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4hv.org :: Forums :: Tesla Coils
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High speed Tesla spark photos

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Tesladownunder
Wed Sep 06 2006, 02:15PM Print View
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
I have now setup my rotating mirror setup on a small Tesla coil. The related threads on this topic that I started that got me here are "End-on spark view - broken sparks" and "High speed spark photography".


The Tesla coil is my junk coil running on half of a 12kV 30mA NST. It has a few ceramic caps and a 3 segment static gap. Primary is 15 turns and secondary is 260 turns in 11 inches. There is usually no toroid but I used one to intensify the sparks by putting an old tin on top.

The motor and first surface mirror is as shown and rotates at about 2900 RPM. It is a synchronous motor with ground flats that doesnt quite sync at 3000 RPM with the extra load.

Mirror to spark distance is 38cm which means that the image moves at about 100m/s. The picture represents about 2cm width and 4cm height ie vertical scale is 40us. (just over 100ns/pixel)

So what do we see and how to interpret it?
There is a ladder of sparks with each spark being fairly discrete and without any obvious parallel sparks. All sparks seem complete and there are no discontinuities. Almost all sparks are bright at the ends but less bright in the centre third. This also corresponds with what you see when it is running. I am not sure what it means, however, if each spark is a single cycle then the negative one third may brighter each half cycle, leaving the centre dim.

Peter







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HV Enthusiast
Wed Sep 06 2006, 02:39PM
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Could you explain in more detail how the mirror works and how you are capturing the arcs image in a brief period of time? With a rotating mirror, i can't see how that is occuring especially considering that the minimum shutter duration for your camera is only 125us and you are actually using a shutter speed of 1/2 second.

I would think you would only want to rotate the mirror once extremely fast to capture a brief period of time as continously spinning the mirror will just integrate the image over the full 1/2 second as opposed to capturing say only a 5us fragment.
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Tesladownunder
Wed Sep 06 2006, 04:00PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
EastVoltResearc wrote ...

Could you explain in more detail how the mirror works and how you are capturing the arcs image in a brief period of time? With a rotating mirror, i can't see how that is occuring especially considering that the minimum shutter duration for your camera is only 125us and you are actually using a shutter speed of 1/2 second.

I would think you would only want to rotate the mirror once extremely fast to capture a brief period of time as continously spinning the mirror will just integrate the image over the full 1/2 second as opposed to capturing say only a 5us fragment.



The mirror spreads the image over time and due to the mirrors speed, it will only show a 4cm section out of each circumference of 2*pi*38cm = 238cm. This means that only 4/238 = 1.7% of the time is the mirror aligned to allow a spark image to the camera. It is however travelling at 238cm*2900RPM = 238*48cm/s=11500cm/s. It will do this repeatedly over the duration that the shutter is open.
Typically with camera shutter speeds of perhaps 1/10 sec you only get to see single sparks. This is 4.8 mirror rotations only with a 1.7% chance of hitting a given spark. About even odds for a 10Hz spark.
I did do some 1/400 shutter speeds and never saw a spark once in about 10 shots.
Each spark image will be overlaid but if there are few enough, then they won't overlap too much.

So no, this isnt a high speed shutter but a way of introducing a high speed time axis to events. If it were looking at stepped leaders and other lightning stuff, it would be very interesting as the spark crosses from left to right in a stepwise manner.

I need to try this on a bigger TC to show the banjo effect, and perhaps on a DC spark through an inductor to show the spark develop into an arc. As it is these small TC sparks are too small and fast to show structure on a microsecond timescale.

Pic shows a typical single spark seen with a 1/10 exposure and slow TC firing rate.

Peter






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Terry Fritz
Thu Sep 07 2006, 10:36PM
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Very cool Peter!!

This is like the "streak" cameras used to see high speed arcs, but your pictures are 1000X better than other pictures like this I have seen!! Probably due to them being old film based things developed by people that had no idea what they were doing :o) The very high speed arc makes the pictures very nice to as opposed to things that a streak camera really blures out.

