field under the clouds), an electrical leader starts from the top of the tower. If this occurs when the clouds are beginning to discharge, this leader initiates a lightning strike. By using laser beams to create plasma channels at the tops of the towers, Ushida's group has, to the best of their knowledge, reported the first laser-plasma-triggered lightning.

Timing is critical to successfully drawing a lightning strike in this system. The laser system takes advantage of an incipient strike to create a leader in the desired spot. Before a lightning strike occurs, a preliminary breakdown lasting a few hundred milliseconds occurs in the thunderclouds. At the test station, the group uses diagnostic equipment, including corona-current probes and broadband capacitive antennas, to detect a preliminary breakdown. When the breakdown is detected, the equipment triggers the lasers that create a plasma current.

Ushida's group uses a combination of a CO₂ laser, Nd:YAG laser, and Nd:glass laser. The 10.6-µm CO₂ laser is a two-beam system that provides 1 kJ of energy in 50-ns pulses in each beam. One beam is focused to a line on a dielectric target at the top of the tower, generating a hot electron plasma even in strong winds. The second beam is focused into a 2-m-long line above the target to form a plasma channel that can guide the leader out of the corona region. The glass laser is a master-oscillator power-amplifier system that delivers 600 J in a 50-ns pulse, and is also focused on the target. The YAG laser is converted to the fourth harmonic (266 nm) in the UV, offering powers of 100 mJ/pulse at 70 Hz, and aimed well above the target to provide weakly ionized channels between the target and the clouds. Presumably, further research will work on increasing the success rate of the system.

Lasers are not the only method for triggering lighting. A group in New Mexico reached a success rate of about 60 percent for triggering lightning by launching rockets with ground wires attached.³ However, Schwarz said, "The drawback is that you waste an expensive rocket every time."

"Furthermore," she said, "rocket parts from the destroyed rocket pose a hazard in residential areas and rockets can't really be fired at a high rate, meaning they can't follow the moving charges in the cloud fast enough." Most of the lasers, in contrast, can be fired tens of times per second.

The group of researchers in New Mexico that Schwarz works with use 248-nm UV lasers, rather than the combination of IR and UV beams. This method is expected to have the benefit of allowing much lower powers to be used -- tenths of a joule rather than hundreds or thousands of joules per pulse. Even when using these low energies, the very short pulses (about 500 fs) cause very high peak powers; at the moment, peak powers of 20 GW are being used for lab tests.

In the atmosphere, the ionizing power is leveraged. Schwarz said, "The UV beam creates a path of electrons that can move in the ambient electric field (the one between cloud and ground) and multiply via avalanche ionization."

In addition to studying the properties of air under the influence of intense laser beams and plasmas, the New Mexico group is primarily focused on two main areas: improving the laser system (by increasing the reliability and power), and making a more portable system, which will be useful for field experiments.

1. W.R. Newcott. "Lightning -- Nature's high voltage spectacle," National Geographic. 184(1), p. 80-103, 1993.

2. S. Uchida, et. al., "Laser-triggered lighting in field experiments," J. Opt. Tech., 66(3), p. 199-202, 1999.

3. P. Hubert, et. al., "Triggered lightning in New Mexico," J. Geophys. Res. Vol. 89, pp. 2511-2521, 1984).