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Monday, March 27, 2023

Physics – A Laser-Based mostly “Lightning Rod”

• Physics 16, 12

Experiments on a Swiss mountain reveal {that a} high-powered laser can affect the trajectory of lightning strikes—a step towards laser-based lightning safety.

TRUMPF/M. Stollberg

A laser fired into the sky close to a communications tower has been proven to information lightning strikes. To make the laser gentle seen within the photograph, the researchers transformed it from infrared to inexperienced wavelengths utilizing a frequency-doubling gadget.

When Benjamin Franklin launched his well-known kite right into a stormy sky in 1752, he supplied a preferential path for the cost collected within the clouds to achieve the bottom. A twenty first century replace on Franklin’s kite is a laser fired up into the ambiance. Stationed on a Swiss mountain, researchers have proven that the beam from a high-powered laser supplies a preferential path for lightning bolts [1]. Such a laser system might in the future present lightning safety to delicate amenities, reminiscent of airports, rocket launchpads, and wind farms.

The harm brought on by lightning—estimated to trigger billions of {dollars} of injury per yr within the US—will be decreased by controlling the place the electrical discharge goes. “At present, the one out there safety in opposition to lightning is a classical Franklin lightning rod,” says Aurélien Houard, a member of the analysis workforce from the École Polytechnique in Paris. These tall steel rods are good conductors that present a path of least resistance from the clouds to the bottom. However this safety is barely afforded to things inside a sure distance of the rod, given roughly by the rod’s top. Constructing a better rod would defend a bigger space, however this answer shouldn’t be at all times sensible.

“The concept of utilizing a laser is to create an extension of a steel rod that might, in concept, be a number of a whole lot of meters or a kilometer excessive,” Houard says. Not like a steel construction, the laser “rod” might be turned on solely when the skies are threatening.

Laser-based lightning steerage was first proposed in 1974, and a number of other experiments have confirmed the fundamental idea in a lab setting. Nevertheless, demonstrating the system within the subject has confirmed harder. Some exams have been unfortunate and by no means had a storm come shut sufficient to the laser. “Lightning may be very unpredictable,” Houard says. “You’ll have to attend a really very long time to see a strike.” And simply seeing one occasion isn’t sufficient: researchers must construct up the statistics to ensure that the laser is having a noticeable impact.

To extend their probabilities, Houard and his colleagues carried out their laser experiment in one of the crucial lightning-prone spots in Europe: Säntis Mountain, on the northeastern aspect of Switzerland. This 2500-m-high (8000-ft) mountain is capped by a 30-story-tall telecommunications tower that’s struck by lightning about 100 instances per yr. Through the summer time of 2021, Houard and colleagues put in a car-sized terawatt laser on the mountain with the purpose of testing whether or not lightning paths might be guided by the laser gentle.

At first of their marketing campaign, the researchers relied on climate reviews to warn them of approaching thunderstorms, however the forecasts have been usually improper, and the workforce missed some alternatives. “We determined to sleep on the mountain to be prepared to begin the laser if lightning got here within the night time,” Houard says. In addition they needed to alert the close by airport at any time when they deliberate to fireside the laser, as there was a danger that the sunshine might harm the eyes of individuals in airplanes flying overhead.

Laser Lightning Rod undertaking

This photograph from July 24, 2021, reveals a lightning strike that follows a straight trajectory according to the laser path.

When the meteorological situations have been ripe, the workforce aimed their laser into the sky and fired picosecond pulses at a repetition price of 1 kHz. By focusing the infrared laser gentle at a degree close to the highest of the tower, the researchers reached the depth obligatory to provide a nonlinear optical impact wherein the laser gentle divides up into a number of, skinny “filaments” that propagate with out spreading out (see Viewpoint: Air Waveguide from “Donut” Laser Beams). These high-intensity streams warmth the air, creating channels of ionized fuel, or “plasma,” that may be so long as 50 m. Such plasma channels are conducting, like a steel rod, so they’re anticipated to supply a preferential path for lightning.

Over the course of two months, the researchers operated their laser for a complete of six hours. Throughout that point, the tower was struck by 16 lightning strikes. Based mostly on visible and radio observations, the workforce discovered that 4 of those strikes have been guided by the laser for a portion of their trajectories. Photographs of considered one of these occasions clearly present a lightning bolt with a straight part that corresponded to the 50-meter plasma channel from the laser. The opposite three strikes have been measured with antennae on the mountain that report radio emissions from lightning discharges. The workforce’s evaluation of those radio knowledge revealed that, once more, these strikes have been aligned with the laser path.

The outcomes point out that the laser can affect the trail of lightning on its method to a steadily focused object (the tower). The subsequent step is to point out {that a} laser can defend extra delicate objects by “drawing fireplace” from the thunderclouds. “We wish to reveal that we will set off lightning,” Houard says. For this utility, he imagines a extra highly effective laser, capable of generate a plasma channel that extends 500 meters or extra into the sky. Such a tall laser rod might steer lightning away from a row of wind generators or a rocket readying for takeoff.

“It’s a promising expertise for lightning safety,” says lightning researcher Adonis Leal from New Mexico Tech. He questions how price efficient it will likely be, on condition that conventional lightning rods require no vitality to function. “We’d like extra subject campaigns reminiscent of this [experiment] to have dependable statistics on the effectivity of the laser system in opposition to the usual lightning-protection methods,” Leal says.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics Journal primarily based in Lyon, France.


  1. A. Houard et al., “Laser-guided lightning,” Nat. Photonics (2023).

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