LARGE SCIENTIFIC PROJECT
CONTROLLING lightning with lasers
Figure 3. Images of the laser guided lightning flash recorded by two cameras over Mont Saentis on July 24 2021. Reproduced from [ 4 ]. CC BY 4.0
was also necessary to build a 3 m high telescope shelter to direct and focus the laser beam, and to cover the entire laser path between the building and the sending telescope. The installation of this shelter required the helicopter transport of two 10 ' high containers and 20 tons of concrete to stabilize the structure, which had to withstand winds of 80 km / hour. In addition, a special device was designed to protect the telescope ' s exit optics from the heavy rain that accompanied the storms, and the whole experiment had to be connected to a generator during each storm, to protect against frequent power cuts. The laser was then activated whenever thunderstorm activity was forecast, between July and September 2021. Beforehand, the area had to be closed to air traffic for a 5 km radius around the laser beam. Once all these problems had been resolved, the experiment could begin in mid-July 2021. A week later, the first bolt of lightning struck the tower while the laser was in operation. Luckily, that evening, the cloud ceiling was very high and the lightning was perfectly visible from the neighboring peaks. Using two high-speed cameras positioned at different angles, the images shown in Figure 3 were recorded. It can be clearly seen that over 60 m, the lightning bolt, which starts at the top
The laser system was located in the radome building, sheltered in an air-tight, air-conditioned and thermally isolated tent( see image). After exiting the tent, the laser beam was directed downwards by a conduit through the radome wall to the terrace, where a 4” folding mirror directed the beam into a beam-expanding sending telescope. The entire laser path toward the telescope, was protected by an isolated aluminium housing to prevent any beam leakage and to reduce the perturbation from the environment.
Layout of the experimental setup installed on Mont Saentis( Adapted with permission from [ 4 ].) CC BY 4.0 of the tower, follows the path of the laser beam. This is precisely the length over which the laser beam produces plasma filaments with this telescope configuration. Subsequently, a similar guiding effect was observed during three other lightning events, using a VHF detector capable of tracking the movement of lightning tracers in clouds, when the cloud prevents the lightning from being seen with a camera( see Figure 4). This experiment therefore demonstrated that an intense ultrashort laser beam was capable of repeatedly guiding lightning over a distance of more than 60 m, even under difficult weather conditions, as the mountain was regularly shrouded in dense fog. Many questions remain unanswered, concerning the reproducibility of this effect, or the ability of the laser to stimulate the number of lightning discharges. But these questions
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