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Re: Streamer growth and filming it all

Some ideas, based solely on literature and not first hand measurements..
>
>a.) How exactly do they form. Do they form nm for nm or
>     does the initial streamer "lash out" a few inches and grow
>     the rest of the length?

The literature says that the leaders progress in discrete jumps, the size of
which is determined by the charge reservoir (tiny for the few pF in a TC,
hundreds of meters for a lightning stroke).  As near as I can figure, charge
moves down the "wire" formed by the existing leader until enough accumulates
at the end to raise the E field above breakdown. Then, it breaks down and
jumps forward, which reduces the charge on the whole wire (a travelling wave
propagates back up the leader as charge flows into the new segment, which is
gradually getting hotter and more conductive (due to the current flow).
During this current flow time, the voltage at the tip is reduced (IR losses,
if nothing else), so the E field is lowered.. Current keeps flowing because
once the channel is formed, it's resistance drops dramatically (that
negative V/I curve).

>
>b.) Why do they stop growing. Is it really *just* a matter of
>     temperature. In other words, does the ion channel simply
>     cool off too fast?

I think that as long as you have a source of charge to keep the current
flowing in the leader, which keeps it hot, it will keep growing.
Eventually, IR losses will be enough that it can't extend, and inevitably,
it cools down and disappears. The random dendritic nature of the spark means
that it is branching out as it goes (even if most of the side branches are
very short and don't go anywhere), which consumes ever more charge and
current to keep the whole thing hot, as it gets longer.
>
>c.) Using two coils, how are streamers attracted to each other?
>     The point where these two connect must get pretty hot. Why
>     donīt (or do they?) branch out from here?

Two coils or a single coil and a pointy ground should be similar. The
streamer is very low resistance, and so acts like a localized field
concentration. Since the direction of the "next jump" is determined (in a
statistical sense) by where the E field is strongest, and the streamer
coming the other way also contributes to the E field, it seems that the
"segment" of air between the leaders will be the likely place for the next
leader jump.

Once the channel has reached 7000K, the conductivity is so high that the IR
heating drops off.. Essentially, the entire leader is "regulated" by a sort
of negative feedback (heat loss proportional to T^4, heat input proportional
to I^2R, which is inversely proportional to temperature) to a temperature
just high enough to keep it "well ionized". So the point where two leaders
meet is probably no hotter than anywhere else along the leader.


>e.) How (if it) does the ion channel change, once the streamer
>     hits something grounded (i.e. turns into an arc)? I think an
>     infrared film would help as one could analyze the spark
>     and ion channel temperature.

Now the spark channel can get current from both ends and it can get bigger
and fatter...


>
>
>g.) Why and how do streamers branch out. Is it really the same
>     streamer or is it just an optical illusion, (i.e. not really the
>     same streamer), because our eyes arenīt fast enough to
>     detect this.

They really do branch.. High speed photos of spark development (or corona)
show a whole family of leaders proceeding from the end of the spark channel.
Sometimes only one of the little starter leaders is selected, othertimes
it's several.

>
>> Original Poster: "Jeff W. Parisse" <jparisse-at-teslacoil-dot-com>
>> Well... I've got a bunch of industry contacts that I could call about
>> such a project. I'll see what I can do...

There is a guy whose name I forget who has a vacuum drum camera (spinning
film, not spinning mirror) called the Millisecond camera. He is represented
through Panavision.  He might be interested in an experiment, esp if you
were to pay for film and processing.  Each take is only about 5 feet of
film.  The system has suitable synchronisation electronics where you might
be able to sync it up with the "bang" on a TC rotary gap (or the line
frequency on a static gap).  I don't know how fast his capping shutter is,
but it is probably in the millisecond range.

>
>Over here, you CANīT get your hands on such a camera,
>unless you are a professional. Iīm sure there are
>companies who would rent you a camera AND a crew for
>bags of money.... ;o( Of course, I hadnīt thought of the
>film costs. This really would a reason to use an electronic
>camera.
>
>Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
>>Bazelyan and Raizer, "Spark Discharge", has a number
>>of streak camera photos of developing HV sparks (1
>>MV+). They might have used an image intensifier (i.e.
>>night vision goggles), or an electronic streak camera
>>(fastest of fast).
>
>Hmm, the idea of using an image intensifier hasnīt crossed
>my mind. Wouldnīt it blur a fast acting subject, like a
>spark discharge is?

Nope.. the image intensifier is a very fast device. Essentially, photons hit
the photocathode and knock electrons loose which then get multiplied (sort
of like a PM tube, in a channel) and then hit a phosphor screen (like a crt)
.  The transit time through the device (from photon on the front to electron
hitting the screen is essentially fixed. You could have several photons very
close together (<nano or pico seconds) hit the photocathode in sequence and
trigger separate electron multiplications, etc.

For high speed use, you need to make sure that there is charge on the
photocathode (so the electrons have somewhere to come from), likewise in the
multiplication section, and there is some response time for the phosphor,
but these are all really fast.

Now.. I don't know if the surplus $500 night scopes have this sort of high
speed performance.. They certainly have low resolution (mostly a function of
how many channels there are in the intermediate amplifier stage)

The fastest cameras (picochron) change the basic image intensifier to add a
set of plates to deflect the electrons (like in an oscilloscope). By putting
a sweep voltage on the plates, you can move the image on the output screen
to achieve very high "scan rates". Of course, creating a sweep voltage or
field that varies at several GHz is no easy matter, either.

Fast analog oscilloscopes use a similar technique to make the trace
brighter.. The microchannel plate is just behind the phosphor, and as the
electron beam is sweeping (very rapidly), it hits the plate and gets
amplified.  Otherwise, when you get to those 100 pSec/cm sweep rates, the
trace gets pretty darn dim (just not very many electrons hit a given area of
the screen)
>
>