Is Terawatt Laser A Revolution in Lightning Protection? Opinion of Prof. E.M. Bazelyan

So, here is one more article, which, according to the authors “… ...paves the way for new atmospheric applications of ultrashort lasers and represents an important step forward in the development of a laser based lightning protection for airports, launchpads or large infrastructures”. The experimental material provided in the article conclusively demonstrates the result of the active effects on the lightning. This time, it was done with a laser beam (trigger lightnings) rather than a grounded wire, which does not clog the lightning channel with the vapors of the evaporated wire and does not require the tedious preparation of the rocket start. Do modern science and technology need all this?

We have no doubts that science does. Lightning is the most efficient natural electric event that has not yet been studied completely. The problem is in rare frequency of lightning discharges: in Russia, on average, 3-4 strikes per year occur per 1 km2 of ground surface. The lightning should be excited in the location in the required time point with a millisecond accuracy, which is a researcher's chimera. Surely, we can climb to the high-rise structure wherein lightning strikes frequency is significantly increased, but the lightnings there have a different nature, i.e. upward. They start from the high-rise building top and go to the thunderstorm. In physics, these are different lightnings. They significantly differ from conventional (downward) current and transferred charge parameters which pre-define all major dangerous effects of the storm electricity. It is annoying that the laser effects described in the article demonstrate efficiency only in terms of upward lightning start.

It is worth thinking of the mechanism of such a control effect. It allows at least evaluating its prospects. Here, all of these are defined by the opposed discharge development process from the top of the onground facility in a thunderstorm electric field. This field builds up rather slowly, within several or even several hundred seconds, and excites the opposed discharge as a streamer-free ultracrown with a very narrow ionization zone directly adjacent to the structure's top. Slow ions are emitted to a space over a structure from this area. They reduce electric field behind the wavefront and thus prevent the development of a conducting plasma channel of the opposed leader which can independently develop to become, as a result, the upward lighting. To transfer the streamer-free crown to a more powerful phase of the opposed discharge, its current should exceed some critical value at least 10 mA. This crown current may not be provided by a slowly building-up electric field of the thunderstorm even from high building such as the Ostankino TV Tower. An additional effect of a rapidly increasing electric field is required. In real-life natural conditions, it is provided by a charge of closely located downward lightnings. They do not strike high-rise structures, although they promote upward lightning start by strengthening the electric field nearby.

The word "rapid" in the previous paragraph is a key. Only in case of rapid transfer before the tip, a new portion of the volume crown discharge cannot be formed. Therefore, it is useless, e.g. to lift the electrode manually. However, the meteorological rocket with a flight speed of about 100 m/sec is suitable for this purpose. Laser essentially operates instead of such rocket by rapidly creating a well-conducting long plasma filament. Note that we can also look for an easier method by addressing to purely mechanical systems. We should only reliably evaluate the minimum movement speed of the extended conductor in particular conditions. There are reasons to believe that it can be implemented using relatively simple devices, e.g. a pressurized jet of conductive liquid. It is worth remembering the price of the terawatt laser with a 1 kHz pulse repetition rate.

Considering high price of the device, we should begin to evaluate the potential of using the laser equipment in practical lightning protection. The price is certainly important, although it is not defining in the problem. It is more important that the laser efficiency is yet defined for upward lightnings only. Their control is of low interest for the lightning protection specialists. All upward lightnings with high probability start from the top of the facilities having the strongest electric field. We should not particularly bother about them. The more so, there is no sense to artificially promote the upward lightning development. How will the laser control act in relation to the downward lightnings? Here, we need serious additional research. They have not yet been conducted.

Now, there is a fundamental issue. Modern equipment requires a very high reliability to protect from the direct lightning strikes, up to 0.999. Imagine for a moment that the developed laser effect may provide it. But can the laser equipment operate with such a high reliability? I am afraid that today, this question cannot be answered positively. The more so that the reliability is more than laser radiation emission but it is its formation at a required time moment when the downward leader actually approaches the protected structure and becomes really dangerous for it. This is when we should promote the start of the opposed leader from the lightning arrester to catch a dangerously moving downward lightning.

Certainly, we can stay ahead by starting the laser system in advance. In these conditions, the emerged opposed leader may not recognize the downward lightning and may propagate along the thunderstorm field. As a result, it will become an artificial upward lightning that strikes the lightning arrester. This is not very good since this lightning current in the lightning arrester will become an additional source of electromagnetic interference dangerous for neighbouring facilities containing numerous microelectronic devices. With this regard, note that the laser system of the authors has operated, in total, for more than 6 hours before 4 induced upward lightnings have been recorded. This can hardly be allowed in a practical lightning protection wherein the reliably synchronization system will be required. This is one more problem affecting the protection reliability.

Finally, the issue of the protection radius of the laser-controlled lightning arrester needs serious consideration. Without this, it is hard to hope for their practical use to protect large structures such as air fields or spaceport launching pads, as proposed by the authors of the article.

It seems like there are many problems, and most of them require that the serious scientific research would be continued.

Optical beaming of electrical discharges 2. Energy consumption of control actions on a lightning How Does A Lightning Orient and Can We Control This?