DRSSTC Arc at 1.75M FPS - See the Resonant Frequency

[Max]

Hello everyone,


The Slow Mo Guys filmed Mehdis DRSSTC at up to 1.75M FPS and I found the results quite interesting. F.ex. I wouldn't have thought that you can actually see the arc going on and off with (twice) the resonant frequency. I somehow thought the airs thermal capacity would keep it hot enough to glow more or less continuously. I wonder if that's a part of the explanation why QCW coils require (much) higher frequencies for non-branching arcs.

Curious to hear your thoughts and interpretations!


Kind regards,
Max

[AstRii]

It's very impressive.
I'm not an expert in thermodynamics but I think that most of the arc's light won't come out of the heat but rather due to excitation of mostly hydrogen gas. In above troposphere we have so called "red sprites" discharges which are glowing red due to excitation of
oxygen. In around 400km above the surface the atmosphere consists of mostly nitrogen which makes auroras glow green. None of those glowing colors are given by the temperatures.
Of course, correct me if I'm wrong.

Think about it this way; DRSSTCs pump several tens even hundreds of kilowatts of peak power to the arcs. If we would like to cool down the surrounding air back to the ambient temperature between each resonance cycle, we would need the same cooling power. Where would we get that?

[davekni]

I'm not an expert in thermodynamics but I think that most of the arc's light won't come out of the heat but rather due to excitation of mostly hydrogen gas.

Agree that light isn't coming from heat.  Arc is plenty hot enough.  It's lacking any black body to radiate that heat.  Transparent material doesn't radiate significantly due to heat.  That is why gas lanterns require a mantel.

AFAIK, arc brightness doesn't depend on humidity, the only source of hydrogen.  It is combined glow of nitrogen and oxygen and a little argon.  At high pressure and temperature spectral lines are broadened.  (That is why high-pressure sodium vapor lights are orange and broader spectrum than the almost-monochromatic yellow low pressure sodium vapor lights.  Not counting the little bit of mercury sometimes added for further spectrum increase.)

I wouldn't have thought that you can actually see the arc going on and off with (twice) the resonant frequency.

The 500kSPS shot was interesting, showing the relatively low frequency aliasing of the 2 x 250kHz light generation.  Surprised it took the higher-speed video before the one guy realized his missing 2x factor.

Would be interesting to have a longer clip starting when the arc is only partially formed.  I wonder if one could detect significant arc tip advance from one half-cycle to the next.

Marx generator shots were also interesting.  From the video angle, the last gap appeared slightly smaller, likely explaining why it triggered first.

It's very impressive.

I quite agree.  Thank you, Max, for posting!  Amazing that it has 500k views in first 12 hours.

BTW, anyone else been looking at Steve Ward's and other recent argon plasma toroid videos?  They are fascinating too.

[Mads Barnkob]

It is kind of interesting that more details does not show (apparent, as it now takes a long time to view) the initial step leader grounding and sub-sequent pulsing of energy into the arc. I got this captured pretty well at 8000FPS, however only with 1-2 frames of the really bright first grounding of the step leader.

The videos are not quite ready yet.

[Uspring]

As David pointed out, transparent materials are poor light emitters. And air is transparent until it becomes hot enough to become conductive. At this point it can emit light. TC arcs are rather cold compared to arc lamp arcs and much less conductive. So they are much less effective in light generation.

I find most interesting the glow around the tips of the growing arcs. They indicate an abundance of free electrons. They must have come from the tip of the arc, since free electrons usually are scarce due to their short lifetime of less than 100 ns until they become attached to air molecules. I presume, that arc branching takes place inside these regions and determines, whether we have a straight and unbranched or a branching and more bent arc.