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"Runaway breakdown" for creating *really* long sparks

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Steve Conner

Sat Mar 31 2007, 10:58AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

bill beaty wrote ...
But if we wanted to see the huge long discharges, we'd have to put the whole cyclotron atop a tesla coil!

Sounds like a great plan, except my apartment is kind of full already

Seriously, if my mental model of these things is right, I think all that would happen is that the particle beam would stay the same length and would function as a seed for a regular looking Tesla coil discharge. I don't think it would be any different from what happens when you put a candle on a Tesla coil:
Candlezap

Of course I would love to be proved wrong

Sulaiman

Sat Mar 31 2007, 09:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I remember reading that long arcs occur when dV/dT > 2.06 kV/us,
presumably to achieve some critical stay-alive arc current (I=C.dV/dT)
and 5 MV helps!

Hope this helps more than it confuses.

Bored Chemist

Sun Apr 01 2007, 09:14AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I read this bit "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." and was a bit surprised.
There's stacks of data for beta particle ranges in air on the net so I collected some.
KeV cm range
18, 0.6
100, 11
156, 25
167, 26
249, 50
257, 50
500, 150
1000, 370
1710, 790
2000, 850
If you plot that out you will see there isn't a sudden rise, the data are pretty much on a nice smooth curve.

Since the effect doesn't seem to exist, I think discussing it is redundant.

Hazmatt_(The Underdog)

Sun Apr 01 2007, 07:38PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

So we can see the plot.

bill beaty

Mon Apr 02 2007, 12:45AM

Banned on April 8th, 2007.
Registered Member #597 Joined: Thu Mar 22 2007, 03:33AM
Location: Seattle, WA
Posts: 16

Fascinating graph! Something is certainly wrong somewhere, eh?

Bored chemist wrote:
> Since the effect doesn't seem to exist, I think discussing it is redundant.

Heh. I suggest that your confidence is misplaced. Your response strongly indicates that you haven't bothered to read anything about the topic before stating that it doesn't exist. This is suprising when considering the links I provided.

Also, do I really have to point out that theories are falsified by experiment and not the reverse? If some phenomenon is well-verified by the plasma physics research community, then any theory-based falsifications such as your graph are flawed, by definition. To me they even resemble an attempt at the logical fallacy of "well-poisoning."

Did you click on any of the links? I made it easy by giving google-search links on keywords, and on the article in Physics Today. You'd have found a large variety of physics papers and abstracts about runaway breakdown and relativistic electrons. Yes, the thunderstorm effect is still controversial, but runaway breakdown itself is not. But if you are genuinely a "chemist," bored or otherwise, I suggest avoiding all the plasma physics papers. Instead take a look at the popular article about thunderstorms. Particularly consider figure 2. Here it is again:

wwwphy.olemiss.edu/~jgladden/phys510/ spring06/Gurevich.pdf

And here again are those google searches.

Runaway breakdown

(9K hits)
Runaway electrons

(47K hits)

Looking at the graph, I probably misinterpreted Figure 2 in that article as not only determining the braking force experienced by individual electrons, but also having an effect on penetration distance. Doh!

Steve Conner wrote:
> Seriously, if my mental model of these things is right, I think all that would happen is that the particle beam would stay the same length and would function as a seed for a regular looking Tesla coil discharge.

Perhaps, but then you're ignoring avalanche effect, and it's avalanche which determines spark length. I hope you're at least slightly curious. I note that pseudoscientists preserve their pet theories, and they look for excuses to avoid empirical testing, while (amateur)scientists instantly want to see pet theories falsified (or even verified, but verification is far less common.) Faraday: "Let the experiment be made".

Even if the effect has been long demonstrated in plasma physics experiments, there's a good chance that the number of MeV seed-electrons attainable by hobbyist equipment is too low to trigger the long discharges or affect the look of TC streamers.

I have high-vac equipment, also glassblowing skills long unused. I have some new ideas for preventing the glass burn-through problem that Tesla was working on. But the largest TC I've built was a 30" desktop model (rotary synchro gap, NST-based, sold commercially.) My first TC I built in 1981 using an oil burner transformer.

Make no mistake, this drift-tube experiment has some dangers, at least similar to the "Atom Smasher" linac in old Scientific American.

[Edit: Removed off topic content]

Hazmatt_(The Underdog)

Mon Apr 02 2007, 04:37AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I keep asking myself where is this going? Certianly we are chatting about a phenomonon, but is this turning into a project thread?

And if you want to prove an avalanche charge effect by use of a LINAC in a vacuum system, how does that experiment have any relavence at 1-ATM?

And if you're talking about particle beams, emerging from a vacuum system through a window of some type now you're dealing with two totally different sytems. Any particle or wave encountering two different substances encounters two very different impedances. We talked about this in Physics, along with the dispersion relations. This brings up many more complications, like particle particle interactions at 1-ATM, and friction.

