End-on spark view - broken sparks
Tesladownunder, Sat Sept 02 2006, 05:51AMHere are some interesting shots of sparks taken end on. I used my MO-SIDAC-ignition coil setup which gives 4 inch sparks.
With 2 fine electrodes and aligning the camera down the axis you get some unusual effects.
Peter
Re: End-on spark view - broken sparks
Part Scavenger, Sun Sept 03 2006, 01:01AM
Cool. Reminds me of medusa.
Part Scavenger, Sun Sept 03 2006, 01:01AM
Cool. Reminds me of medusa.
Re: End-on spark view - broken sparks
Tesladownunder, Sun Sept 03 2006, 03:04AM
I was wondering about the discontinuity of the sparks with apparent gaps. Others have observed this and commented on it in Tesla shots.
I offer this explanation. An area of hot gas will act as a lens (concave type) due to the central rarefied area being of lower refractive index. This will act in mirage like fashion to diffuse the spark area beind it so that you get to see only a fraction of what you might otherwise.
This effect will be seen more on longitudinal shots which is exactly what you are seeing here and much more than in the standard right angle view.
The second pic shows the faint connecting segment. So this is the real black plasma
Any comments? I haven't seen this lensing effect given before.
Peter
Tesladownunder, Sun Sept 03 2006, 03:04AM
I was wondering about the discontinuity of the sparks with apparent gaps. Others have observed this and commented on it in Tesla shots.
I offer this explanation. An area of hot gas will act as a lens (concave type) due to the central rarefied area being of lower refractive index. This will act in mirage like fashion to diffuse the spark area beind it so that you get to see only a fraction of what you might otherwise.
This effect will be seen more on longitudinal shots which is exactly what you are seeing here and much more than in the standard right angle view.
The second pic shows the faint connecting segment. So this is the real black plasma
Any comments? I haven't seen this lensing effect given before.
Peter
Re: End-on spark view - broken sparks
Bjørn, Sun Sept 03 2006, 12:15PM
Take two pictures at the same time from different angles.
Bjørn, Sun Sept 03 2006, 12:15PM
Take two pictures at the same time from different angles.
Re: End-on spark view - broken sparks
Electroholic, Sun Sept 03 2006, 08:21PM
great idea but how?
oh heres and idea, how about a mirror?
that would give you another angle,
not exactly at the same time, but should be close enough.
Electroholic, Sun Sept 03 2006, 08:21PM
great idea but how?
oh heres and idea, how about a mirror?
that would give you another angle,
not exactly at the same time, but should be close enough.
Re: End-on spark view - broken sparks
Steve Conner, Sun Sept 03 2006, 09:18PM
I think it's because of the timing of the spark interacting with the timing of the camera shutter. This can lead to some weird results in cameras with focal plane shutters (a travelling slit that doesn't expose the whole frame simultaneously) I've posted about this effect on the TCML quite a few times.
Of course, if it was a time exposure, please ignore me :P
Steve Conner, Sun Sept 03 2006, 09:18PM
I think it's because of the timing of the spark interacting with the timing of the camera shutter. This can lead to some weird results in cameras with focal plane shutters (a travelling slit that doesn't expose the whole frame simultaneously) I've posted about this effect on the TCML quite a few times.
Of course, if it was a time exposure, please ignore me :P
Re: End-on spark view - broken sparks
Bjørn, Sun Sept 03 2006, 09:37PM
I was thinking of a mirror.
These pictures seems to be taken with a 2 second exposure time. The shutter may still generate some artifacts if something happens at just the right time.
Bjørn, Sun Sept 03 2006, 09:37PM
I was thinking of a mirror.
These pictures seems to be taken with a 2 second exposure time. The shutter may still generate some artifacts if something happens at just the right time.
Re: End-on spark view - broken sparks
HV Enthusiast, Sun Sept 03 2006, 11:06PM
Definitely not the shutter. If the camera captures the arc from beginning to end, and the gap is in the middle, than it definitely isn't the shutter.
