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Banned on April 8th, 2007. Registered Member #597
Joined: Thu Mar 22 2007, 03:33AM
Location: Seattle, WA
Posts: 16
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.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
I think he's just trying to raise awareness and generate interest, which is fair enough... I subscribe to SciAm and hadn't caught the article on cosmic rays/lightning, any idea which issue?
Banned on April 7, 2007 Registered Member #277
Joined: Fri Mar 03 2006, 10:15AM
Location: Florida
Posts: 157
Bill:
I appreciate you mentioning topics which you think might be of interest, such as the 1MV spark. I didn't know the above. Thanks. The following is strictly my personal opinion to help you survive newbie bootcamp here at 4hv. There seems to be some sort of ritualistic initiation rite that some newbies are put through here especially if you bring up topics that aren't generally found in a college text book. Progressive thinking isn't prohibited here, but it isn't encouraged much either (again, my opinion). Middle-of-the-road topics seem to be the safest topics because nobody gets upset at them or lables them pseudo-science... but if you choose to mention topics that are not already generally known science or can't be goggled... be prepared for a few members or moderators to give you a hard time. I concur that true nut-cases should be policed in order to keep this site sane, but sometimes the policing seems to be applied unevenly, or threads locked down on topics that members are still quite interested in. The good news is that 4hv.org has a level headed owner and this site is worth putting up with the ritualistic newbie arse-spanking. However, I think yours will be abbreviated since you are already well known in science circles. That being said, this is a great site and the run-away spark topic you posted is of interest to me since I have been working with sparks daily for the last couple years on my sky voltage antenna project. In fact, somewhere (if I can find it), there is a picture of my sky voltage antenna operating your 5kV 'soda' bottle corona motor. CM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Are you trying to get our opinions? Our criticism? Random flames?
Perhaps he is simply trying to inspire some thought. (and dude, quit being the (first) one to nag)
This is why those big Russian marx generators were capable of enormous (100 meter IIRC) discharges from only 5MV output. I never even had a clue as to why until now.
I still suspect that your voltage source (creating the field of > 1MV) must still have considerable energy behind it, in order to actually create a proportionally longer spark. I mean, people have built little marx generators, capable of MV level discharges (of course, its always less than they thing due to stray capacitance having an initial charge of 0), but they dont generate 50' sparks or something. So what exactly happens in this case? Is the path bridged by a few electrons, but without sufficient energy we just dont get the bright spark?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
I agree that Bill's post poses some thought provoking questions. It is well known that ionising radiation can initiate discharges in gases (after all, this is how a G-M tube works). It would be nice if he did more than offer us a few scientific buzz-words and electrons that "think."
Registered Member #69
Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116
Steve Ward wrote ...
I still suspect that your voltage source (creating the field of > 1MV) must still have considerable energy behind it, in order to actually create a proportionally longer spark. I mean, people have built little marx generators, capable of MV level discharges (of course, its always less than they thing due to stray capacitance having an initial charge of 0), but they dont generate 50' sparks or something. So what exactly happens in this case? Is the path bridged by a few electrons, but without sufficient energy we just dont get the bright spark?
The bulk of the electrons actually have to gain ~1MeV of energy for the relatavistic effects to 'show' which would not happen with a 1MV accelerating potential since the electrons will collide with air and lose energy many times before crossing the potential drop. Of course you could do it in a vacuum if you built a DC linac.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
It's not enough to have a very high E field to generate long sparks, you have to have enormous currents as well. You could have a lot of electrons, but without the current, they're not going to be moving with enough velocity to produce really big sparks.
My TC (provided that dielectric breakdown is 30kv/inch) can output ~4 foot arcs, which is over a million volts in my own garage, but they don't just suddenly explode into a huge 8 foot arc across the room because the current is just too low to saturate the volume. The only way I could achieve an 8 foot spark from a low power Tesla coil would be to have an enormous E field causing a stream of current inbetween the two charged surfaces, like a flyback causing a cloud of charge between the terminals but without arcing. This would require a huge current at ~2MV DC over the 8 foot span.
The way I see it, ionized paths only occur inbetween two highly charged surfaces, with a substantial lekage current inbetween them. In the case of a source being one type of polarity (+ or -), and the sink being 0, you're uncovering opposite charges in the sink as you charge the source, so in effect you still have two highly charged surfaces, you're just moving charges around.
