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Re: theory(?) for long sparks
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
http://www.coe.ufrj.br/~acmq