Tikalon Blog by Dev Gualtieri

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Laser Lightning Rod

March 23, 2012

When my daughter was in high school , she did a science fair project on the radio detection of meteors (details here ). Her apparatus, as she found, was also a very good detector of lightning . Lightning generates radio frequency interference . You can hear this on the AM broadcast radio band , and also on the 97.7 MHz frequency she monitored. The following figure is a data plot of a fifteen minute period during a particularly intense thunderstorm in our area.

Radio observation of lightning strikes

Radio detection of lightning strikes during a storm in northern New Jersey on June 12, 2003. Each peak is the radio signature of an individual lightning bolt . These data are from 10:00 PM to 10:15 PM. (Via FM Radio Detection of Meteors Web Site).

Karl Jansky , the father of radio astronomy , owed his good fortune in being the first to detect an extraterrestrial radio source to lightning. His experiment was conceived as a means of tracking down and quantifying all sources of radio interference. The interference he found was from the expected sources, nearby thunderstorms and distant thunderstorms, but there was also the faint hiss of Sagittarius A .

Lightning strikes are infrequent, but when lightning does strike, there might be serious consequences. A recent fire at a magnesium plant in Ohio is thought to have originated in a lightning strike.[1] Nearly all of us have experienced flight delays because aircraft are not refueled during lightning storms; but a flight delay is less of an inconvenience than having your plane hit by lightning.[2].

Since Benjamin Franklin's time, people have been devising means for lightning protection . Franklin invented the lightning rod as a follow-up to his experiments on electricity . Franklin's observations showed that lightning rods should have a pointed tip, but the contrary British thought they should have a ball at the tip. I, and all physicists who have studied electrostatics , would uniformly vote for pointed rods, but our spherical cow model might be wrong.

One good experiment is worth quite a few theoretical papers , and such experiments with lightning rods were done by Charles B. Moore and his colleagues in 2000. They found that lightning rods with a moderately rounded, or blunt tip, performed a little better.[3]

Although sharp-tip lightning rods have a much stronger electric field strength at the tips, the field strength decreases quickly with distance. At a few centimeters distance, the slightly blunt rods have the higher field strength.[3] There must be a way to do this with textured surfaces to get the best of both worlds, although a wet surface might not behave as you would hope.

There are quite a few lightning rod patents . Nikola Tesla had his own design in 1918.[4] Since lasers have been applied to nearly everything , it's no surprise that a patent on a "Laser Lightning Rod System" was issued to Leonard M. Ball in 1977.[5]

Ball's idea, which has been pursued in other systems, is quite simple (see figure). The laser is used to ionize a column of air to prime a discharge path for a lightning strike. In this way, lightning can be directed to one area, and the surrounding areas are rendered safe.

Fig. 1 of US Patent No. 4,017,767, 'Laser lightning rod system,' by Leonard M. Ball, April 12, 1977

Fig. 1 of US Patent No. 4,017,767, "Laser lightning rod system," by Leonard M. Ball, April 12, 1977.

The laser beam (5) is directed through a conducting tube (4) by a mirror (10).

There's a magnetic field (12) to ensure that the lightning bolt doesn't go too far down the tube before contacting it to make a ground connection .

(Via Google Patents). [5]

The critical technology in such a device is the laser. Historical experiments on laser lightning rods in the 1960s, and later, used CO₂ and YAG lasers , since a high power laser is required. The problem with such lasers is their long optical pulse, which produces a discontinuous filament. Physicists from several French organizations, including the Laboratory of Applied Optics of the Ecole Polytechnique (Palaiseau, France), have published the results of experiments on the use of a more effective femtosecond laser to direct electrical discharges.[6-7]

The laser used was a Ti:sapphire laser capable of delivering a 7 terawatt pulse of 350 mJ in 50 femtoseconds at a repetition rate of 10 Hz. Chirped pulse amplification was used to prevent damaging the optics, and a laser beam of 40 mm in diameter was produced. The conductance path is produced by ionization of oxygen by the beam. The laser beam was shown to be effective at guiding a discharge at distances of at least fifty meters .[6]

Laser lightning rod testing apparatus

Using a femtosecond laser to short-circuit a more direct discharge path.

An experiment by B. Forestier, et al. [6]

(Rendered by author using Inkscape).

One interesting experiment is illustrated by the above figure. Discharge from the flat upper plate would naturally connect to the high, pointed rod. Instead, the laser was capable of directing it to a round electrode, instead. Not only that, but triggering the laser when an initial current was detected through the pointed rod, the laser redirected the discharge to the round electrode.[6] This is the first time that a laser was demonstrated to be capable of redirecting an electrical discharge away from its intended target to another electrode.[7]

This research was supported by EADS (noted for the Airbus ), with assistance in the high voltage testing by the Direction générale de l'armement .

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