What is happening is as TDU stated. When an arc channel forms, it superheats the air leaving a nice heated air channel which rises and thus blocks light from new arcs causing them to appear "broken" and "segmented." With larger Tesla coils (bigger arcs), the effect also occurs, but since the arc length is very large and not confined to such a small area, you don't see it as readily.
With some single-shot discharge photos i've taken, i've actually captured the shadow of the channel (after the arc occurred), on the wall behind it with the flash.
HV Enthusiast, Sun Sept 03 2006, 11:06PM
Definitely not the shutter. If the camera captures the arc from beginning to end, and the gap is in the middle, than it definitely isn't the shutter.
What is happening is as TDU stated. When an arc channel forms, it superheats the air leaving a nice heated air channel which rises and thus blocks light from new arcs causing them to appear "broken" and "segmented." With larger Tesla coils (bigger arcs), the effect also occurs, but since the arc length is very large and not confined to such a small area, you don't see it as readily.
With some single-shot discharge photos i've taken, i've actually captured the shadow of the channel (after the arc occurred), on the wall behind it with the flash.
Re: End-on spark view - broken sparks
Tesladownunder, Mon Sept 04 2006, 12:06AM
On further reflection, I don't think lensing is the effect since it occurs at only one point along the spark.
I now suspect that it is just a Crooke's space effect in a predominantly DC spark.
Note the brightness variation and where the dropout occurs.
Now also note the same effect with a TC. which to me implies some constant effect of polarisation of the spark. ie only positive or negative strokes make it. That is not surprising as a spark from smooth surface to point is affected by polarity by a factor of two if I recall. I did some measurements on this a couple of years ago.
Peter
Tesladownunder, Mon Sept 04 2006, 12:06AM
On further reflection, I don't think lensing is the effect since it occurs at only one point along the spark.
I now suspect that it is just a Crooke's space effect in a predominantly DC spark.
Note the brightness variation and where the dropout occurs.
Now also note the same effect with a TC. which to me implies some constant effect of polarisation of the spark. ie only positive or negative strokes make it. That is not surprising as a spark from smooth surface to point is affected by polarity by a factor of two if I recall. I did some measurements on this a couple of years ago.
Peter
Re: End-on spark view - broken sparks
HV Enthusiast, Mon Sept 04 2006, 01:12AM
Interesting effect, although I am still not convinced its an actual effect occuring within the arc. Do you have other cameras you can use to attempt to capture this?
I've taken literally tens of thousands of high voltage images ranging from low voltage DC discharges, to spark gap coils, DRSSTCs, high power Marx Generators, to lightning, and never have seen this effect even once.
Seems weird you have caught it numerous times, and in the last three photos all to the right side of the image.
HV Enthusiast, Mon Sept 04 2006, 01:12AM
Interesting effect, although I am still not convinced its an actual effect occuring within the arc. Do you have other cameras you can use to attempt to capture this?
I've taken literally tens of thousands of high voltage images ranging from low voltage DC discharges, to spark gap coils, DRSSTCs, high power Marx Generators, to lightning, and never have seen this effect even once.
Seems weird you have caught it numerous times, and in the last three photos all to the right side of the image.
Re: End-on spark view - broken sparks
teravolt, Mon Sept 04 2006, 03:54AM
Indeed that is an efect that is real I have seen this with my own eyes. I you can see it with ignition coils, VGraph, any blue arch. Maby has it something with different charges meating or a step leader maeting the arch. I would like to know why do VTTC's have steight archs. N.B.
teravolt, Mon Sept 04 2006, 03:54AM
Indeed that is an efect that is real I have seen this with my own eyes. I you can see it with ignition coils, VGraph, any blue arch. Maby has it something with different charges meating or a step leader maeting the arch. I would like to know why do VTTC's have steight archs. N.B.
Re: End-on spark view - broken sparks
Tesladownunder, Mon Sept 04 2006, 12:49PM
To get a two axis view of a spark, I have set up a first surface mirror to give a 90 degree view. I have kept the aperture small to accomodate the different focal lengths.
Look at the 5 spark example with 3 discontinuities present in both images.