Apparently, old laser printers also have a very nice eight sided front surface mirror on a stepper motor. It scans the laser beam across the paper. I guess you could just mount the camera so the mirror would reflect the coil's arcs... With rechargable digital cameras, one does not have to worry about wasting 10,000 pictures now

I was going to try it once but it all seemed to messy. However, you have shown it to be easy after all!! It would be very cool to see streamer growth with this method!!

Dan - Imagine if the camera were spinning and you took a picture of the arc. The fast arcs would be recorded one after another next to each other on the film. Since the arc is "moving", the small sub oscillations are recorded next to each other in a row. Since the arcs are sharp bright and fast, they each show up clearly as opposed to being a long blur. I tried to find a nice explanation on the net, but they are all goofy high level crap... In out case we don't need a slit or anything like that.

The pictures are normally only good for a "thin line" of sight smeared arcoss the film. But the sparks take care of that for us in this special case

Cheers,

Terry
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Tesladownunder
Fri Sep 08 2006, 01:24AM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Terry Fritz wrote ...

This is like the "streak" cameras used to see high speed arcs, but your pictures are 1000X better than other pictures like this I have seen!! Probably due to them being old film based things developed by people that had no idea what they were doing :o) The very high speed arc makes the pictures very nice to as opposed to things that a streak camera really blures out.

I hadn't heard of "streak" cameras before. Doing this with film would have been a real pain. As it is. my mirror is too small to allow my camera to autofocus and manual focus is not accurate enough. I would like to increase spark intensity and length and push the boundaries in regard to speed. My motor is awfully unbalanced with the mirror despite a small counterweight so I have to physically hold it down to prevent it going out of alignment while it picks up speed.

Terry Fritz wrote ...

Apparently, old laser printers also have a very nice eight sided front surface mirror on a stepper motor.

I used on of those in a laser scanner project just recently. The mirror is only a few mm wide but they are accurate first surface mirrors and you get 8 times the chance to see a spark And you dont have to hold them down...

Terry Fritz wrote ...

I was going to try it once but it all seemed to messy. However, you have shown it to be easy after all!! It would be very cool to see streamer growth with this method!!
The pictures are normally only good for a "thin line" of sight smeared arcoss the film. But the sparks take care of that for us in this special case

I would like to increase to speed/distance of the spark from the mirror although I need to maintain distance between the spark and the camera so I might need to change the setup to a vertical one. so the mirror will reflect at 90 degrees rather than only 20 or so.

The related threads on this topic that I started that got me here are "End-on spark view - broken sparks" and "High speed spark photography".

The pic shows how the less bright middle section feathers out with small streamers sometimes. The very flat lines are edge of mirror effects.

Peter


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Sulaiman
Fri Sep 08 2006, 08:19AM
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2717
Peter,
as I'm sure you realise - you must stop this line of investigation immediately

If not you will reduce Tesla Coil design to
A) Choose input power
B) Choose ark type desired

crunch-crunch-crunch ........

The design for your coil is ...............

That would be boring - so for the sake of the TC community
secrets of the arc must remain secret
stop now before it's too late!

YOU HAVE BEEN WARNED
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Tesladownunder
Fri Sep 08 2006, 07:31PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Sulaiman wrote ...

.. secrets of the arc must remain secret
stop now before it's too late!

LOL
But there will always be the dark gap to make sure this remains a Dark Art....

I have now done a LED calibration which shows my geometry is a bit out.
Nevertheless with the motor in sync at 3000RPM (better balancing), and two LED's at 2.9m running at 100kHz, this is the result. The green LED is brightest and a white LED beside it is an older type and not very bright. The 100kHz is a square wave but the op amp gain drops off markedly so it might be a bit sinusoidal. I need to check it on a CRO.

Nevertheless there are 10 flashes in 1100 pixels at 100kHz. ie One cycle of 110 pixels occurs in 10us. Each pixel represents about 100ns.

Peter

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Terry Fritz
Fri Sep 08 2006, 07:38PM
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Posts: 282
Hi TDU,

Focus is an interesting thought. I am thinking that (for a Tesla coil) the distance would be say 5 to 10 feet so that helps. I will have to look at my cameras to see if they have any other settings besides "auto"

An 8 sided mirror would be nice since the probablitlity of the thing being in frame are 8 times higher. But your right about the very thin mirrors (nice scanner project you did BTW ).

