Short video clip of 18" x18" Lichtenberg Figure being discharged (fwd)

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Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Fri, 23 Feb 2007 10:27:47 -0600
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Short video clip of 18" x18" Lichtenberg Figure being discharged

Hi All,

I just uploaded a short video clip of an 18" X 18" X 1" Lichtenberg 
Figure being discharged to You Tube. This particular specimen was 
brought by local coiler Jeff Larson during our 2005 "Captured Lightning" 
production run. The video was captured and edited by Terry Blake:
http://www.youtube.com/watch?v=FWOst4VwwEU

We used a commercial 5 million volt 150 kW CW linear accelerator to 
inject a layer of electrons into the Plexiglas (PMMA) specimen. Prior to 
discharging, the internal charge plane had an estimated potential of 
~2.2 MV. The estimated stored electrostatic energy within the specimen 
was 1800 Joules, and the estimated peak discharge current was between 2 
- 4 kA. The video does not fully capture the brilliance and sound of the 
discharge. It sounds similar to a high voltage capacitor discharge, but 
the sharp report is somewhat muffled by the acrylic.

The charged specimen behaves like a "plateless" capacitor, with a huge 
negative charge on the interior and a similar amount of temporarily 
bound positive surface charges on the specimen's outer surface. Other 
than a bit of darkening by solarization, the PMMA looks virtually the 
same as it did prior to being irradiated. By poking the the specimen 
with a sharp conductive object, the resulting E-field concentration 
initiates dielectric breakdown within the PMMA. Branching positive 
streamers and leaders rapidly propagate throughout the PMMA as the 
excess charges are rapidly neutralized. You can sometimes see blueish 
air sparks jumping towards the excess positive surface charges on the 
exterior of the specimen. And example is this 12" x 12" x 1" specimen 
(737 kB):
http://205.243.100.155/photos/HVStuff/Lich/IMG_5695Xa.jpg

Localized heating and gases generated within the discharge channels 
create small fractures in the PMMA, leaving a permanent "fossil" record 
of the discharges behind (an acrylic fulgurite?). The roots of the 
discharge often become darkened, even charred, by higher current, longer 
duration leaders as they channel the combined current from multitudes of 
propagating tributaries. Curiously, although the injected charge is 
negative, the resulting figure is a positive Lichtenberg Figure since 
the discharge is propagating from the positive-most "electrode".

In the video, you can also see a batch of secondary discharges that 
persist after the main one. These cause the specimen to sparkle and 
sizzle for perhaps 15-30 seconds afterward as residual charges are 
redistributed within the PMMA. Specimens still retain significant 
residual charge (easily raising the hairs on your arm) for weeks later.

During our December, 2006 run, we managed to capture the discharge 
current from a 4" x 4" x 3/4" specimen using a special holding fixture, 
a Pearson 410 wideband CT and a Digital Storage Oscilloscope (borrowed 
from Jeff Larson). The current peak was about 200 amps, and the total 
discharge event was about 600 nsec. Lower amplitude pulses from 
secondary discharges could also be observed (140 kB):
http://205.243.100.155/photos/HVStuff/Lich/4inchSquareDischarge.jpg

During the latest run we also were able to capture some video of the 
actual irradiation process as the specimens were being bombarded by the 
5 MeV electron beam. Terry Blake positioned a video camera so that it 
could view the irradiation process with the image reflecting off a 45 
degree metal reflector. Direct viewing would have destroyed the camera's 
image sensor and solarized the optics. Although positioned behind over a 
foot of concrete, the camera was still hit by high energy X-rays that 
disturbed its operation. During irradiation, specimens glow a brilliant 
blue color from fluorescence (and perhaps also some Cherenkov 
radiation?). Specimens that prematurely self-discharge look as though 
they have a raging lightning storm within.

I hope to add more video and discharge pictures soon. BTW, did I mention 
that making these is a real blast.... :^)

Bert
-- 
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We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, Lichtenberg Figures (our "Captured
Lightning") and out of print technical Books. Visit
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