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.
(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.
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]
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.
References:
- Jack Shea, "Lightning Strike May Have Sparked Magnesium Fire," Fox 8 News, March 13, 2012.
- Mike Glenn, "Lightning strikes 4 planes near Houston airports," Houston Chronicle, March 9, 2012 .
- C. B. Moore, William Rison, James Mathis and Graydon Aulich, "Lightning Rod Improvement Studies," Journal of Applied Meteorology, vol. 39, no. 5 (May 2000), pp. 593-609,
- Nikola Tesla "Lightning-Protector," US Patent No. 1,266,175, May 14, 1918.
- Leonard M. Ball, "Laser lightning rod system," US Patent No. 4,017,767, April 12, 1977.
- B. Forestier, A. Houard, I. Revel, M. Durand, Y. B. André, B. Prade, A. Jarnac, J. Carbonnel, M. Le Nevé, J. C. de Miscault, B. Esmiller, D. Chapuis and A. Mysyrowicz, "Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament," AIP Advances, vol. 2, no. 1 (March, 2012), Document No. 1.3690961 (13 pages).
- Charles Blue, "Laser lightning rod: Guiding bursts of electricity with a flash of light," American Institute of Physics Press Release, March 12, 2012.
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