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1/4 Wave Theories - Trash Them!
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From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent: Saturday, June 06, 1998 6:55 AM
To: Tesla List
Subject: Re: 1/4 Wave Theories - Trash Them!
Tesla List wrote:
>
> ----------
> From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
> Sent: Friday, June 05, 1998 2:55 PM
> To: Tesla List
> Subject: Re: 1/4 Wave Theories - Trash Them!
>
> Tesla List wrote:
> >
>
> > Terry and Malcolm,
> >
> > Good point! Streamer propagation requires a source of "instantaneous"
> > charge on-tap to provide the energy required to extend the ionized path
> > as well as charge the incremental capacitance associated with the newly
> > increased streamer length. Voltage collapse at the source (toroid in
> > this case) stops the streamer growth process until the toroid voltage
> > can "recharge" enough (through to coil's inductance) to resume the
> > process anew.
> >
> > -- Bert --
>
> This phenomenon is well described in most works on spark discharges.
> This is why the rise time of the pulse and the available C has a huge
> effect on leader propagation.
>
> In fact, some experimenters have noticed that the current at the base of
> the leader can actually flow both ways. First, charge flows out into the
> leader, as long as the terminal voltage is greater than the leader's.
> Then, if the terminal voltage drops (say due to its C being discharged),
> the current can actually flow back through the leader into the terminal.
Jim and all,
I fully agree! Most HV research has focused on using a lighning
simulator/Marx generator to generate a rapid rise/slow tail
lightning-like waveform, or a crowbarred source to generate a fast rise,
faster cutoff waveform to generate arrested streamers. Either of these
generate what is basically a unipolar pulse. The effect you described
tends to show up in the latter configuration - and in some cases, the
brightness of the streamer during initial formation and "charging" of
the arrested streamer was significantly less than the brightness (i.e.,
current) seen when Vout was suddenly clamped to ground, rapidly
discharging the charge stored in, and around, the arrested streamer.
The GOOD news is that a Tesla coil's oscillatory, underdamped damped
waveform should enhance this phenomenon since the underdamped
oscillations force polarity reversals at the toroid, alternately driving
charge into, or or sucking charge back out of, the conducting streamer.
This helps reginite a predescessor streamer's root path, and helps to
keep it "hot" (conductive) from resulting current flow. Complicating all
this is the fact that streamer propagation for disruptive Tesla coils
appears to occur only in alternate half-cycles, when the top terminal is
positively charged.
One of Greg Leyh's measurement of sphere-to-streamer current taken at 20
uSec/division [http://www.lod-dot-org/electrum/sphere20us.jpg] may actually
provide some evidence for these effects. In this measurement, the first
positive current peak forming an initial streamer is then kept "alive"
by relatively steady arc-current (charge being sucked out of the
streamer) during the subsequent negative transition of the top terminal.
This is then followed by an even higher amplitide current peak as the
NEXT positive transition seems to be neutralizing the negative space
charge around the streamer (from the previous negative half-cycle) and
then further propagating the positive streamer. This is why I questioned
whether a critically damped (non-reversing) current flow would in fact
be optimal for long sparks.
Unfortunately, there's relatively little in the literature that covers
the RF case for long sparks, and virtually NOTHING covering actual
streamer currents under RF conditions - this reinforces why Greg's
physical measurements of actual streamer currents so very valuable...
Excellent and stimulating discussion, Jim!
-- Bert --