Re: theory(?) for long sparks
From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: bert.hickman-at-aquila-dot-com
Sent: Friday, November 28, 1997 3:33 PM
To: Tesla List
Subject: Re: theory(?) for long sparks
Tesla List wrote:
> From: Antonio Carlos M. de Queiroz[SMTP:acmq-at-compuland-dot-com.br]
> Sent: Thursday, November 27, 1997 10:56 PM
> To: Tesla List
> Cc: 'Tesla List'
> Subject: Re: theory(?) for long sparks
> Barry wrote:
> > When I finished designing and building the 2 Megavolt Marx generator at
> > the Lightning Laboratory I did some voltage and current comparison
> > measurements. The voltage rose, leveled off like a square wave, and then
> > collapsed at which time the current started to rise. This took only about 2
> > microseconds for a 10 foot point plane arc (2.2 MV) with a current peak of
> > about 14 kiloAmperes. The erected Marx capacitance was around 3 nanoFarads.
> > The terminal capacitance of a Tesla coil is several orders of magnitude
> > less than this. Is it possible that the source charge feeding the streamer
> > effects the propagation velocity?
> What I would interpret from this description is that the levelling occurs when
> charges start to leak from the terminal to form the spark, at the point where
> the surface field reaches about 30 kV/cm. This makes me think about the following:
> In a Tesla coil the voltage would rise more and more at each cycle of the RF,
> until charges start to leak from the terminal and form a spark. The channel of
> the spark is then electrically connected to the terminal, and is effectively part
> of it. It would charge and discharge at each RF cycle, along with the terminal.
> Subsequent RF cycles would extend the spark, by causing more ionization at the
> tip (or tips) of the spark channel. This process would persist, if the spark
> does not touch a conducting surface, until no more energy is pumped into the
> secondary (the "first notch"). Note that the appendix formed by the spark channel
> in the terminal can change its capacitance, and detune the secondary resonant
> circuit. The increasing length of the spark in formation would also require more
> power at each semicycle. The effect of this is difficult to predict without
> complex calculations.
> This idea can be verified by measuring the voltage waveform at the terminal
> (How? A capacitive divider?). It shall exhibit peak leveling after some cycles,
> and maybe a visible change in frequency while a spark forms. The detuning can
> also explain strange transients in the primary when sparks form.
> Do someone have these waveforms measured?
> Antonio Carlos M. de Queiroz
Antonio and all,
I tend to agree with most of your explanation above. The output waveform
of operating Tesla Coils can be (and has been) monitored by many of us
by using a short "pickup wire" and an oscilloscope placed far from the
reach of the streamers. :^)
The most obvious secondary voltage waveform changes have to do with much
reduced secondary circuit Q as energy is increasingly lost to
fully-formed streamers. The envelope of the secondary voltage shows a
much more rapid ring-down, with the effective secondary Q typically
dropping an order of magnitude or more. The increased energy coupled to
the streamers also assists in quenching the primary gap. Systems which
initially quench at the 3rd or 4th notch may begin quenching at the 1st
or 2nd notch after heavy (but non-connecting) streamer discharges are
Streamer capacitive loading can be roughly estimated to be about
1-2pF/foot depending upon the degree of branching. However, this
distributed capacitance is in series with in imperfectly-conducting
spark channel, whose resistance climbs as we go from the root outwards.
Additional capacitive coupling to the ion cloud surrounding the torroid
has also been speculated. Most coilers find they need to decrease their
primary tank circuit frequency perhaps by 1/8 to 1/2 turn to compensate
for the detuning effect.
While this change is pretty hard to see on the scope, it's fairly easy
to observe in actual coil behavior. On my system, for example, I need to
change the tap by about 1/4 turn for optimal fully-loaded running. This
implies an additional secondary load of only about 2 pF, even with 63"
Safe coilin' to you!
-- Bert H --