A European consortium, coordinated by the Paris Polytechnic Institute Applied Optics Laboratory in collaboration with the University of Geneva, the Ecole Polytechnique of Lausanne, and TRUMPF Scientific Lasers, has succeeded in deflecting the path of lightning with a powerful laser mounted atop Mount Santis in the Swiss Alps. The system, called “Laser Lightning Rod”, can help better protect people, buildings, and large infrastructure during thunderstorms. Today, only ordinary lightning rods can protect the highest structures. Invented in 1752 by Benjamin Franklin, the system – which has remained largely unchanged since – consists of a high-altitude metal rod connected to downward conductors that transmit electrical energy to the ground, where it is dissipated. However, the protected zone is limited to a few meters or tens of meters, no more. According to satellite data, the total frequency of lightning strikes in the world is between 40 and 120 strokes per second. Every year, lightning strikes cause thousands of deaths and tens of billions of dollars in property damage. As extreme weather events, including thunderstorms, become more frequent, it becomes necessary to find a better protection system, especially for critical facilities such as airports, wind farms, or nuclear power plants. Therefore, physicist Aurélien Houart and his colleagues developed a new protection system using laser filamentation. At the top of Sentis, there is a large telecommunications tower 124 meters high – one of the most frequently struck structures in Europe, which is struck by about 100 lightning strikes every year. Therefore, it was an ideal place to test a new type of lightning rod. Laser to create ionized air channels The goal was to create channels of ionized (i.e. conductive) air to guide lightning. To do this, the researchers fired powerful laser pulses into the atmosphere: this changed the refractive index of the air, causing the laser pulse to shrink and intensify, ” like a self-generating series of increasingly convergent lenses,” the researchers explain. The laser pulse eventually becomes intense enough to ionize the surrounding nitrogen and oxygen molecules, a phenomenon called laser filamentation. Then a chain of “threads” is formed, inside which air molecules are rapidly heated under the action of the absorbed laser energy and then fly out at supersonic speed, leaving behind channels of ionized air of reduced density. ” These low-density millisecond channels have higher electronic conductivity and therefore provide a preferred path for electrical discharges,” the team says. The length of ionized filamentation can reach hundreds of meters when the power of the initial picosecond pulse is on the order of a terawatt (1012 W). In the summer of 2021, the team took advantage of a thunderstorm to experiment: they used a laser with an average power of one kilowatt, one joule per pulse, and a pulse duration of one picosecond, developed by TRUMPF Scientific Lasers. The device, eight meters long and weighing more than three tons, was installed at an altitude of 2500 meters on Mount Sentis, which is already well equipped with various sensors and devices for monitoring lightning. The laser beam was directed over a telecommunications tower equipped with a traditional lightning rod and activated whenever lightning activity was forecast. ” The goal was to see if there was a difference with or without the laser. So we compared the data obtained when the laser filament was produced over the tower and when the tower was naturally struck by lightning, ” explains Aurélien Houart. Lightning is deflected by several tens of meters Between July 21 and September 30, 2021, the laser operated for a total of 6.3 hours during a thunderstorm that passed within a radius of three kilometers from the tower. During this period, at least 16 lightning strikes hit the tower, four of which occurred during the operation of the laser. Since the first laser lightning, researchers have found that the discharge can follow the beam for several tens of meters before reaching the tower, thus increasing the radius of the protective surface. On July 21, the conditions were suitable for filming: the researchers were able to follow the path of lightning in two directions using high-speed cameras at a distance of several kilometers. The images show that the lightning did indeed follow the laser’s trajectory, upward, to a distance of about 50 meters.
” While this area of ​​research has been very active for over 20 years, this is the first result experimentally demonstrating laser-guided lightning, ” the team notes in a paper describing the experiment published in the journal Nature Photonics. This idea was first proposed in the 1970s by Leonard M. Ball. However, the few field tests carried out so far have not produced any evidence of laser guidance or the initiation of a lightning discharge. The laser used here fires up to a thousand pulses per second—two orders of magnitude faster than previous attempts—allowing the beam to intercept any lightning precursors that form over the tower. Curiously, although over nine years of observations at the Santis Tower in the absence of lasers, 84% of negative flares, 11% of positive flares, and 5% of bipolar flares were recorded, all four recorded laser events were positive flares, connecting the top of the tower with the center of positive charge in the cloud. This may be due to the electric field conditions required to create a discharge that fills the gap between the lower tip of the filament and the metal rod at the top of the tower. The consortium’s next step is to increase the height of the laser. The ultimate goal is to extend the 10-meter lightning rod by 500 meters. More powerful lasers operating at different wavelengths could send lightning farther and even fire it before it becomes a threat, Huard says.