Banned on April 7, 2007 Registered Member #277
Joined: Fri Mar 03 2006, 10:15AM
Location: Florida
Posts: 157
Wave:
Not sure some us follow your entire post, except the 'rant' part is pretty clear. I for one, think Bill has taken sufficient effort in explaining his line of reasoning. If you tell him what part you don't follow, I bet he might explain. Bill innocently commited the dreaded deed of a double-post because he has so much info to share. Reducing or characterizing his insightful comments down to a series of buzz-words simply isn't fair to him or accurate and tends to support my post above typical of the harsh and/or rough treatment some newbies receive who dare to stretch the 'thought' envelope. Here's a friendly challenge to you... if you sit back and listen to what Bill has to say over the next month, assuming he doesn't decide this has already become more trouble than it's worth, I'll bet you a nickle you will learn some very interesting things along the way, many have, including me. Back to the topic of sparks, when a storm is within 50 miles or so of my 3500 foot long antenna, the thick sparks produced are easily 2 inches, sometimes 3 inches, between spark electrodes, and are a bright blue color, discharging between 2-3 times per second, accompanied by sizzling sounds like bacon on a skillet and loud bangs! There doesn't have to be visible lightning in the air, just some dark clouds off on the horizon. I do 95% of my research during fair weather conditions, when the above begins to happen, that is usually when I close down the RV (aka my lab) and go seek safer shelter. CM
Banned on April 8th, 2007. Registered Member #597
Joined: Thu Mar 22 2007, 03:33AM
Location: Seattle, WA
Posts: 16
Eric wrote: > The bulk of the electrons actually have to gain ~1MeV of energy for the relatavistic effects to 'show'
Oops. Good catch. I was assuming electrons in vacuum.
So, what would some hobbyist-level "electron runaway" equipment look like? The basic requirement is to create a large, space-filling e-field, then provide a "seed" of high energy electrons to start the "runaway" avalanche. The phenomenon might occur at nanosecond time scale. So relative to nS, a large TC running at 10uS time scale (or 100uS) would behave as slowly pulsed half-wave DC. Placing a radioactive source on the main terminal of a large TC might produce some new discharge phenomena. But unless the runaway process can be triggered by quite low intensity of ionizing radiation (Torbernite samples, or lantern mantels,) some other means is probably required.
A more silly version: create a very large TC with a meters-wide main terminal. Then build a 1MV VandeGraaff-based particle accelerator having a high-vac drift tube (see the first diagram on Shawn's page brightscience.com , also unitednuclear.com/fwa1.htm Then mount the entire linac inside the TC terminal. If the linac is a high-brightness source of fast electrons when compared to radioactive samples available to hobbyists, it has that much greater chance at sucessfully producing interesting changes in the TC discharge.
But do we need a VandeGraaff generator *and* a Tesla coil? Why not combine them? Let the TC act as a source of pulsed DC. Let's get rid of the VDG, since a TC has higher power output capability. Let's instead connect one end of a high-vacuum drift tube directly to a TC main terminal, with the other end of the tube projecting horizontally outwards from the TC terminal (connect a field-emission electrode within the drift tube to the TC terminal, of course. Or perhaps even use a hot filament.) The TC would need to put out far more than one megavolt, since we'd want to transiently produce at least a 1MV potential drop between the ends of the drift tube. The far end of the drift tube might need a foil window to more easily pass electrons, Lenard-tube style.
And so we've come full circle back to Nikola Tesla's single-electrode X-ray tube atop a tall TC. Very simple: a drift tube with an electrode at the HV end. (Perhaps enough electrons penetrate the glass wall at the far end to produce anomalous discharges?)
Scopeboy, about that photo you posted of a cyclotron beam. In the thread that was shut down? In answer to your question, yes, fast electrons only go a few tens of cm through air, so the obvious solution is to apply a 100M wide, volume-filling e-field to the yards of space outside the cyclotron. The cyclotron beam provides the fast-electron seeds. But if we wanted to see the huge long discharges, we'd have to put the whole cyclotron atop a tesla coil! That, or use a grounded cyclotron inside an enormously wide metal shell, then apply high voltage to that shell.
Anyway... during a negative half-cycle of the TC output, as the PD present along the drift tube rose high, some relativistic electrons would begin leaving through the foil barrier at the far end of the tube. There they would encounter the remainder of the TC's e-field, as well as encountering air molecules. They'd accelerate, and they'd also spawn more fast electrons via collisions with air molecules. Rather than producing a conventional plasma streamer, with luck the TC would produce an example of a "new kind of spark."
One problem I see in the above is that the relativistic avalanche might tend to bend and follow the e-field pattern near the TC. It might curve immediately downwards. On the other hand, particle beams in gas environments rapidly neutralize, so they tend to ignore e-fields, see papers about it: Finally, since the electron beam is composed of mobile electrons, it behaves as a long thin conductor connected to the TC. It would automatically develop an e-field along its length, as does any resistor. Also it would probably experience self-organization effects. What would this produce? Maybe the discharge would have some interesting structure. Like Red Sprites, etc.? Or ...since the particles move at nearly C, perhaps it would more resemble the beam from a searchlight. But why pursue theoretical prediction when empirical testing is more fun?!
One other person who intends to experiment in this topic is colleague Greg Leyh. See his page about this at wwwlod.org/LightningLab/LightningLab.htm
Note again that physicists say that these phenomena occur at a length scale of many tens of meters. They weren't noticed until recent years, since most plasma physics experiments don't employ chambers 50M across. If a "runaway discharge" spark can be produced by a backyard TC, the discharge should end up being that long, at the very least.
And about the Wikipedia article. I pointed it out because it has a brief description as well as article links, including the link to that great article in Physics Today. But PT has removed their article from public view. That's probably why somebody tagged the WP entry as lacking references. Here's a mirror copy:
Runaway Breakdown and the Mysteries of Lightning wwwphy.olemiss.edu/~jgladden/phys510/
spring06/Gurevich.pdf
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"Runaway breakdown" for creating *really* long sparks
