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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
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