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DRSSTCs and Long Pulse arcs (corkscrew effect)

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LAN_403

HV Enthusiast

Sat Jul 15 2006, 01:40AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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I've recently been playing around with DRSSTCs utilizing long pulse mode of operation and exploring the so called corkscrew effect of the arcs. My present system is designed to produce output arcs up to 20ms and PRFs of 20Hz with a maximum duty cycle of 40%, although i typically limit runs to 10% for obvious reasons.

Regarding the corkscrew effect, one conclusion i have come to is that this corkscrewing effect is evident regardless of the pulsewidth. Short (50us) as well as the long pulsewidths exhibit this corkscrewing effect, and the it only becomes more evident with long pulses because the arc tends to rotate more revolutions due to the longer pulsewidth and this causes an apparent increase in brightness.

I'm not sure if anyone has come up with a reason for this effect yet, but i suspect it may be due to the magnetic field created by the arc (AC current) and its causing the arc itself to rotate. From my photography, i have determined the direction of rotation is clockwise which tends to correspond with the right hand rule.

Anyways, if anyone else has some thoughts on this, or if there is an actual explanation already for this effect, i'd love to hear it.

Here are some photos of a 20ms pulsewidth long arc striking a 36" target. Note the many loops, and "knots" in the photos as well as the primary arc channel which can be clearly seen. This is a single pulse (one arc) and although the main channel appears rather straight, i suspect the "loops" actually move through out the pulse and are simply overlaying the main channel when they occur.

Bjørn

Sat Jul 15 2006, 02:27AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
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Plasma has a tendency to do that at all scales from tiny discharges to gigantic structures in the universe. My guess is that it is Birkeland currents. Just currents flowing in the plasma. The current will create an opposing magnetic field that makes corkscrews and knots in the plasma.

Dr. Drone

Sat Jul 15 2006, 02:28AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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HV Enthusiast

Sat Jul 15 2006, 03:20AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Thanks. Looks like it just might be a contact area where the arc is hitting. If you look vertically to that, there seems to be some sort of boundary where the arcs seem to stop and this weird squiggley thing is right on it. There is a similar one down below. Thats just my guess.

...

Sat Jul 15 2006, 05:04AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
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You had me for a second with that first pic, until I clicked on it I thought you actually had a spark that had the red line down the center

In any case, those are some pretty awesome pics

I almost doubt that the corkscrew is from the plasma's magnetic feild, as I remember seing a pic looking in line with the ark off a marx generator that looked alot like your sparks do (not as fat of course)

@chris
I would almost put money on that little corkscrew thing was a bug that flew past the coil, got struck, then started to corkscrew arround

Steve Ward

Sat Jul 15 2006, 06:19AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I almost doubt that the corkscrew is from the plasma's magnetic feild, as I remember seing a pic looking in line with the ark off a marx generator that looked alot like your sparks do (not as fat of course)

So the spark from a marx doesnt produce a b-field around it? huh...

Dan:

I think we really need some high speed camera's to see exactly what is going on with the corkscrew production. I noticed that when i got my coil up to 5' sparks like that, the effect was even more pronounced. My mental image was that it was simply corona "growing" on the base streamer, but as to why it corkscrews, i had no idea. I agree that its likely its own b-field distorting the path, but i wonder about discharges between say 2 large spheres, where the arc is almost completely straight. And also, the B-field should be oscillating back and forth at a rather high frequency, so im not sure i see why it would only rotate in one direction over the other.

It seems that VTTCs produce a much more uniform effect. But sometimes the oddness of the SSTC sparks are more interesting to look at

.

Oh, and fantastic pictures btw.

Bjørn

Sat Jul 15 2006, 06:26AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Hey, can you tell me what this is as I wonder if it is the cork thingy? It just appeared and went away?

The tiny corkscrew seems to be part of something larger that did not record completely on the picture. You can see that the camera has moved during the exposure and that there is a line of missing arc segments to the left of the bar. So it is not possible to say it it is a corkscrew or not from the photograph.

Reaching

Sat Jul 15 2006, 11:33AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

if you convert a pic to negative you can notice this effect on most every pic of sstcs,drsstcs and vttcs. i think it has to do with the big E-M field of an arc induced back in the secondary coil which modulates the almost sinusodial waveform to a much more complex one which is part of this corkscrew effect.

Negative

normal

i looked at a few of my pics and noticed this effect on every of my drsstcs and sstcs independend of dutycycle,pulsewidth etc

another thought, it has to do with air flow between the hot plasma, .if you have a look at this corkscrew in detail you can see that it consists of many streamers with nearly the same distance to each other but with the same direction and corkscrew appearance. maybe the hot air is circulating between this arc which causes the plasma to rotate on the air flow, just a thought

...

Sat Jul 15 2006, 02:09PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

@steve ward

my logic was that the spark from a marx doesn't 'grow' like a tesla coil, which is running current through the streamer all of the times (and cuasing a magnetic feild around it) that gives it the corkscrew appearance. The marx spark spark doesn't have very much energy running through it until is is fully formed, so it couldn't have formed as a corkscrew from its own magnetic field.

HV Enthusiast

Sat Jul 15 2006, 05:32PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Thanks for the comments. Good information regarding the Birkeland currents. I never heard of that term before.

Steve,

I never thought about corona forming from the arc itself, but that is an interesting observation, at least visually.

Regarding high speed photography, i don't have access to a high speed video camera, but do have a Canon 50mm f/1.0L lense which is one of the fastest 35mm lenses ever made.

I may try taking some shots with this (i can probably shoot at f/1.0 and up to 1/2000 or 1/4000th) and still get a good exposure of the arc. This way, i can get a closer look to actual streamer channel and see what happens on a smaller time scale. Still even 1/1000 is stilll only 100us, quite long. Maybe a better example would be to shorten the pulsewidth and use that to control the exposure, except now arc length will be relatively short.

... wrote ...

@steve ward

my logic was that the spark from a marx doesn't 'grow' like a tesla coil, which is running current through the streamer all of the times (and cuasing a magnetic feild around it) that gives it the corkscrew appearance. The marx spark spark doesn't have very much energy running through it until is is fully formed, so it couldn't have formed as a corkscrew from its own magnetic field.

Any moving distribution of charges causes a magnetic field. This includes the discharge from a marx generator.

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