Making Lichtenberg Figures... (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 30 Jun 2004 10:52:46 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Making Lichtenberg Figures...

All,

A group of fellow experimenters from Stoneridge Engineering and GCL
Laboratories recently made a number of Lichtenberg Figures using a 5
MeV electron beam accelerator (LINAC) earlier this month. While most
pieces were smaller, we also made a number of 6" x 8" x 3/4" and 12" x
12" x 1" figures that were among some of the best we've ever seen.

Of potential (no pun intended!) interest to this group - we were able
to capture some digital images of the figures as they were being
discharged. The larger piece required the use of a heavy neutral
density filter to avoid saturating the camera's image sensor. E-beam
irradiation produces a "stranded" region of negative space charge
within the acrylic (polymethyl methacrylate - PMMA). Once the internal
E-field exceeds the dielectric strength of the acrylic, or if the
E-field is concentrated by externally poking the block with
a sharp conductive object, dielectric failure ensues, and an internal
positive Lichtenberg figure is formed as the discharge propagates
through the dielectric, dissipating the stored charge within.

When a charged specimen is manually discharged, the flash is a
surprisingly brilliant blue-white color. The sound of the discharge is
similar to that of a HV capacitor discharge but it's strangely
muffled. It had significantly less brisance than a regular HV
capacitor discharge. I suspect this may be due, in part, to most of
the discharge occurring deep within the plastic instead of directly in
the air, perhaps dampening the spark's shockwave.

Per the literature, the internal electrical field within irradiated
PMMA prior to self breakdown is estimated to be of the order of
1.8 MV/cm, and charge density about 1 uCoulomb/square-cm. Once
breakdown occurs, the excess charge rushes out to any nearby
conductive object (people included, and we found out the hard way on
some of the smaller specimens!). Some discharges were also observed
going to the outside surfaces of the acrylic as spidery surface
discharges. I guess this is to be expected, since the internal space
charge forms one "plate" and the positive charges on the surface of
the dielectric the other. Also in the literature, the duration of the
main discharge for small specimens has been measured at ~20 nsec.

Because of the larger physical size, the breakdown "front" will take
longer to propagate in our much larger specimens - perhaps as much as
200-240 nsec. Based upon a back of the envelope calculation, the
absorbed charge in one of our 12" x 12" x 1" specimens is estimated at
~890 microCoulombs. This implies a discharge current in the range of
3.5-7 kiloAmperes! Not bad for a "plateless" capacitor - and based on
the brilliance of the discharge and the resulting heavy fracturing
within the PMMA, it sounds in the ballpark.

After the huge main discharge, there are often a series of secondary
discharges that randomly occur over the next 10-30 seconds as residual
charges are redistributed and bled away through the recently formed
channels in the acrylic. On 12 inch specimens, the entire discharge
region seemed to scintillate with 1/4" - 1" discharges for the first
few seconds, making a distinct crackling and sizzling sound. The
activity gradually declines and finally ceases after about 30 seconds.

I hope to add a short video clip on my site showing the main discharge
followed by secondary discharges for a 2" cube. Many of the specimens
still retain considerable internal charge over two weeks later. I can
easily feel the residual stranded charge with the hairs on my arm,
especially on the 12" specimens.

Anyway, here are some "before" and "after" images for a 4" x 4" figure:

http://205.243.100.155/photos/For_Sale/June04/4InchSq/Discharge1a.jpg
and
http://205.243.100.155/photos/For_Sale/June04/4InchSq/Square1a.jpg

A 12" x 12" figure being discharged:
With no neutral density filter:
http://205.243.100.155/photos/For_Sale/June04/12InchSq/12Inch_Discharge2b.jpg
Through a dark neutral density filter:
http://205.243.100.155/photos/For_Sale/June04/12InchSq/12Inch_Discharge1b.jpg

Note: A couple of surface discharges can also be seen snaking along
the top surface. The discharge to ground is mostly obscured under the
metal tool used to discharge the panel.

The result may be one of the prettiest figures of this size anywhere:
http://205.243.100.155/photos/For_Sale/June04/12InchSq/12Inch1a.jpg

To my knowledge, the only other specimen of this size on the web is at
the Oak Ridge Associated Universities' Health Physics Historical
Instrumentation Museum:
http://www.orau.org/ptp/collection/Lichtenberg%20figures/Largemendonca.htm

We also did multiple-shot specimens with two or more layers of charge,
including some with "cubic" discharge regions - these can be seen on
our main and mirrored sites.

Hope you find these of interest!

-- Bert --
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We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
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