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Re: Streamer colour

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

Original poster: Ed Phillips <evp@xxxxxxxxxxx>
I have a comment.  In running my VDG and observing sparks in a dark
room I see an interesting effect.  Often I get a segmented spark which
looks about like this:
First part, coming from the top terminal, is a fairly straight 4" spark,
ending in

This is the "leader" portion of the discharge. The leader conducts the sum total of current into the cone-shaped region that's actually made up of tens or hundreds of hair-like "streamers". The leader has a higher current density, is hotter (making it a true plasma), and it is a relatively good electrical conductor. The brighter discharges that most coilers call streamers are, in fact, leaders. Leaders propagate in one or more jumps, each jump reflecting a brief transfer of charge from the VDG terminal. Once initially formed, a leader will continue to propagate as long as the E-field at the tip of the leader remains above ~30 kv/cm. In a VDG (and also a Tesla Coil), each sudden transfer of charge out of the top terminal during leader propagation reduces the topload voltage. This may reduce the E-field at the leader tip below 30 kV/cm, choking off further propagation.

Second part, a point from which am "umbrella" of purplish sparks emerge
and strike whatever.

These fainter discharges are actually the streamer portion of the discharge. Streamers can only be seen in low ambient light, and they often appear as a purplish-blue "hazy" region from the tips of the leaders in photos. Unlike leaders, streamers are cold discharges (the air is heated only a few degrees above ambient). Although they are sometimes mistakenly called corona, they have markedly different characteristics and properties. A photo showing leaders grading into a hazy streamer region (complements of Sue Gaeta) can be seen here:

The streamers transfer charge into nearby air region, creating temporarily charged (space charge) regions. If the leader channel cools down and becomes nonconductive before the terminal voltage can recover (typically 10-20 msec), the next leader will follow a different path than its predecessor.

I don't know what causes this phenomenon (bet Antonio does), but it is
obvious that the current density for the bright blue part must be higher
than that of the whole bunch of purple coming front of it.

A leader represents the sum of currents from hundreds of streamers. Each streamer typically winks into and out of existence in 10's of nanoseconds), and the streamer current, temperature, and energy density are all much lower than in the hotter leader.

P.S. I wonder how much of what we see is real and what is introduced by
things associated with how the eye sees stuff of very wide brightness

Your dark-adapted eye may actually be better suited to seeing the myriad of hairlike streamers than many cameras...

Best regards,

-- Bert --
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