- Research
- 27/11/2014
A thunderstorm in the lab
Lightning is more than just a flash of light. A thunderstorm appears to release gamma rays and X-rays, and perhaps even neutrons. PhD candidate Pavlo Kochkin imitated lightning strikes in the TU/e high-voltage lab, and sent along an X-ray detector in an Airbus so as to see what happens exactly when we fly through a thunderstorm.
Mysterious things are going on above our heads. As early as the 1990s a NASA satellite –built to explore space to look for gamma ray flashes – saw short outbursts of such radiation, which, to everybody’s surprise, originated from the earth. Terrestrial Gamma-Ray Flashes they were called, which means ‘earthly’ gamma ray flashes, although it soon became clear that they did not come from the surface of the earth, but were generated high up in the atmosphere during thunderstorms. What was remarkable was that these gamma ray flashes always preceded the lightning.
In the meantime it has become known that also X-rays are generated during lightning. Both gamma rays and X-rays consist of photons with a much higher energy than the light particles we perceive as visible light. In principle, both forms of radiation could be harmful to passengers and equipment in airplanes, for instance. “Pilots already try to avoid thunderstorms as much as possible because of the risk of icing, turbulence and the risk of being hit by lightning”, says Pavlo Kochkin, PhD candidate at Electrical Energy Systems. “Exposure to radiation can be an extra reason to be careful when there’s a thunderstorm.”
“When the X-rays form a beam, you wouldn’t want to be hit by that”
How harmful the radiation is, depends on the energy of the photons and on the directions in which they are emitted, Kochkin explains. “When X-rays are emitted evenly in all directions, they can’t have such a big impact, but when they form a beam, you wouldn’t want to be hit by that.” Reason enough, then, to find out how and where the radiation is generated exactly.
As it was known that X-rays are also released when long sparks are drawn in the lab, the Ukrainian created ‘lightning’ himself in the high-voltage lab of Electrical Engineering in the past few years, studying this phenomenon at close range. “We have a so-called Marx generator that makes it possible to create very short voltage pulses of a few megavolt. This allows us to draw sparks of more than a meter. I analyzed those sparks by means of a special high-speed camera and X-ray detectors. If you look at the sparks that way, they turn out to have a striking similarity to special discharge phenomena in the atmosphere, the mysterious sprites that can be seen above thunderclouds.”
However, the sparks in the lab are some ten thousand times smaller than those in the atmosphere, and in addition they are generated at a much higher pressure and temperature. “That does make the comparison controversial”, Kochkin admits. “After all, the genesis of such discharges in the atmosphere is explained precisely by the low pressure and temperature reigning there. For that reason some people say that the origin of sprites cannot possibly be the same as those of the sparks in the laboratory. Nevertheless I have devoted attention to this in my in PhD thesis, because I want to open the discussion about this topic.”
By drawing countless sparks, while always making photos at slightly different moments during the discharge process, Kochkin created a kind of film of the process. Each photo has an exposure time of less than one-millionth of a second. Never before have meter-long sparks been made visible in that way, he believes. “The camera was positioned three meters away from the discharge. We have had to consider the construction very carefully indeed in order to prevent the camera from being hit by the discharge. For that would set you back sixty thousand euros in one fell swoop. Fortunately we have extensive expertise in designing such a constellation in the proximity of high voltage.”
By comparing the photos with the signals of the X-ray detectors, Kochkin could find out the precise moment of genesis of the radiation. And indeed –as suggested by the satellite measurements- this was just before the real spark is generated. In the run-up to the spark you first get so-called streamers emerging from the negative electrode, thin plasma channels with ionized gas which appear to function like trailblazers for the eventual discharge (see pictures). When the streamers get in the proximity of the positive electrode, they meet specimens of the same sort that travel the reverse route.
Just before the positive and negative streamers hit each other, enormously strong electric fields are generated between them. As a result, the electrons in the plasma are accelerated and subsequently slowed down by collisions with ions in such a way that they begin to emit X-rays. That whole process lasts no more than a billionth of a second. As soon as the streamers hit each other, a conductive channel between the electrodes is created. As a result, they are discharged in one blow: that is the ‘lightning’ that we know. So just before that discharge, a large dose of X-radiation is emitted for a very brief moment.
Conducting measurements of lightning ‘in the wild’
The spectacle described above takes place entirely inside the lab. This spring, though, Kochkin and some colleagues got a unique opportunity to conduct measurements of lightning ‘in the wild’. Airplane manufacturer Airbus regularly carries out test flights under extreme weather conditions. For this purpose they use a system, ILDAS, which makes it possible to determine the impact of lightning strikes on the airplane. “My PhD thesis supervisor, Lex van Deursen, has developed sensors for ILDAS. In April that system was expanded temporarily by two X-ray detectors. Altogether eight test flights were made during which we have recorded 62 lightning strikes.”
In order to elaborate the results of the test flights, the Ukrainian will after obtaining his PhD degree – on December 2 – continue at Electrical Energy Systems as a postdoc for another year. In addition to the painstaking analysis of the gigabytes of measurement data, he has meanwhile also started a new experiment, as he explains with great enthusiasm. “I’m going to charge a cloud of steam electrically with a plasma in order to imitate a thundercloud. I then want to suspend that cloud over a scintillator, which is a detector that is used to detect cosmic radiation as well.”
The doctor-to-be has far from finished his work with lightning, then. Indeed, there is still so much to be researched. Recently a Russian group reported that they had seen neutrons in a discharge that is similar to the one Kochkin created. “We have not been able to reproduce that, but this may be due to us. Unlike those Russians we have little experience with neutrons. On the other hand, nobody has any idea how those neutrons are generated then.”
Meanwhile the Eindhoven research of lightning has aroused extensive interest. “I get invited everywhere now to speak at conferences. We are in fact reasonably unique as electrical engineers among all the scientists studying the atmosphere.”
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