Optics & Photonics News - July/August 2010

When they connect, a current up to several hundreds of kilo-amperes flows from the ground to the cloud: The so-called return stroke, which lasts from a few tenths of a second to a few seconds, constitutes the visible and audible part of the lightning strike.

Since stepped leaders are typically 10 m long, it would not be realistic to fully reproduce the mechanism of lightning in the laboratory. Even field experiments are challenging, given the random character of lightning and the need to accumulate reproducible data.

Researchers have developed a technique to trigger lightning on demand by launching a rocket that pulls a thin metallic wire toward thunderclouds. The rocket and its wire act like a lightning rod, which triggers the lightning strike and guides it to the ground. A more realistic effect can be obtained by limiting the metallic section of the wire to a few tens of meters, so that the discharge hitting the ground is very close to the natural one.

However, the number of rockets available during a thunderstorm is limited by the launch pad, which can host 5 to 10 rockets in typical facilities and cannot be refilled before the end of the thunderstorm. Furthermore, the success rate depends strongly on the instant when the rocket is launched. A continuously operating technique to trigger lightning would therefore be highly desirable. Lasers have very early been identified as candidates for this purpose. The first tests were performed almost as soon as high-power lasers were available in the 1960s.

These experiments, which used “long” laser pulses of several nanoseconds or more, were not successful. The leading edge of the pulse ionizes the air and accelerates the free electrons, resulting in avalanche ionization and finally a dense plasma that absorbs the trailing edge of the pulse. As a consequence, a large part of the pulse energy is lost, so that the generated plasma column is limited to a few meters in length.

Stepped leader Corona discharge Elevated point ++ +

+
+ Ascending leader
Return stroke

Lightning strike guided by a wire-pulling rocket at

Langmuir Laboratory (New Mexico Tech, N.M.)

In contrast, ultrashort (sub-100 fs) laser pulses are too short to initiate electron avalanches. The electron density therefore stays lower than in the case of “long” pulses, and the plasma remains transparent to the laser pulse. As a consequence, ionized self-guided filaments can exceed 100 m in length.

Using the Teramobile laser system, we first demonstrated their capability to trigger high-voltage discharges. By connecting two electrodes that were several meters apart, the laser filaments reduced the breakdown voltage by 30 percent, triggering the discharge in conditions that would not have allowed them without the laser. Furthermore, these triggered discharges are guided along the laser filaments rather than following the erratic path typical of a classical electric discharge.

Getting through
stormy weather

Besides the fact that the
mechanism of meter-scale discharges in the laboratory is
not fully representative of real lightning, two conditions are
required to extend the above laboratory results on megavolt
discharges to the real atmosphere.

First, laser filaments must propagate through the turbulent medium of the thundercloud and even through rain. This is made possible because the filamentation process is particularly robust. Filaments do not bear the whole energy of the laser beam. They are surrounded by a “photon bath” that regenerates them if they have been blocked by an obscurant such as a water droplet. We observed that this mechanism allows filamentation to be transmitted even through dense clouds.

Similarly, atmospheric turbulence does not block filamentation because refractive index gradients due to atmospheric inhomogeneities are too weak to block the dynamic balance at play in the filament. Furthermore, filaments transmitted through perturbed atmospheres retain their main properties. In particular, we have shown that they can still trigger high-voltage discharges under an artificial heavy rain.