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Ions n' arcs
All,
The continued study of arc and gas ion dynamics has had me pouring over
a number of references. By far the best remains my first love "Ionized
Gases" by Alfred von Engal. There is a reason for this enthralment by
myself with this text. I am not alone in this adulation,either! The
AVS (American Vacuum Society) has thrown this book on its extremely
short heap of "permenantly-to-be-printed" books known as the AVS
"classics series". (maybe 20 books total in the history of all printing
that made their list- I have most).
In short, von Engals was a recognized expert in the field. If this were
the only criteria he would have been relegated to just an E Pluribus
Unum amoung countless other "ion/plasma experts" so litering the field,
not to mention publishing their own "definitive tomes". What makes von
Engal's work so different, was the mentality and timeless nature of his
effort. Furthermore the following litany of what makes something
classic to do'ers in any field of endeavor......... The clear graphs
and diagrams of emperically derived data........ The relative paucity
of tedious theoretical equations so locked to rigidly fixed and
controlled environments......... His open admission through out the
book that the equations he does offer up are for specific cases and his
supplying caveats regarding there use in real systems......His treatment
of a full range of ionization pressures in spite of everyone else in the
field talking of vacuums and reduced pressure ionization as a
"given".........open admission of ionic processes that are still
mysterious in spite of their observational mechanics being well known
and engineered........ supplying real world, empirically based
application rules of thumb with the same cautionary caveats he supplied
for the equations earlier. Writing in a scientific but friendly
mentor-like advisory tone as opposed to one of complete
authority................All of these make the book an easy read though
complete and thoroughly on topic, leaving it in a class by itself and
thus, a classic!
Regarding my recent discussions on Tesla coil air arcs, I quote this
following paragraph from page 209 of his work where the discussion
involves detailing high pressure ionization in intense electric fields
as relates to emboldened current channels (glows to arcs)...........
"There have been recent attempts to measure the longitudinal spread of
luminosity in a gap and to relate it finally to the spatial variation of
the electric field and its change with time. (ref) The transition
mechanism from avalanches to the more condensed form of discharge remain
still obscure. An obvious step in the direction of explaining the rapid
multiplication and rise of current is to include secondary processes in
the gas such as ionization of excited species which would enable slower
electrons in the distribution to play an active part besides charge
transport. (ref.) Once a finite length of path has a high density of
ionization (which because of time cannot diffuse) the electric field
along this path length must drop to a very low value. As a result of it
the field in the remaining gap will rise and the multiplication process
will speed up. However, there is no satifactory treatment which deals
with this problem."...................
I am now on my fifth cover to cover re-read of his book in three years.
I haven't figured out whether I am just slow on the uptake or whether
his work so complete that it is like a superb Monty Python movie where
more is picked up after each re-viewing items often missed in the
shuffle to follow the plot in prior viewings. I fear it is a little of
both. I have yellow highlight marked so many key passages and thoughts
that the book is looking more like the Bell System yellow pages than a
scientific text.
To pick the above salient paragraph apart, we see:
1. He starts the thought with the statement of what was attempted by
recent experimenters to codify and their subsequent failure. Gives
reference.
2. He states that some observation evidence exists whereby the gap is
filled by successive ionizations, the field drops along the arc channel,
and provided the ion liftimes are long enough or the energy persists for
at least that period, a further enhancement of the channel is seen due
to SECONDARY processes. Gives another key reference.
3.Finally closes with the sad admission that no formal mathematical or
physical treatment of this observed process exists.
He has supplied info on how it appears, from real observations, gaps in
high pressure gases are crossed by heavy current channels via multiple
ionizations and re-ionizations in strong electric fields. Then, he
finally admits that no real theoretical mathametics exists for this
treatment. It is rather obvious why, to the informed reader, this is
so. The entire first 209 pages have brough us to this point with a
myriad of useful but limited equations, numerous qaulitative,
observational tips with their associated caveats, along with the von
Engalian truth about numerous little understood processes. A daunting
task for would-be modelers especially in view to the numerous possible
secondary processes mentioned. This is often more than the staunch and
often rabid phyicist can bear. However, folks who actually work in the
real world (AVS members), elevate this effort full of maybes,
non-commitals, qualitative tips and addmissions of ignorance to
"Classic" status. I might be just an old grizzled electronics engineer,
but I see why they honor von Engals as they do.
We can now understand a lot of why our casual observations of big
terminal loads, lots of power with lots of arc current and moderate
voltage make long hot arcs. Also why potential might not be all that
important beyond supplying an electric field (hopefully intense) around
the coil.
The key factors for our Tesla coilin' efforts are that air ionization
can occur with relatively low fields as most atoms only require 10-20 ev
of energy to lose an electron. While a long reaching, intense electric
field is strictly a function of terminal potential, ion lifetimes and
volumetric ion production demands current!! More current makes for more
ionizationsat any given field intensity and higher levels of excitation
and ionization also follow current. The mean ion lifetime goes up
rapidly also as the channel expansion reduces the internal channel
pressure. Still, for our purposes, the max lifetimes of super excited
ions are still on the order of maybe, at most, 100milliseconds. Mean
liftimes of the bulk of the really hot gas jumps from under ten
milliseconds to around 50 milliseconds. Thus, more energy pulses
(higher rep rate) of lesser energy (smaller cap) but overall increased
average energy (plenty of input power from the wall socket and efficient
conversion) can, to a point, make maximum use of air ionization to make
arcs go where no real potential difference with low channel current
could pass. This is concomittent with a moderately high electric
field (large xl secondary) and high channel current (large terminal
loadings).
Single chanel arcs which I have stressed, funnel and focus the
available field (as noted by von Engal above) and allow the ion
re-enhancement to move out unabated until the ion channel gets too long
for the repetitive energy to support not only growth, but continued
existance due to increasing temperatures in the gas dispersing
mechanically, the ions in the channel.
I hope this helps with the understanding of the object of our "off coil"
desires.....long hot air arcs! As this is a totally off coil process we
must reverse engineer for this process alone in the Tesla coil engine.
This will mean a lot of tradeoffs and a load of multivariable and
covariant items to juggle. A daunting process which is only hinted at
and dimmly understood in the best of HANDS and MINDS daily involved with
Tesla coils. (read- a handful of people on the entire planet)
Richard Hull, TCBOR