RE: [TCML] Question about vacuum tube Tesla coils

["Phil Tuck" <phil@xxxxxxxxxxx>]

An excellent well explained post Bert, something I have long wondered about
myself.

Regards
Phil Tuck

www.hvtesla.com



-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Bert Hickman
Sent: 05 September 2011 18:49
To: Tesla Coil Mailing List
Subject: Re: [TCML] Question about vacuum tube Tesla coils

Hi Ryan,

The rate that the peak RF output voltage increases during resonator ring-up
(called the voltage "envelope") and the repetition rate both have a lot to
do with spark growth and appearance.

A fast-rising envelope (such as from a spark-gap or DRSSTC) will tend to
cause lightning-like sparks that branch numerous times. A coil with a
slower-rising envelope (such as VTTC's, older base-driven SSTC's, or newer
Quasi-CW coils) tend to have less channel branching, and more arc-like
discharges. The difference is thought to be due to the way that the tip of
the sparks (leaders) propagate from one RF peak to the next during ring-up.

If the coil has a fast-rising envelope (quick primary-to-secondary energy
transfer), the peak RF voltage (and leader tip E-field) will be
significantly higher it was at the previous RF peak. Spark propagation
proceeds by forming fuzzy streamers far ahead of the current leader that
allow the leader to rapidly grow in a series of jumps. It is also known that
the higher the leader tip E-field, the greater the probability that the tip
will split, causing the main leader channel to branch into two propagating
leaders.

By slowing the rate of energy transfer (which a VTTC tends to do naturally
and which a QCW system can do via precise envelope control), leader tip
splitting can be significantly reduced or even eliminated. 
This permits leaders to grow (albeit, more slowly) without branching,
resulting in sword-like sparks. The overall trajectory of a given spark
tends to generally follow the direction of maximum E-field divergence,
mostly tracing out the E-field lines that connect the toroid to its
surroundings. These can be mostly straight (from the top flat regions of the
toroid) or more curved elsewhere.

The course of individual streamers is also influenced by residual space
charges left by previous discharges, adding some randomness to the position
and shape of the discharges. Under the right conditions, a QCW appears to be
able to generate a slow-growing, virtually streamerless leader that appears
to be more arc-like than spark-like.

Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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Ryan QF wrote:
> Hi everybody,
>
> I've got a question about vacuum tube driven Tesla coils.  Every one of
> these I've seen produce distinctive-looking sparks - the link below to
Roger
> Smith's TC is a great example:
>
> http://www.flickr.com/photos/29248051@N02/6104333777/in/photostream
>
> What is it about using a vacuum tube to drive a Tesla coil that creates
> these sparks?I am guessing it has something to do with thermionic emission
> speeding up the electrons as they leap across the tube.  Please correct me
> if this is wrong.
> I'm curious because I haven't seen any other type of Tesla coil produce
> these kinds of sparks, so it stands to reason that there is something
about
> using vacuum tubes which creates this.
>
> Thanks,
>
> Ryan
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
>


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