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High Speed Spark Photography 3

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HV Rotating Mirror Streamer Hit1

This technique looks at microsecond events in the Tesla coil sparks. Here are some Tesla shots with the rotating mirror setup described here. The TC is my 4 inch one. 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 30 cm and from mirror to TC 140 cm.
 

The left photo shows the the setup (taken with my older camera) and shows the TC at left. The camera (center) picks up the image from the rotating mirror on the right. The right photo shows the TC running with spark just behind my shoulder.

 

The left photo shows the reversed image through the rotating mirror (stationary for this photo) showing the toroid on the left. The right photo shows the single spark with a series of up to 5 parallel sparks. Each space between sparks is 50 pixels which is 5 us period or 200 kHz. This implies a 100kHz waveform if there are two sparks per sine wave. Seems in the ballpark for the running frequency of this coil.
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 asynchronous gap which was running flat out as I didn’t have a third variac setup). This is 100 – 1000 times faster.
Very high speed observations of spark growth can be made with streak cameras which use a photomultiplier tube to displace and magnify the image. It is about 3 orders of magnitude faster than what I am doing. It gives propagation rates of spark 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 left photo shows an arc with no following 100 kHz ring down like in the last photo. The right photo shows a bright arc with faint ring down.

 

The left photo shows gaps in the bright white arc channel filled with faint purple arcs. The right photo shows detail of the initial spark which has a clear central channel on the enlarged view.

 

The left photo 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. Thecenter photo shows that the ionization is sometimes delayed by 5 us after the initial spark strikes. The right photo shows an unusual streak that I suspect is the spark channel hitting a dust mote and burning it up.

 

The left photo shows a streamer which is 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. The center photo shows two different streamers which are unrelated but overlapping. It shows the variability in intensity of subsequent spark channels and the gap before the main arc forms. Perhaps this is a harmonic effect and the spark channel is actually of greater energy than the channel before The right photo shows the streamer ring up sparks of as many as 8 sparks in a row.
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.

Photo Date: 2009

High Speed Spark Photography 1

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HV Broken Sparks Rotating Mirror Tesla Single

Now this is interesting. This is taken through a rotating mirror.

 

I joined a first surface laser mirror to one of my motors. Running at 2250 RPM and with the spark 16 cm away the radial velocity of the spark is 37 m/s. With the image being only the negative 2 cm of a total 7 cm spark width, the vertical distance of the photo is 500 us. So you should see events in the region of 10 us easily enough. There doesn’t seem to be any structure at that level around the discontinuity.


 The Tesla coil above is my junk coil running on half of a 12 kV 30 mA 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 gap is only about 2 cm to all fit in the mirror view. You can just see the spark in the mirror in this photo.
Of course, with each spark lasting microseconds or less it becomes harder to catch a spark in the mirror. Even with 2 second exposures and the spark firing at perhaps 20 Hz you only get a spark in view occasionally. It should be easy to increase the resolution by a factor of 10 – 20 to see events at microsecond level. It may take many minutes of exposure to get a spark though.
This would be of great interest to Tesla coiling to get sparks seen on that time frame.
Note that this is not a true high speed photograph. Vertical movement of the spark on the image may be due to irregularity of the spark or due to events happening in time. Multiple spark channels should show up well or stepped leaders perhaps. 

  

 

The left photo shows a single spark and the centre photo shows multiple sparks captured with a longer exposure. Mirror to spark distance is 38 cm which means that the image moves at about 100 m/s. The picture represents about 2 cm width and 4 cm height i.e. vertical scale is 40 us. (just over 100 ns/pixel). The right photo shows a view of a LED being flashed at 100 kHz hence the distance between each LED is10 us.
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.

 

The left photo shows a Royer ZVS circuit firing a rewound inverter MOT transformer to give perhaps 2 kV at 15 kHz. It wasn’t bright enough to show so I later added a diode, resistor and .06uF mica cap to give a brighter spark which was rather irregular due to the low firing voltage. The right photo is 100 vertical pixels = 10us showing 3 sparks of less then 1us duration, which appear to deviate from a vertical line. Going back to the setup photo, you can see that one of the electrodes is vibrating changing the spark position.
The 3 sparks suggest that there is a resonance at about 10 us period – 100 kHz due to the .06 uF cap and the effective series inductance of the cap itself plus the two 8 inch crocodile clip leads. As you can judge by the pixilation (automatically smoothed by the software) plus the noise, the camera is being pushed to the limit. Very small sparks still seem to be point events. Hopefully a 2 foot TC spark will have more structure.
To see speed of light events I would need to have 500 foot events which would be 1us. In fact it would not be too hard to bounce a laser over a path this length to show the speed of light. Hmmm… I have a corner cube prism and two eight inch parallel first surface mirrors. Add a beam splitter or two, line it all up and go. Ohh, and it needs to have picosecond switching. Did I mention that? Maybe my scanner Hex mirror assembly could rotate the laser beam to give fast enough effective switching. Head is starting to hurt here.