A am thinking of taking a small block and drilling and pressing a shaft through it. Then one could accurately mill off four flats on it in relation to the shaft. Then one could slobber glue all over it to hold the mirrors on and mess up the ballance But the block could also be easily drilled for setscrews for ballancing weight. Then you would have a pretty easy to make four sided mirror that would be pretty well ballanced.

Then, you could hook it to a high speed motor somehow and spin it really fast till the mirrors fly off into the camera lens to win one of those photography versions of the Darwin Awards Ok, it would need a lexan shield too

I don't think vibration would be a giant problem since everything happens so fast. It would be easy to put it all on a frame the could mount the camera too and then it would all go on a tripod. The motor could just be a plain DC one to make it simple since the speed would not be very critical at all. Some parts would have to be a bit massive while others are light.

I am not sure how fast it should or could turn. It would be cool to catch each cycle of a bang and see how the leader forms. "Theory" is that they form very quickly in relation to to top voltage of that cycle.

Cheers,

Terry


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Tesladownunder
Sun Sep 10 2006, 12:10PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Well there was a break in the weather so I go to set everything up to run the TC.
The TC is my 4 inch one. I'll look up the details later but it was set up for 4 then 6 inch sparks between pointed electrodes to a grounded object. Power was 4 MOT's and current draw about 10A 250 V so enough to have a reasonable power arc rise in the centre if it got going.

The distance from camera lens to mirror was 30cm and from mirror to TC 140 cm. First two pics show the setup (obviously taken with my older camera).

Third pic shows the through the non-rotating mirror showing the toroid on the left with the reversed image.

Fourth pic shows a single spark with a series of up to 5 parallel sparks. Each space between sparks is 50 pixels which going by the photos before is 5us period or 200kHz. This implies a 100kHz waveform if there are two sparks per sine wave. Seems in the ballpark.

Note that this is not the Banjo effect seen on a windy day which is just the spark gap firing rate of 100/120Hz for a synch gap (or 1100Hz with my fast asynch gap which was running flat out as I didn't have a third variac setup). This is 100 - 1000 times faster.

I've taken a few hundred shots in the last few days (and lots of blanks) so there is a lot more there.
I have not investigated streamers yet.

Comments? Has anyone done this before?

Peter







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Terry Fritz
Sun Sep 10 2006, 06:36PM
Registered Member #393
Joined: Tue Apr 18 2006, 12:30AM
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Hi TDU,

WOW!!!!

That is wonderful!!!

As far as I know, these are the first high speed streamer pictures like this!!! We have often wished, but never have "had" them before!!

I PDFed some stuff about streak cameras and using them for high voltage arcs from Bazelyan and Raizer here (3MB):



But they are using slits and all so their stuff is sort of fuzzy (the pictures are that bad ). But yours are beautiful!! I am working on getting parts for mine

This is probably the most important tool to come along in years for Tesla coil studies!!!

Cheers,

Terry
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Steve Conner
Sun Sep 10 2006, 11:05PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Excellent stuff TDU! Can we see a picture of an air streamer?

BTW, I'm sure it says in the Colorado Springs Notes "Must examine sparks with rotating mirror" but Tesla never got round to it, and neither did anyone else that I know of
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Tesladownunder
Mon Sep 11 2006, 01:01AM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
The streak photography article is fascinating. It is also about 3 orders of magnitude faster than what I am doing and requires a photomultiplier tube to amplify it. It gives propagation rates of leaders of 10^9 cm/sec (approx 1/30 of speed of light) whereas I can only achieve 10^4 cm/sec.
Still, I was never expecting to be able to see things like that with equipment found around the home.
On the other hand, streamer growth has structure on very slow timescales which is why they are interesting to look at. In short, you can see them move so there are things happening at all sorts of timeframes from nanoseconds to seconds. Streamer brightness is much lower however but should register some interesting images.
Interpretation of streak camera stuff is easy if sparks are a straight line but become difficult if angled or branched so a blurred mess is a possible outcome when I try this with streamers.
I'm not sure how "useful" this will be but I hope to get some streamer data sometime.

