Bill Beaty wrote this
@all Bill Beaty wrote this here: 4hv.org: Forums / High Voltage / "Runaway breakdown" for creating *really* long sparks What do u think?
Sparks leap between electrodes because of progressive ionization of the air. Once gas-breakdown has been triggered, the plasma contributes bare electrons via avalanche, and also creates UV radiation, both of which ionize the next bit of air into spark-stuff.
But there is a second little-known kind of spark. While in 1-atm air, electrons normally have very short trajectories, and can travel a cm or two before being halted. However, if electrons should travel across a voltage drop of approximately 1MV or larger, they suddenly are able to travel a hundred times further in air. At kinetic energy of around 1MeV or higher, electrons go relativistic (travelling at nearly the speed of light) and the collision rules are different. The air seems more transparent.
If such "fast electrons" should travel through an electric field, they gain far more energy than normal electrons would, since normal electrons experience far more 'air friction' via multiple collisions with air molecules. In other words, the fast electrons think that our air pressure is 0.01 atmosphere, and they behave more like a particle beam rather than an outbreak of fractal spark-plasma. With 100x less 'friction,' fast electrons are easily accelerated by fairly weak e-fields.
In addition, if they strike air molecules, fast electrons can create more fast electrons. This opens up the possibility of a different kind of spark, a spark based on an outbreak of a different kind of electron-avalance. Physicists refer to this by several names:
Runaway breakdown
Electron runaway
Runaway electrons
Also see , and the short wikipedia entry I wrote on this.
This bit of physics is increasingly in the news because it may explain some of the continuing mysteries of lightning. Lightning is not a conventional spark, since it occurs at far too low a voltage. But if cosmic background radiation (the geiger counter clicks) can easily supply a tiny amount of fast electrons, an immensely long spark might form via runaway breakdown rather than the usual UV and avalanche ionization. And this spark might grow despite a very weak environmental e-field present in storm clouds. Or said differently: because cosmic rays are present, lightning in a storm would strike at much higher frequency because the e-fields would not have to grow very large before a spark appeared to short them out again.
@all Bill Beaty wrote this here: 4hv.org: Forums / High Voltage / "Runaway breakdown" for creating *really* long sparks What do u think?
Sparks leap between electrodes because of progressive ionization of the air. Once gas-breakdown has been triggered, the plasma contributes bare electrons via avalanche, and also creates UV radiation, both of which ionize the next bit of air into spark-stuff.
But there is a second little-known kind of spark. While in 1-atm air, electrons normally have very short trajectories, and can travel a cm or two before being halted. However, if electrons should travel across a voltage drop of approximately 1MV or larger, they suddenly are able to travel a hundred times further in air. At kinetic energy of around 1MeV or higher, electrons go relativistic (travelling at nearly the speed of light) and the collision rules are different. The air seems more transparent.
If such "fast electrons" should travel through an electric field, they gain far more energy than normal electrons would, since normal electrons experience far more 'air friction' via multiple collisions with air molecules. In other words, the fast electrons think that our air pressure is 0.01 atmosphere, and they behave more like a particle beam rather than an outbreak of fractal spark-plasma. With 100x less 'friction,' fast electrons are easily accelerated by fairly weak e-fields.
In addition, if they strike air molecules, fast electrons can create more fast electrons. This opens up the possibility of a different kind of spark, a spark based on an outbreak of a different kind of electron-avalance. Physicists refer to this by several names:
Runaway breakdown
Electron runaway
Runaway electrons
Also see , and the short wikipedia entry I wrote on this.
This bit of physics is increasingly in the news because it may explain some of the continuing mysteries of lightning. Lightning is not a conventional spark, since it occurs at far too low a voltage. But if cosmic background radiation (the geiger counter clicks) can easily supply a tiny amount of fast electrons, an immensely long spark might form via runaway breakdown rather than the usual UV and avalanche ionization. And this spark might grow despite a very weak environmental e-field present in storm clouds. Or said differently: because cosmic rays are present, lightning in a storm would strike at much higher frequency because the e-fields would not have to grow very large before a spark appeared to short them out again.
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