| MadSci Network: Physics |
Hi,
"Dielectric" is a broad term-it can involve both gases, liquids and solids. I've just read 908725376.Ph, and there the discussion of focused on gases, so I'll restrict this discussion to gases.
There are in fact two main mechanisms by which light can influence the breakdown processes: the first is called photo-ionization, and the second is called tunnel ionization. I'll briefly outline both processes. I'd like to note beforehand that the second process is rather exotic. Furthermore, I'll simplify things a bit, so
First, let's define what we mean by breakdown. A normal gas at typical atmospheric conditions consists of uncharged molecules. In order to be able to conduct current, this gas will have to be at least partially split in negative and positive charges. Normally, this involves ripping electrons from the molecules, leaving us with a positive core, the ion, and a negative electron. Subjecting these particles to an electric field lets them flow, and these flowing charges form an electric current. Hence, splitting some of the molecules in ions and electrons is sufficient for breakdown to occur.
Normally, breakdown in the atmosphere starts when a single electron, that perhaps originated from cosmic radiation, gets accelerated in the electric field, knocks other electrons free from their atoms, starting a Townsend avalanche [1]. Breakdown can only occur if this multiplication is faster than the rate by which electrons recombine or get attached to surfaces. This typically requires fairly high voltages. which is the reason you don't see sparks in your wall sockets. It's important to note that not all collisions between an electron and a molecule result in ionization; rather, in many cases, the energy of the electron is too low, and the electron is not knocked completely away from the atom, but rather knocked into a higher orbit. This does lower the energy of the electron.
One way in which light can facilitate this process is by exciting or ionizing the molecules [2]. Light can be seen as a stream of photons, and each of these photons has a certain amount of energy. Typically, this energy is not enough to directly ionize the molecules. What can happen, however, is a two stage process, in which for instance the electrons excites the atom and the photon ionizes it, or vice versa. The "boost" that is given by the light can greatly enhance the ionization rate, because there is an exponential dependence of the ionization rate on the energy of the electrons. Even a small "leg up" can mean an increase in ionization rate by orders of magnitude.
This process may be enhanced even further is the light is aimed at the metallic electrodes. This light, combined with the electric field at the electrode, might lead to the liberation of many free electrons [3]. This can also make breakdown occur far more easily.
A second process by which light might influence the breakdown is by means of a process called tunnel ionization. If an enormously powerful light source is focused between the electrodes, the gas might be ionized by the light alone. If the electric field of the light becomes comparable in strength to the electric field that ties the electrons to the molecules, it is possible that the electrons are liberated from their molecules en masse. This leads to an almost instantaneous breakdown, in theory even in the absence of an electric field [4,5], apart from the field from the light. The required light intensities can most easily be generated by means of a highly focused Ti:Sapphire laser, that has an output in the order of 100 GW [6].
In summary, light can dramatically change the breakdown of gases. The light provides energy that can liberate electrons, either boosting the Townsend avalanche, or in extreme cases ionizing the gas on its own.
Regards,
Bart Broks
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