The photo (one of my "mystery" photos) shows that the smear of light in the path of the spark is an artifact since it disappears when the spark is out of the top of the screen. I am not sure if this is due to mirror edge effects, or is a lens flare or camera UV issue. The green colour is a result of auto exposure and probably auto white balance when the software processes purple sparks.

Peter


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Terry Fritz
Mon Sep 11 2006, 03:15AM
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Posts: 282
Hi Peter,

There is another photo from the book here that shows continuous and flash mode streak pics:



Super fancy streak cameras can do <2pS and some laboratory thngs are into the femto seconds!! I am not sure much "moves" in that case





Pie-in-the-sky stuff for "us", but maybe we can learn some tricks from them.

Before Tesla was born, John Tyndall was doing the expiriment described here with moving a mirror or turning of one's head quickly.







I have a 1860's physics book (lost at the moment) that shows how to use a funnel with a paper diaphram with a bunsen burner to modulate the flame and use a spinning mirror to "see" the sound waves... I am sure Tesla was familiar with this but I am not sure here ever tried it. He probably though the events were too fast to see. Or, he did try it and could not get it to work given the time and equipment he had.

The leaders in the book are made with a Marx generator I think so they are different than ours. Tesla coil leaders might be much slower since they ride the ~100kHz sine wave of the secondary as opposed to being a giant cap discarge thing. We can look and see now

The "smear of light" you mention is where the camera is looking down the leader from the end thus it appears to have a bright area in it. People commonly mistake these for ball lighting in standard arc photos.





You picture seems completely true to me. You can probably "fix" the white ballance on your camera which might help the colors if you don't already have enough camera settings to worry about

The bending of the arcs just makes the pictures more "interesting" since we can argue for years as to how that affects the pictures

Great stuff!!

Probably be a whole week or two before I can get all the parts for mine and get it going

Cheers,

Terry



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Tesladownunder
Mon Sep 11 2006, 11:09PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Terry has a nice setup (see the next post). It should have a good chance of working well and it has a lot more design about it than my unbalanced shaft and small mirror. Having the camera nice and close is a big plus and the lexan shield is close enough to the lens not to worry about internal reflections.
I'm not sure the ISO is really increased by 4 times since you are just increasing your chance of catching a single event in the mirror by 4 times.

A few more pics for comment

Photo 1 shows that sometimes the spark has no following ringdown sparks.

Photo 2 shows some mild ringdown only.

Photo 3 shows the bright white arc channel interrupted with faint purple arcs.

Photo 4 shows detail of the initial spark which has a well defined bright central channel on the enlarged view.

Photo 5 shows the ionization around the stainless steel electrode which does glow red hot at the end of a run although that is too faint to see.

Photo 6 shows that the ionization is sometimes delayed by 5 us after the initial spark strikes.

Photo7 shows an unusual streak that I suspect is the spark channel hitting a dust mote and burning it up.
















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Terry Fritz
Mon Sep 11 2006, 11:36PM
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Hi,

I got the motor and mirrors ordered for my streak camera. I also made the frame, mounted the parts, and machined an aluminum block to hole the mirrors. Here is the camera system so far:





Just simple wood construction. The bottom has a tripod 1/4-20 adaptor for my heavy tripod. The big pillow block will hold the motor. The aluminum cube will have the mirrors expoxied to it and then it will be pressed on the motor shaft. The lexan shield protects the camera in case it blows apart. There is a 1/4-20 brass bolt on the bottom of the camera and spacer with a wing nut to hold the camera to the base. Took about two hours to build up.

Here is a picture (850k) through the camera as mounted:



So just need to drill the block for the motor shaft and press it on and glue the mirrors in place.

It should do 0.5in/uS at 20,000 RPM at ten feet. So a 100kHz coil should have the + - cycles separated buy 2.5 inches at full speed. With four mirrors and having the camera so close it should have a wide field of view so I don't have to take 10,000 pictures

The camera has manual zoom, focus and ISO (100,200,400) settings and has 2048 x 1536 pixels. At 24000 RPM that is 400 RPS or four frames so the ISO really should be 1600. But I think a friends camera will do that if it turns out to be a problem. The mount is easy to adjust to any camera with a drill and saw

Should be cool!!

