Re: exploding wire

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Gomez Addams <gomezaddams@xxxxxxxxx>


On Jun 13, 2006, at 5:25 PM, Tesla list wrote:

> Original poster: "Kyle Sandbornhesium"
> <sandbornenterprises@xxxxxxxxxxx>
>
> How does it shrink a wire by running all that energy through it?

It's the "Z Pinch" effect, resulting from Lenz's Law.  A single wire in
free space has a magnetic field.  That magnetic field compresses the
conducting material, whether it is copper heated to the melting point
or copper vapor, or plasma.  The strength of the magnetic compression
is dependent on the current.  Instabilities in the magnetic field cause
some parts of the conducting path to be compressed more than others,
ultimately leading to a breakup of the conducting path.

It's the same phenomenon which causes streamers from the top of a Tesla
coil to be confined into fine filaments of plasma, rather than disperse
as a diffuse cloud or glow.  But of course, the currents are many orders
of magnitude higher, so the pinching effect is much greater.

Since the copper (or plasma, depending on the operating regime of the
experiment) is being squeezed, the current per unit of cross-sectional
area goes through the roof, and so must the temperature of the
conductor.
This is a non-linear effect, and there are several different mechanisms
at work, which explains why you get different looking effects at
different
energy levels.

When I still had my 5kJ / 50kV ultra-fast capacitor bank, I found very
marked differences in the nature of the explosion at different energy
levels.  All of them were very loud, and all but the lowest voltages
resulted in no trace of the wire being left. However, at the higher
charge levels, the exploding wire began to resemble a lightning bolt
channel, with no trace of sparks.  Even though my bank was "only" 5kJ,
the concussion was deafening and despite wearing both ear plugs and
hearing protectors (I also tried one or the other alone), after half a
dozen shots, my sinus and chest would begin to ache.

Even though my new bank has 18kJ of total energy, it's only 10kV and
the capacitors, while also low-inductance pulse caps, are more like
the Aerovox and Maxwell caps most other amateurs are using today -
low inductance, but not ultra-low inductance.  As a result, I wonder
whether I'll be able to achieve as fast rise times as I did with the
old bank.

PS: exploding wire experiments are VERY hard on pulse capacitors.
I've been considering building a low-self-inductance power resistor
from copper pipe (using a stainless steel rod for the resistive
element) to try to damp some of the high frequency oscillations
(voltage reversal) which are the most harmful to pulse caps.  The
trick is to achieve near-critical damping without drastically
harming rise time or peak current.

Somewhere around here I've got a white paper on the design and
construction of such a resistor designed to pass hundreds of
kilo-amps and to absorb peak power levels on the order of tens of
megawatts without harm.

 - Bill "Gomez" Lemieux

..........................
WWTD: What Would Tesla Do?