Laser lightning rod and artificial fog dissipation

Significant efforts have been dedicated to control lightning with lasers [1] to protect sensitive sites like rocket launching pads and airports. We report on a dedicated European large framework project towards this goal, called Laser Lightning Rod (llr-fet.eu) [2] . In this project, a unique laser system has been developed, providing 720 mJ, < 1 picosecond pulses at 1 kHz repetition rate, i.e., TW-class peak power and kW average power [3] . This thin disk laser has been further frequency doubled (SHG) and tripled (THG) to provide high intensity outputs at 1030 nm, 515 nm and 343 nm, with conversion efficiencies up to 45% and 30%, respectively. Nonlinear propagation and filamentation of this laser could be characterized over 130 meters in a dedicated hall (LAL in Orsay), both in a collimated geometry, and using a 40 cm diameter beam expander. The whole system will be transported to the top of the Santis Mountain (2481 m altitude) in Switzerland in April 2021, one of the locations with the highest lightning occurrence in Europe. The site has been fully equipped to host the laser transmitter in the harsh meteorological conditions encountered on the top of the mountain. The site has also been fully equipped with state of the art lightning detection and measurement devices (a lightning mapping array, an interferometer, high-speed cameras, current sensors, a field mill, etc.) to register the triggering and guiding effects of the laser on the lightning process. This site is especially interesting as it generates mostly upward flashes from the 123 m tall tower communications tower. The laser will be aimed to the top of this tower in order to extend its height by the length of the filaments , realizing a "laser lightning rod".