For Peter's pictures:

Photo 1 shows that sometimes the spark has no following ringdown sparks.

I wonder if the hot streamer arc channel was alread in place. Or it might have just gone in one shot like a Marx spark. It does seem more streaked at the right if that is were the spark began.

Photo 2 shows some mild ringdown only.

I would think the dim part would be the ring "up" and then the arc would drain the system and be dark immediately after that.

Photo 3 shows the bright white arc channel interrupted with faint purple arcs.

I have no idea!! Maybe the dim parts happen to be hotter and more conductive.

Photo 5 shows the ionization around the stainless steel electrode

Interesting that the ionization is fairly constant as one would expect

Super cool! Prolly far more questions than answers

Cheers,

Terry

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Bert
Wed Sep 13 2006, 03:33PM
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Posts: 71
Terry and Peter,

It appears that the direction of increasing time is downward, not upward.
Peter can you confirm?

Bert
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Tesladownunder
Wed Sep 13 2006, 06:09PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Yes. The bright spark at the top is the initial one, then the next 5 or so progressively weaker sparks below are subsequent events so down is advancing time.

With this photo I have increased the contrast to show two things.
Firstly that there are two types of electrode glow on the right with the greenish one appearing only after a few cycles.
Secondly, look closely at the ringdown sparks at the left. The space between the sparks is not empty but shows another line in between. I wonder if this is a second harmonic.

Peter

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Terry Fritz
Wed Sep 13 2006, 09:38PM
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Posts: 282
Hi,

Yes. The bright spark at the top is the initial one, then the next 5 or so progressively weaker sparks below are subsequent events so down is advancing time.

Ok... This is what I thought was going on which is apparently wrong:



This is a pretty typical top voltage of a coil when an arc reaches ground:



So this should be the correct situatiion now.




Cheers,

Terry




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Tesladownunder
Sat Sep 16 2006, 11:56AM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Here is the "first" streamer picture which forms an arc that hits.

Pic is of about 12 inches of an 18 inch spark from the toroid side on the left. I was throttling the variac back to try to just get streamers and few hits.

It is quite different. Time axis is downward. The initial streamer sparks (the top one) can be broken into perhaps 6 consecutive channels (5us apart = 2 pulses per 100kHz). Although it is difficult to be sure, only the last one makes it across the screen then a 10us gap then the main arc hits. Interestingly there is no ring down on the main arc, however the distances are greater and intensity is down.

I've taken lots of black shots with information on them but need to process them.

I guess the new information from the rotating mirror stuff is that streamers enlarge with successive cycles and ring up leading to a spark that connects. Sparks that connect (often) have a ring down. Not really unexpected from the CRO pics but nice to see it directly. So streamers ring up and sparks ring down - easy to remember.

First pic shows the streamer with ring up then an arc connects. The yellow blur on the right is my kitchen window seen through the mirror when it is horizontal and mostly out of view.
Second pic is lots of overlapping sparks just for fun. Conventional sparks to a grounded object like I have shown before with the ringdown visible. The sparks are somewhat out of focus at the bottom of the pic.

Peter






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Terry Fritz
Sat Sep 16 2006, 03:50PM
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Hi Peter,

Here is the "first" streamer picture which forms an arc that hits.

What a stunning picture!!!!!!!!!

I enhanced and commented it here:



You can see every known leader phenomena in it!!!!

Here is a reference from Bazelyan/Raizer.



It looks like the leader tip stalls a bit probably as that cycle reverses and cuts off the power. The next cycle reheats it and eventually it goes onward and breifly stalls again.

Amazingly, you can see the streamer branches(!!!) at the very tip on the right where it is darker.

Interestingly there is no ring down on the main arc

That is a hit or miss thing. Often the power is totally removed from the streamer so the arc is dead right there. Models predict that based on stuff that still needs to be studied. It is probably fairly consistant (if it will occur or not) with a given coil setup.

I've taken lots of black shots with information on them but need to process them.

Looking forward to more!!!