I don't believe that this is a camera effect or a lensing effect. The brightness of the spark to the right is visible to the eye and often seen for thin low power sparks but not higher power sparks. EVR's sparks are much brighter than mine most of the time I suspect.
The spark discontinuity is not usually visible to the eye though, due probably to the rapidity of the event to the eye.
I will try a different camera and a rear view to show reversal of sides to confirm that it is not a camera view issue.
Peter
Tesladownunder, Mon Sept 04 2006, 12:49PM
To get a two axis view of a spark, I have set up a first surface mirror to give a 90 degree view. I have kept the aperture small to accomodate the different focal lengths.
Look at the 5 spark example with 3 discontinuities present in both images.
I don't believe that this is a camera effect or a lensing effect. The brightness of the spark to the right is visible to the eye and often seen for thin low power sparks but not higher power sparks. EVR's sparks are much brighter than mine most of the time I suspect.
The spark discontinuity is not usually visible to the eye though, due probably to the rapidity of the event to the eye.
I will try a different camera and a rear view to show reversal of sides to confirm that it is not a camera view issue.
Peter
Re: End-on spark view - broken sparks
Steve Maurer, Mon Sept 04 2006, 06:59PM
Peter,
Thanks for sharing these interesting photographs. Maybe we can offer some additional insight if you would tell us what camera you are using for these pictures. Is it a film or digital camera? What are the camera settings (shutter speed, aperture (f-stop), film ISO rating, lens type and focal length, distance from the sparks to the camera)?
- Steve
Steve Maurer, Mon Sept 04 2006, 06:59PM
Peter,
Thanks for sharing these interesting photographs. Maybe we can offer some additional insight if you would tell us what camera you are using for these pictures. Is it a film or digital camera? What are the camera settings (shutter speed, aperture (f-stop), film ISO rating, lens type and focal length, distance from the sparks to the camera)?
- Steve
Re: End-on spark view - broken sparks
Bjørn, Mon Sept 04 2006, 10:32PM
The last picture contains this EXIF information:
Nikon D70s
70 mm
f/16
1s
The shutter moves from bottom to top or possibly from top to bottom in the frame.
Bjørn, Mon Sept 04 2006, 10:32PM
The last picture contains this EXIF information:
Nikon D70s
70 mm
f/16
1s
The shutter moves from bottom to top or possibly from top to bottom in the frame.
Re: End-on spark view - broken sparks
Nik, Mon Sept 04 2006, 11:43PM
I have seen this broken spark effect from my tesla coil when I just started. The gap did not quench properly and the tank circuit was fed with 2 doubled mots, this made the coil output more like an induction coil then a tesla. I could see the broken spark pretty often untill I switched to a 4 stack of mots for a power suply.
Nik, Mon Sept 04 2006, 11:43PM
I have seen this broken spark effect from my tesla coil when I just started. The gap did not quench properly and the tank circuit was fed with 2 doubled mots, this made the coil output more like an induction coil then a tesla. I could see the broken spark pretty often untill I switched to a 4 stack of mots for a power suply.
Re: End-on spark view - broken sparks
Tesladownunder, Tue Sept 05 2006, 01:15AM
Perhaps one in 3 or 4 sparks have a discontinuity in my setup.
Some more photos with different settings result in the spark being a bit overexposed but still show the same effect.
Also a reversed photo from the rear of the same setup to show it's not a camera side thing. Most of these were 1 second exposures. None showed any sign of camera interference. Some of the earlier ones were taken with my Sigma 180 lens from about 3 feet and some with the 18-70 lens from closer.
I guess I need to pull out my 300kV diode and check polarity stuff.
Peter
Tesladownunder, Tue Sept 05 2006, 01:15AM
Perhaps one in 3 or 4 sparks have a discontinuity in my setup.
Some more photos with different settings result in the spark being a bit overexposed but still show the same effect.
Also a reversed photo from the rear of the same setup to show it's not a camera side thing. Most of these were 1 second exposures. None showed any sign of camera interference. Some of the earlier ones were taken with my Sigma 180 lens from about 3 feet and some with the 18-70 lens from closer.
I guess I need to pull out my 300kV diode and check polarity stuff.
