Re: Exploding wire (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 02 Dec 2006 12:21:37 -0800
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>, hvlist <hvlist@xxxxxxxxxx>
Subject: Re: Exploding wire (fwd)

At 11:36 AM 12/2/2006, High Voltage list wrote:
>Date: Tue, 28 Nov 2006 09:19:32 +0000
>From: Mike Harrison <mike@xxxxxxxxxxxxxxx>
>To: High Voltage list <hvlist@xxxxxxxxxx>
>Subject: Re: Exploding wire (fwd)
>There is certainly a huge difference  between exploding copper and 
>steel (guitar strings are good) -
>with the latter you get a really spectacular shower of sparks as the 
>metal burns.
>
>There is some 1000fps high-speed camera footage of this on my site :
>http://www.electricstuff.co.uk/fastdestruct.html (bottom of page)
>
>Also some frame sequences here :
>http://www.electricstuff.co.uk/explode.html

Those look like "just on the verge of exploding" wires, but certainly 
high power input.  When you don't have quite the di/dt for an 
explosion, the wire melts, then surface tension (and the magnetic 
field) pulls the liquid column into droplets, which then burn as 
they're scattered around, creating that nice fireworks effect.

You have to dump the energy into the wire fast enough that it boils 
before it can move (similar to inertial confinement in an fission 
weapon), which is a real challenge.  We're talking a few microseconds 
at most. My exploding wire rigs had a total inductance of about 1-2 
uH and 10-20 uF..which I measured by measuring the resonant 
frequency, and knowing what the C is. The self resonant frequency is 
in the 100-200 kHz area, so it reaches peak current at about 2-3 microseconds.

An exploding wire that's well into the "exploding" regime has a very 
uniform appearance.  It just disappears with a flash into smoke, with 
no "glowing particles", with a very characteristic "crack" sound (as 
opposed to a bang or thump).  If you've ever heard a supersonic 
bullet pass by you, you know what the shock wave sounds like (without 
the buzzing sound of the bullet), although the exploding wire does 
sound a bit different. On a fast framing camera (even at 10,000 fps), 
I would think that you'd see wire in one frame and a column of 
glowing plasma in the next.  The entire event is all over in 10 microseconds.


Your destructotron rig is 220 uF at 5 kV, I think, so I don't know if 
you can get the power out of it fast enough.  There's a sort of 
tradeoff in voltages, currents, and L and C.  You want high voltage 
(because di/dt = V/L... big V means big di/dt)  you also want small L 
(because of the sqrt(L*C) thing for the self resonant 
frequency).  And big C (because that's where your energy comes from), 
but big C usually means big L (because of how capacitors are 
constructed).  Estimating your loop inductance at 2 uH (a couple 1 
meter leads), your self resonant frequency is probably in the 5-10 
kHz area, so the energy is dumped in a hundred microseconds or so.

If you rewire so that your capacitors are in series, that helps 
(assuming that the external circuit L is dominating) because you 
reduce C and increase V, while keeping the energy constant.



Jim