This is fantastic stuff!!! Your pictures are some of the best available of any leader arc and probably far better!!!

Cheers,

Terry
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Tesladownunder
Sun Sep 17 2006, 06:53AM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Terry Fritz wrote ...

... I enhanced and commented it here:

You can see every known leader phenomena in it!!!!
Here is a reference from Bazelyan/Raizer.

It looks like the leader tip stalls a bit probably as that cycle reverses and cuts off the power. The next cycle reheats it and eventually it goes onward and breifly stalls again....

I don't think these are leader tips and my interpretation is a little different. What you have labelled as the leader tip on the left looks to me like a simple change in direction for the spark. In the two dimensions we see it in there is a sudden change in direction downwards. This will overlap the streamers and make that area bright. If the area is much brighter than expected (as it appears to be visually) then this is due to a third dimensional effect, ie the spark goes in or out of the plane of view for a distance which will relate to the perceived brightness.
The second leader tip that you indicate on the right occurs where a streamer branches. Again there is streamer overlap.
In both cases the streamer channel continues although all the 4 or 5 channels may be "stepped" so it looks like one or two were stopped.
The diagrams from Bazelyan/Raizer, while not well labelled with time axes, indicate much faster events analysing what happens within the first strike that I just see as a single event.

Terry Fritz wrote ...

Interestingly there is no ring down on the main arc

That is a hit or miss thing. Often the power is totally removed from the streamer so the arc is dead right there. Models predict that based on stuff that still needs to be studied. It is probably fairly consistant (if it will occur or not) with a given coil setup.

The presence or absence of ring down is seen in different sparks in the same photo in the (second photo in my last post) so it is not just a coil setup issue. I would have thought that it is hard to remove all energy in one cycle from a resonant circuit without some ringing.

Pic #1 below shows variation in the intensity of each ring up spark channel.
Pic #2 below shows tthe two most central sparks of similar intensity. Can you tell which is a streamer and which is a spark? (Sparks ring down and streamers ring up) On the full photo the streamer fades but the spark doesn't.
Pic #3 Shows many streamer channels

Peter







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Terry Fritz
Tue Sep 19 2006, 02:48AM
Registered Member #393
Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 282
Hi All,

It works!!





Here are the setup details before I forget them

Zoom - To fit the image nicely to the picture. Does not seem to affect light gathering. Optical zoom only as opposed to stupid software zoom.

Shutter speed - Does not matter much. Long enough that you get a good chance of "catching" an arc. 1/10 and 1/20 sec does well here.

Background sensor noise - Don't worry about trying to fix noise by subtraction of a dark frame or anything. Oddly, that seems to hurt more than help. If your camera does this automatically, I think it should be turned off. The key is a sensitive low noise sensor to begin with!!

Aperture - As big as possible!! You need to gather as much light as you possibly can at all costs!!

Len's - If you have a choice, one that gathers the most light.

ISO - As high as possible!!! 1,000,000 would be nice!!! Mine only goes to 400 at that is barely enough. One of those cameras that can be forced up to 16,000 whether it is good or not would be nice. You really need this one!!! As Dan and other's say, getting data out of the camera in a deep RAW format would really help here too. The cool stuff is just barely above the noise level in my case.

Distance - As close as possible just to gather more light!!

Speed - My four rotating mirror block is running at about 5000 RPM. 2000 - 6000 RPM would be fine. I think slower is a bit better since more light hits the sensor pixels that way, but you want it to be fast enough to separate the pulses.

The room should be very dark to eliminate stray light.

Flash - Be sure to turn that **^*&%* thing off!!! There you are in the dark fiddling with the camera and blammo!! Almost knocked me over.

Remote shutter is ultra nice!! If the camera can take pictures automatically at say every second that would be very nice too. But you can do the shutter button too.

White Balance - I used daylight, but it is turning out black and white mostly in my case as I super pump up the brightness and contrast.

EV - Set it up high to brighten the picture as much as possible, but it had no effect in my camera's manual mode.

Reset - Set the camera so it does not loose all the settings when the power is turned off

Focus - Manual. I just measured it and set the camera to that distance. This could be tricky, but I did not have much trouble with it.