Peter
Re: End-on spark view - broken sparks
WaveRider, Tue Sept 05 2006, 08:21AM
You may be witnessing the "magnetic pinch" effect. See
and
Essentially, the current in the arc creates a magnetic field that encircles the discharge. However, this creates a force on the moving charges in the arc which tends to constrict the flow...hence the "pinch"
Some experimentalists have measured the discharge currents and observed current decreases suddenly after the initiation of the discharge as the pinch effect causes current to drop off. Then the current increases again as the pinch effect is reduced or the discharge finds another path...
WaveRider, Tue Sept 05 2006, 08:21AM
You may be witnessing the "magnetic pinch" effect. See
and
Essentially, the current in the arc creates a magnetic field that encircles the discharge. However, this creates a force on the moving charges in the arc which tends to constrict the flow...hence the "pinch"
Some experimentalists have measured the discharge currents and observed current decreases suddenly after the initiation of the discharge as the pinch effect causes current to drop off. Then the current increases again as the pinch effect is reduced or the discharge finds another path...
Re: End-on spark view - broken sparks
Tesladownunder, Tue Sept 05 2006, 10:43AM
Here is a DC spark with left side negative. Hence this is a negative side effect.
Tesladownunder, Tue Sept 05 2006, 10:43AM
Here is a DC spark with left side negative. Hence this is a negative side effect.
Re: End-on spark view - broken sparks
WaveRider, Tue Sept 05 2006, 12:21PM
TDU, Looking closely at your last photo, you can just see channels of ionisation (leaders) widely avoiding the pinched off region (as a result of a concentrated magnetic field "ring" near the pinched off zone..). The current is becoming choked off at the center, so the potential drop in the gap becomes large, generating a corona "pre-discharge" which circles outside of the concentrated magnetic field region...
My 2 cent theory...
Lovely photos BTW!!
WaveRider, Tue Sept 05 2006, 12:21PM
TDU, Looking closely at your last photo, you can just see channels of ionisation (leaders) widely avoiding the pinched off region (as a result of a concentrated magnetic field "ring" near the pinched off zone..). The current is becoming choked off at the center, so the potential drop in the gap becomes large, generating a corona "pre-discharge" which circles outside of the concentrated magnetic field region...
My 2 cent theory...
Lovely photos BTW!!Re: End-on spark view - broken sparks
Tesladownunder, Tue Sept 05 2006, 03:04PM
My guess is that it isn't a magnetic pinch effect. The main reason is that the discontinuity occurs in only part of the spark near the negative electrode. Secondly Crooke's spaces vary greatly in site and number depending on pressure of the gas rather than current.
I have now taken pics in a high speed rotating mirror withe perhaps 10us resolution and am not seeing any other "structure" at that level.
Peter
Tesladownunder, Tue Sept 05 2006, 03:04PM
My guess is that it isn't a magnetic pinch effect. The main reason is that the discontinuity occurs in only part of the spark near the negative electrode. Secondly Crooke's spaces vary greatly in site and number depending on pressure of the gas rather than current.
I have now taken pics in a high speed rotating mirror withe perhaps 10us resolution and am not seeing any other "structure" at that level.
Peter
Re: End-on spark view - broken sparks
WaveRider, Tue Sept 05 2006, 05:15PM
You may be right... I am no expert in the theory of discharges. However, if I am not mistaken, the Crooke's dark space is space-charge driven effect in low-density plasmas (in vacuum environments, etc.) Since we are talking about a developed discharge (dense plasma "tube") with considerable current density, I would be inclined to look to current-driven effects first...
My theory could be full of BS, too.....
WaveRider, Tue Sept 05 2006, 05:15PM
You may be right... I am no expert in the theory of discharges. However, if I am not mistaken, the Crooke's dark space is space-charge driven effect in low-density plasmas (in vacuum environments, etc.) Since we are talking about a developed discharge (dense plasma "tube") with considerable current density, I would be inclined to look to current-driven effects first...
My theory could be full of BS, too.....