So is this a research project which goes in the project thread? A chatting thread where we talk about theories that may be substantiated at a much later time? Or a general science thread or high voltage thread?

Steve Conner

Mon Apr 02 2007, 10:16AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

We need to do is reconcile Bill's pleas that runaway breakdown allows huge discharges, with BC's empirical data, assuming neither is lying or wrong.

I think there is a simple explanation: BC's figures are for beta particles in air with no electric field. If a 2MeV particle can travel 8.5 meters before being brought to a halt, then a naive calculation suggests that if it were in an electric field of 235kV/meter, the field would add energy to it as fast as it was drained by collisions, and it would never stop. Of course, there are probably all sorts of relativistic horrors I don't know about, but I bet the fundamental argument still holds.

235kV/m is easily attainable by a hobbyist Tesla coil, but 2Mev beta particles are a different ball game. An electron accelerator built into a Tesla coil secondary might produce, say, 500keV particles and a 500kV/m field. You could build such a device and see whether the streamers got longer or changed direction when the electron gun was turned on.

bill beaty

Mon Apr 02 2007, 10:57AM

Banned on April 8th, 2007.
Registered Member #597 Joined: Thu Mar 22 2007, 03:33AM
Location: Seattle, WA
Posts: 16

Steve wrote:
"then a naive calculation suggests that if it were in an electric field of 235kV/meter, the field would add energy to it as fast as it was drained by collisions,"

Exactly. The thunderstorm physics article uses Runaway Breakdown as an explanation of why lightning can be initiated at far below the 3MV/meter value for dielectric breakdown in air. In that environment, the avalanches would be triggered by *single* cosmic ray particles. Perhaps an HV drift tube can create a robust effect by providing a few more than that.

"Of course, there are probably all sorts of relativistic horrors I don't know about, but I bet the fundamental argument still holds."

There's also Beam Neutralization phenomenon seen in gas environments, also Gas Focusing, and the conductor effect from the presence of mobile carriers. I suspect that all three will tend to aid things, making the discharge even longer. Throw in the nonlinear emergent plasma stuff that makes lightning be fractal, and it's hard to say what it would do.

Hazmatt wrote:
"And if you're talking about particle beams, emerging from a vacuum system through a window of some type now you're dealing with two totally different sytems. Any particle or wave encountering two different substances encounters two very different impedances. We talked about this in Physics, along with the dispersion relations. This brings up many more complications, like particle particle interactions at 1-ATM, and friction."

That's the case if we're theorists. Experimentalists do things differently. Analogy: it's 1880, and I want to know if sticking a filament in an evacuated bulb can act as a light source. Perhaps many problems will arise which careful analysis could avoid. Or perhaps I'll stick a filament in an evacuated bulb and apply power just to see what happens. It's Edison science versus Langmuir science. Maybe any such projects benefit from both types of people?

About "project thread," I have no project to display. The real problem is that there's no section for "Amateur Science Projects" here. That's where proposed hobby setups never before built, and with unknown results, would belong. Otherwise, it's a tossup whether this goes under High Voltage or Tesla Coils.

"If we knew what it was we were doing, it would not be called research, would it?" -- Albert Einstein

Eric

Mon Apr 02 2007, 08:17PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

Making a LINAC that 'fires' into air is not so hard to do. Check out this Lenard tube. It seems like it wouldn't be too hard to build such a thing driven by ~1MV and use it as an electrode for a TC or perhaps a VDG. Arc dynamics are so complex I don't think you could predict how it was going to work without trying it.

Bored Chemist

Tue Apr 03 2007, 05:47PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

OK, I was a bit hasty condemning this. Sorry.
I'm not pretending I read all that Bill wrote. On the other hand what he says (that what I wrote indicates I didn't read his stuff) doesnt make sense since roughly half the text I wrote was a quote from him.

Either there's something magic about 1MeV electrons or there isn't. They can't "remember" that they were spat out of a nucleus or launched from an accelerator. They behave in the same way.
I also think that the data I provided are quite clearly experimental whereas the topic you discuss is (at least from the point of view of most home experimenters) pretty theoretical.
Now I have read (and thought) a bit more about it, I accept that you might be able to get electrons to act oddly if you can launch them fast enough. A hydroplaning boat might be a bad analogy; once you have got it up to speed you can keep it running with a smaller power than you needed to launch it in the first place.

I still stand my my assertion that "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. " is wrong unless you add the rather important detail that having been taken up to a high speed they are not left to coast, but are pulled along by a further potential drop.

BTW, 2MeV betas might be a bit tricky for the amateur scientist. However, 1.3 and 1.5MeV electrons are relatively (if you forgive the pun) easy to get. Unfortunately, unless you happen to have isotopically enriched 40potassium, the yield is lousy. At this point I wonder if that relativistic avalanche helps.

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