Background - Black felt works fine.

I took about 200 pictures today. Mostly figuring out by trial and error what is good. Rechargeable batteries are good too but you don't use the flash so the battery drain is pretty low. I did keep the LCD screen on all the time.

At 5000 RPM you can just glue the mirror to the little motor shaft or whatever. No need for a fancy machined block. No need for four mirrors, just one would do fine with a longer shutter speed to catch a good arc on one of the rotations.

The stray light paths near the mirror should be blocked and all the surfaces should be painted black to eliminate stray light reflections.

So it is pretty easy. I did have to carefully increase brightness and contrast with software to see air streamers. They are barely above the noise level. However, power arcs are very easy to see. I had to go back to all the black frames and lighten them so see if there were air streamers. Power arcs show up right away.

I put a 40.5MB zip file with a lot of pictures here:



At the bottom of the page select "free".
It makes you wait about 2 minutes to read the ads and then a download screen comes up.
Enter the funny letters in the robot trap box and save the file. It is just a typical zip file filled with all the JPG images from the last "lucky run". Some are plain and some have been enhanced. Some of those pictures are just black too... The four read streaks to the left or the calibration LED flashing at 100kHz. Each mirror makes it's own streak depending on the fine alignment of the mirror. If the pulses are blurred out, it is because that mirror scanned twice while the shutter was open.

I "think" time increases from bottom to top. Hard to say since the camera is upside down, inverted, twisted, and the motor turns in some direction... "Hard to say for sure" #;-)) I will try to modify the calibrator so it gradually goes out to point in the right direction. The calibrator is not really needed but it does prove the speed and all.

Thanks to Peter for figuring all this out!!! I think you are even working on an improvement )

I don't know what the pictures "mean" yet since Just taking the pictures is the deal right now.

Cheers,

Terry


To follow up...

I modified the calibrator for 1uS on time pulses at 100kHz but with a diode and cap so it has a "tail" to point in the direction of time.



So time so increasing from the bottom, toward to top in my pictures.

This is most interesting ))

I took one good frame and enhanced it with comments (1.7MB to preserve all the detail):



You can see each ring up pulse now!!!!!!!!

The first pulse is very weak but visible.

The second pulse 5uS latter is stronger and has a significant downward branch but its upward branch almost goes 1/2 the distance.

The third pulse almost gets there and clearly stimulates streamers off the ground point.

The fourth pulse arcs and completes in a very bright area between the previous leader and the corona off the ground point.

Here is a ScanTesla equivalent:



This coil is highly predictable and the trailing oscillations are due to the leftover energy in the primary circuit. But those oscillations are weak in this case and do not show up at all in the photo.

Wild stuff!!!

Cheers,

Terry


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Tesladownunder
Tue Sep 19 2006, 06:01AM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Great pics for first light, Terry even if you people do things upside down compared to here in Australia (the time axis is going up).
The camera is the limitation here even despite your SISG having brighter sparks. You have a larger field than me which will also reduce the available light.
The 100kHz LED works well and fortunately the 4 images don't overlap otherwise you wouldn't see anything. Given your machining tolerances, I guess this is a difference due to epoxy thickness.
How long are the sparks?
I look forward to closer pics with better detail. I will look at 40M file as well.
I have more plans but have the small matter of a day (and part of the night) job.

Peter
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Steve Conner
Tue Sep 19 2006, 10:15AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Wow Terry, that's astounding! You actually caught a Tesla coil streamer in the act of growing and striking a target. Does it grow on both the half-cycles of topload voltage, or only one polarity?
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Terry Fritz
Tue Sep 19 2006, 06:04PM
Registered Member #393
Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 282
Peter wrote:

The 100kHz LED works well and fortunately the 4 images don't overlap otherwise you wouldn't see anything.

Often you can make out the timing light even if they do overlap. I changed the light for only a 10% duty cycle which should really help that.

Given your machining tolerances, I guess this is a difference due to epoxy thickness. How long are the sparks?


The block was pressed on at 4000 pounds with a hammer "helping out" with some impact. I suspect a bent the 1/8 inch motor shaft a little but I wanted to be sure it did not fall off at 30,000 RPM Of course, that speed is not needed. 2000-6000 RPM does fine where just glueing the mirror right to the motor shaft would be fine. The arc distance is 23 inches.

