Re: [TCML] Voltage Question

[Paul Nicholson <tcml88@xxxxxxxxxxx>]

Udo Lenz wrote:
> ...the currents in the streamermight be in the ampere region
> so the voltage drop might well benear your guess.

Maybe.   Sometimes a ball-park figure of 1pF per foot of arc is
used as an estimate of streamer load capacitance.  Say a 5 foot
arc has 5pF - a topload at 300kV, 150kHz would have to push
out 1.4 amps into the streamer to keep it alive.    The streamer
current would reduce the further along the streamer you go.
When 3 foot out from the toroid it only has 2pF to charge and the
current would be down to ~0.6 amps.

(In practice the charge distribution along the streamer would
not be so uniform as this, tending to concentrate to some extent
at the far end.)

> An important parameter seem time scales to me.

The streamers advance in steps which are probably quite rapid
compared with an RF cycle.  Hence the topload has to supply
the sudden amounts of extra charge and that is why I suggest
the topload C matters for arc length.  It is functioning as
a reservoir which banks the slowly arriving charge from the
coil and makes it available at low impedance for streamer
extensions.

How much this matters - I don't know.   I think there has been
experiments with a fibre-optic current probe between toroid
and a breakout point.  Perhaps someone has a reference?

Given a sufficient supply of charge, the streamer should
extend until its tip field strength drops below about 26kV/cm,
assuming the RF stays on long enough (long RF envelope - low
coupling or quasi-CW).   At this point with ideal conditions,
the top voltage will be a little higher than the streamer tip
potential (to cover the voltage drop along the arc) but not
high enough to have started more streamers.

I suggest a point would be reached where increases of top
voltage, topload C, or envelope time, would not extend the
streamers any further (for any given toroid minor diameter).

A popular ratio is the arc length to coil length.  But I don't
think the coil length matters at all here and is therefore a
poor denominator.   Arc length to toroid minor diameter might be
an interesting one to collect data on.   If there's anything to
these suggestions, a scatter plot would show a boundary curve
and that would make a valuable design guide.
--
Paul Nicholson
--

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