Re: Spark-gap sparks vs. solid-state sparks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Ken,
        Energy delivery time I would say is a paramount 
consideration. How does a cap containing, say 10J delivered in full 
to the secondary in, say 10uS compare with the time taken to deliver 
10J from a supply whose continuous output is, say 1kW? Does this go 
any way to bypassing the dVsec/dt explanation you give below? Another 
relevant consideration is the repetitive "letting go" of the 
secondary when considering sparklength. Perhaps the answer lies in a 
single shot comparison (secondary discharge) for both situations and 
doing a length measurement for each.

Regards,
malcolm


On 19 Apr 01, at 18:28, Tesla list wrote:

> Original poster: "Kennan C Herrick by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>
> 
> A number of people have commented, on the List and in separate papers,
> on the seeming fact that spark-gap coils produce longer sparks than
> solid-state or vacuum-tube-driven coils.  There has also been the
> observation that central to that phenomenon is the rapid initial rise
> of secondary voltage that is produced in spark-gap coils.  In those
> coils, the voltage rises to spark break-out in a few cycles at most
> whereas in the other coils, tens of cycles are required.
> 
> What I have not seen commented upon, however, is what I now think I
> see as being the fundamental reason why this is so.  It appears to me
> that the long sparks are engendered utilizing the same phenomenon that
> makes nuclear bombs work; and that is, physical inertia.  And...a
> comment on the bombs later.
> 
> In brief and inexactly put since I am not an expert, a spark, when it
> breaks out of the top electrode, must push away air molecules, by
> heating them, before it can progress.  Those molecules possess
> inertia: it takes a significant amount of time to push them away and
> the energy that must do the pushing comes from the top electrode.
> 
> It is the existence of that time interval that is the key:  The higher
> the rate-of-rise of top electrode voltage prior to break-out, the
> higher the electrode's voltage will be enabled to rise during that
> inertially-created time period.  That voltage will so rise because the
> spark is prevented from proceeding due to the air's inertia.  That it
> is prevented from proceeding means that, briefly, its resistance does
> not get added to the intrinsic resistance of the secondary, thus
> briefly maintaining the secondary circuit's Q in its high state.
> 
> While the secondary's Q is thus relatively high, its current is
> relatively high because its equivalent series L-C-R impedance is low,
> at its resonant frequency.  Thus, its current efficiently continues to
> flow into the top electrode and acts to elevate the voltage there far
> above that which would otherwise appear at spark break-out.  By
> "otherwise" I mean under the conditions imposed in solid-state or
> vacuum-tube coils for example, where the secondary's rate-of-rise of
> voltage can be nowhere near so high.
> 
> It is the much higher electrode voltage, accumulated during one or
> more leading-quarter-cycles of excitation that occur during the
> inertial-containment time(s), that accounts for most of the spark.  It
> may well be that several consecutive quarter cycles are involved, and
> that the spark grows step-wise during a number of cycles of
> excitation--until the spark's added circuit-resistance diminishes the
> secondary's current too much for that process to continue.
> 
> For a riveting account of such a use of inertia, read Richard Rhodes'
> "Dark Sun", about the making of the hydrogen bomb.  In Chapter 24,
> Rhodes provides a microsecond-by-microsecond account--complete with
> construction diagrams--of the process by which the better part of 82
> tons of solid, gaseous and liquid material was turned into photons in
> the space of a few microseconds.  It was only possible because of the
> inertia of its components: they stayed together long enough for the
> numerous consecutive nuclear processes to occur.
> 
> I see now that the challenge for s.s. designers such as myself lies in
> attempting to emulate, to a degree at least, the rate-of-rise
> capability of the common, garden-variety 19th-century spark-gap.  One
> has to chase that spark and well-overtake it, so to speak.
> 
> Ken Herrick
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