A laser can lead lightning discharges to a lightning conductor during a thunderstorm. This is shown by the study by an international research group on a 124 meter high telecommunications tower on the Swiss mountain Säntis. The knowledge could lead to better lightning protection for airports, launch pads and large infrastructure facilities, writes Aurélien Houard's team from the Laboratoire d'Optique Appliquée in Palaiseau near Paris in the specialist magazine "Nature Photonics".
The use of lasers for lightning protection was suggested as early as 1974. The use of lasers to guide lightning was demonstrated in the laboratory at the end of the 1990s. However, outdoor attempts failed in 2004 in the US state of New Mexico and in 2011 in Singapore. The scientists attribute the fact that the experiments on Mount Säntis were successful to the laser pulse repetition rate, which is two orders of magnitude higher than in earlier experiments. The laser used emitted light with a wavelength of about one micron (thousandths of a millimeter) and a repetition rate of 1000 hertz.
The researchers benefited from the fact that the tower on the Säntis was repeatedly used for lightning measurements in recent years. "This tower, which is struck by lightning about 100 times a year, is equipped with multiple sensors that record the lightning current, electromagnetic fields at different distances, X-rays and sources of radiation from the lightning discharges," the study authors write. They installed more measuring devices and two high-speed cameras that recorded lightning strikes at up to 24,000 frames per second.
Flash wraps around the laser beam
These cameras were located 1.4 and 5 kilometers from the top of the tower and only provided usable results in good visibility. This was the case for one of the four recorded flashes where the laser was on. The camera images show that the lightning coils around the laser beam for more than 50 meters and then strikes the tower's lightning rod. The slightly tilted laser beam was aimed so that it came close to the top of the tower.
From a physical point of view, the following is likely to happen: the intense laser pulses heat up the air so much that many air molecules escape into the cooler surroundings; a kind of channel with very low air density, a so-called filament, is created along the laser beam. In this filament, the air is significantly more conductive than the surrounding area, making it easier to conduct lightning. Comparisons with recorded lightning strikes without a laser show that the lightning strikes the tower's lightning rod much more precisely thanks to the guidance of the laser.
"The results of the Säntis test campaign in summer 2021 provide circumstantial evidence that filaments formed by short and intense laser pulses can conduct lightning discharges over considerable distances," the study authors conclude. However, these preliminary results should be confirmed by further test series with new configurations.