Re: End-on spark view - broken sparks
Bert, Thu Sept 07 2006, 03:30PM
Peter,
Very interesting pictures! You may have captured the gaps between isolated "space leaders" and a nearby negative leader. During the propagation of a negative leader, an isolated, bright section (called a "space stem") can be formed within the gap. This may then grow to become an extended "space leader" as the isolated discharge propagates in BOTH directions, being fed by a negative corona (actually a multitude of streamers) at one end, and positive corona at the other. It seems to be a fundamental process during negative long spark propagation
The unique combination of a declining TC terminal voltage and camera setup may be allowing them to be seen. Normally these gaps will be rapidly filled as the discharges propagate and merge with a nearby negative streamer. However, if the topload voltage declines after their initial formation, further propagation may be arrested. I'm not sure if this explains the cases where the gap is entirely bridged, however. See Gallimberti, "Fundamental Processes in Long Air Gap Discharges", by I. Gallimberti, G. Bacchiega, Anne Bondiou-Clergerie and Philippe Lalande, Comptes Rendus Physique, Volume 3, Issue 10, December 2002, Pages 1335-1359. Page 1339 has a picture of ioslated space leader occuring during negative leader propagation - see:
Bert
Bert, Thu Sept 07 2006, 03:30PM
Peter,
Very interesting pictures! You may have captured the gaps between isolated "space leaders" and a nearby negative leader. During the propagation of a negative leader, an isolated, bright section (called a "space stem") can be formed within the gap. This may then grow to become an extended "space leader" as the isolated discharge propagates in BOTH directions, being fed by a negative corona (actually a multitude of streamers) at one end, and positive corona at the other. It seems to be a fundamental process during negative long spark propagation
The unique combination of a declining TC terminal voltage and camera setup may be allowing them to be seen. Normally these gaps will be rapidly filled as the discharges propagate and merge with a nearby negative streamer. However, if the topload voltage declines after their initial formation, further propagation may be arrested. I'm not sure if this explains the cases where the gap is entirely bridged, however. See Gallimberti, "Fundamental Processes in Long Air Gap Discharges", by I. Gallimberti, G. Bacchiega, Anne Bondiou-Clergerie and Philippe Lalande, Comptes Rendus Physique, Volume 3, Issue 10, December 2002, Pages 1335-1359. Page 1339 has a picture of ioslated space leader occuring during negative leader propagation - see:
Bert
Re: End-on spark view - broken sparks
Tesladownunder, Thu Sept 07 2006, 06:24PM
I am glad that someone who knows "stuff" has relieved me of my off the cuff theories.
Interesting indeed.
Just some more detail of interest with the TC/Rotating mirror. This shot shows the spark travelling down the picture. The features are the initial bright spark (uppermost) with a gap then the ionisation fades out over 22 pixels (2.2uS). Interestingly there is a green flash lasting 2.8us at the electrode end, presumably of ionised copper. This is not strong with the initial spark but builds up as the ionisation is fading.
This is the type of time varying detail that I want to capture. If need be the speeds could be increased further but already measuring in 100ns units is pretty cool for home made stuff. May be 10ns timebase might show more stuff
Edit: This rotating mirror stuff leads into a whole fascinasting new topic which I have continued as a new topic in the Tesla coil forum: high speed Tesla photography.
Peter
Tesladownunder, Thu Sept 07 2006, 06:24PM
I am glad that someone who knows "stuff" has relieved me of my off the cuff theories.
Interesting indeed.
Just some more detail of interest with the TC/Rotating mirror. This shot shows the spark travelling down the picture. The features are the initial bright spark (uppermost) with a gap then the ionisation fades out over 22 pixels (2.2uS). Interestingly there is a green flash lasting 2.8us at the electrode end, presumably of ionised copper. This is not strong with the initial spark but builds up as the ionisation is fading.
This is the type of time varying detail that I want to capture. If need be the speeds could be increased further but already measuring in 100ns units is pretty cool for home made stuff. May be 10ns timebase might show more stuff
Edit: This rotating mirror stuff leads into a whole fascinasting new topic which I have continued as a new topic in the Tesla coil forum: high speed Tesla photography.
Peter
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