Steve wrote:

Does it grow on both the half-cycles of topload voltage, or only one polarity?

The timing light is 100kHz and the Fo of the coils is 103kHz. Since the streamer growth pulses are at 5uS, they are clearly both the - and + pulses of the cycle. In this case I can directly compare the pulses to the ScanTesla computer model of this coil here:



I do not know the absolute polarity of the coil this moment though.

Cheers,

Terry


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Steve Ward
Tue Sep 19 2006, 07:23PM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Why not have the strobe light take feedback from the secondary output voltage just like our SSTCs do. That way the strobe can be synched, and you will know which half-cycle you are on in the picture.
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Terry Fritz
Wed Sep 20 2006, 04:00AM
Registered Member #393
Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 282
Hi Steve,

Cool idea!!

One could also use a CT on the secondary base current lead to direct drive the LEDs (Red/Green high-brightness). The base current is 90 degrees off, but it would give the same info. It might get a little odd with ground strikes.

The schematic for my calibrator is here:



This one has the "tails" indicating the direction of increasing time.



But once you know the direction and general time idea, the calibrator serves no function anymore. I am not using it now.

The SISG coil always fires in the same DC top voltage mode and the voltage looks like this:



This is comparable to the model which I now remember I matched for voltage direction:



In the big picture here:



I doubt the first little 50kV nub could have made the first leader shown. So I think the above is all valid.

I am actually using higher scan speeds now. I tried slowing it to make the sparks linger on the pixels longer for more brightness, but that does not help. More spread is what it needs. I also need to cut ambient light way down more. A mirror redesign is in the works. So is a much better camera )


Cheers,

Terry
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Steve Conner
Wed Sep 20 2006, 11:49AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Is there any way you could get an oscilloscope screen to appear in your streak photo, showing the instantaneous voltage? I can't imagine how you would create a trace that wasn't smeared because of the phosphor persistence of the scope.

Maybe you could make a LED bargraph showing the instantaneous output voltage, and the motion of the mirror would turn it into a scope trace. I don't know if those LM3914 thingies are fast enough to hook up to an antenna.
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Tesladownunder
Wed Sep 20 2006, 12:04PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
OK here is my fabulous "improvement". It is so simple it only took me 4 or 5 hours and a few blown high power ultrafast diodes and TVS's. In the end I settled for this. Only 4 components, two of which are red LED's.
Simply, it reads the polarity of a spark. The led closest to the spark is negative ie the spark from the toroid on the left is a negative leader.
They then alternate polarity far further than one can see as a spark.
Lots of interesting stuff is going on here now that I have looked at many of then. More in future posts

Guess how I did it. ie what are the two other components.

Peter



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Terry Fritz
Thu Sep 21 2006, 12:34AM
Registered Member #393
Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 282
Hi,

Steve Conner said
Is there any way you could get an oscilloscope screen to appear in your streak photo, showing the instantaneous voltage? I can't imagine how you would create a trace that wasn't smeared because of the phosphor persistence of the scope.

The scope could be postitioned in the picture outside the spinning mirror. Probably need another mirror so you can put the scope at a distance so it will be in focus. I guess you would have to run in single shot so 50 images combined would not smear it up. Of course, digital scopes display the data far too late to be useful. There might be some odd way to use the camera hot shoe to trigger the scope on just the right spark too.

Maybe you could make a LED bargraph showing the instantaneous output voltage, and the motion of the mirror would turn it into a scope trace. I don't know if those LM3914 thingies are fast enough to hook up to an antenna.

I made an LED scope thing once with them but I am not sure they can do say 500kHz. It would be easy to test if anyone has a LM3914 around. That data sheet did not say.

TDU said:
Guess how I did it. ie what are the two other components.

Cool!

I will "guess" two LEDs a resistor and a MOV. The strike current might be 2000A for a few nS which will kill most TVSs and such. It really helps to keep the device voltage low so the instant power is lower too. A capacitor might also work if one could guess the right value.

Cheers,

Terry
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