So far, metal lightning rods are supposed to protect tall buildings and sensitive facilities from lightning strikes. But there may be another way, as an experiment in the Swiss Alps now suggests. In this, researchers used a pulsed high-power laser to deflect lightning from a 124-meter-high radio tower on the summit of the Säntis mountain. In four cases, the team observed that the laser pulses actually had a deflection effect. The discharges that occur during thunderstorms follow a channel of ionized, thinned-out air molecules generated by the laser. If this procedure proves itself in further tests, it could help to protect particularly vulnerable infrastructures.
Lightning is one of the most spectacular phenomena of our weather. With up to a billion volts, the glaring discharges jump between thunderclouds or between clouds and the earth's surface. The lightning makes its way by ionizing the air and creating a lightning channel a few millimeters thick. It is only through this ionization that the air molecules in the flash channel are stimulated to glow and produce the flash that we can see. The electrical discharge heats the air to 30,000 degrees, causing it to expand explosively - we hear it as thunder. How often there are thunderstorms and lightning in a region depends on the weather and the landscape. Globally, however, satellites register between 40 and 120 flashes per second. These are not without consequences: "The documented number of deaths from lightning strikes is well over 4000 and the material damage is estimated at billions of US dollars per year," report Aurélien Houard from the Polytechnic in Paris and his colleagues.
Lasers pave the way for lightning
So far, buildings and other endangered infrastructures have been protected by classic lightning conductors. Attached to roofs, these grounded metal spikes provide a preferred point of strike for lightning and dissipate the charges underground. Scientists have been experimenting with other methods of lightning conduction, which start directly in the storm clouds, for a long time. Among them are small rockets that are shot into storm clouds and guide the lightning safely to earth via a guide wire. While the process works, it is expensive and generates waste in the form of the spent rockets and wires. Houard and his team have therefore focused on a different method: deflecting lightning using powerful laser beams.
Such laser pulses create a channel of ionized air molecules in the air, a so-called filament. "Along these filamentous zones, the air is rapidly heated by the absorption of laser energy and expands outward at supersonic speeds," the researchers explain. As a result, a channel with reduced gas density and increased conductivity is formed several milliseconds after the laser pulse. It offers lightning the path of least resistance. "In the laboratory, meter-long electrical discharges have already been triggered and conducted by such laser-induced filaments," report Houard and his colleagues. The laboratory tests also showed that the laser pulses can be adjusted in such a way that the filament channel they produce starts up to a kilometer away from the laser source – this reduces the risk of a lightning strike in the deflection laser.
First successes in practical tests
It was previously unclear whether this laser flash deflection also works in practice and during a real thunderstorm. That is why the physicists tested this in the summer of 2021 with an experiment on the Säntis in the Swiss Alps, which is around 2500 meters high. To do this, they transported a high-power laser up the mountain, which can fire high-energy infrared laser pulses into the sky at a frequency of thousands of pulses per second. They aligned this laser so that its pulses created a filamentous channel of air just above the top of a 124-meter-tall telecommunications tower on the mountaintop. "This tower, which is struck by lightning around 100 times a year, is equipped with numerous sensors that can register lightning discharges, electromagnetic fields, X-rays and other radiation caused by lightning," the scientists report. For their experiment, they switched on the laser whenever a thunderstorm was brewing within a three-kilometer radius of the tower.
During the test period, the tower was struck by lightning at least 16 times. The laser was active in four of these flashes, all discharges running from the tower towards the cloud. Analysis showed that these flashes actually followed the channel created by the laser and were thus deflected from their direct path. "The lightning strike follows the laser path over its roughly 50-meter path," the team said. Only after the end of the laser-induced filament did the lightning resume its natural path. "The results of the Säntis experiment campaign provide the first indications that the filaments formed by short, intense laser pulses can direct lightning discharges over considerable distances," state Houard and his colleagues. The method still has to be optimized and tested further, but they still see such laser lightning conductors as a promising way of better protecting critical infrastructure such as airports, power plants or rocket launch pads from lightning strikes in the future. Because unlike conventional lightning rods, the laser beams can deflect the lightning high up in the atmosphere.
Source: Aurélien Houard (Ecole Polytechnique, Paris) et al., Nature, doi: 10.1038/s41566-022-01139-z