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