A fast electrical switch is needed for various operations in physics. It
may need to switch very high voltages and very high currents, and do so
quickly. While there are now more sophisticated things in existence, the
standby triggered spark gap still has a place. This one is intended to
switch about 250 Joules into a microscopic rail gun. The energy
storage capacitor is rated at 2500 Volts, so that will be the voltage, but
I am not sure what the achievable current will be. A pretty rough prototype
of the gap was able to pass enough current quickly enough to shred cheap
speaker wire used as the "rails". Of course the end was blown
off, but that's not what I am talking about. The enormous magnetic field
literally tore the wires apart from one another, and shredded and melted
the insulation in a microsecond-like time frame. Now that we have a real
gap, we will take real measurements and see how well we are doing. We also
have on the shelf one of the xenon thyratrons used in the SLAC accelerator,
but we want this project to have a portable aspect, and will save that
tube for something that stays home. It is probably close to worn out
anyway, which is why we got it surplus. At any rate, spark gaps are one
heck of a lot easier to fix.
Nearly everyone knows what an assembled spark gap looks like, but just
in case, here it is ready to go. Pairs of nuts will adorn the ends and
clamp the wires between the main terminals, and the trigger will come
into the 10–32 screw on top. The trigger spark emits enough
ultra–violet light and probably a little ionized metal to start current
flowing through the main gap, which is set far enough apart to not trigger
without this stimulus. This might be very important for some of our
projects, as the little rail gun this will drive is used as a detonator
for explosives. The idea is to make something safer than mercury
fulminate or lead azide based detonators. Both of those are dangerous
AND poisonous, though we make those too. Our detonator design might
be called a "chip slapper". The idea is to have a short little
rail gun accellerate a small cloud of plasma to a high enough speed to
shock-initiate some HE, which in our case will be PETN. The plasma will
originate as a small piece of wire between the rails at the beginning of
the 1/4" long rails. I have tried 1 mil gold, tungsten, and other
things my sources say work, but frankly, a strand of the cheap speaker
wire that makes up the rails seems to work just as well. Probably because
I don't really need anywhere near this much energy to do the job. Well,
I have it, and reliability is a very good thing
around explosives! In this design, ensuring the capacitor isn't charged
is enough to guarantee no unintended firing, short of getting hit with
a bolt of lightning.
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Here is the gap taken apart. For reasons that should be obvious, the
pipe is only glued to one of the caps. One big reason for doing this
in PVC pipe, other than that it's a good insulator and alignment jig, is
that the prototype gap was VERY noisy, comparable to a .38 pistol
shot in the room. Even with warning, everyone jumped (and there were
more–embarrassing mishaps). Since the planned application will only
need the gap to fire infrequently, there is no need for ventilation and
cooling, as there would be for instance with a gap used for a Tesla coil.
The expected failure mode for this gap will be pitting and evaporation
of the main electrodes onto the plastic, rendering it conductive.
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Here are the separate pieces. As contact evaporation was a big concern
based on work with a prototype, the gap faces here were plated, first
with copper, and then with chromium, quite thickly, not the decorative
stuff. First the zinc galvanizing was stripped off with hydrochloric acid,
then the copper and chrome plating were applied. The copper makes a
good base, and is very thermally and electrically conductive compared to
hardware store iron. Due to skin effect, the current in a high frequency
waveform flows mostly on the surface of the conductor, ignoring the bulk,
so the copper ought to make a difference. The chrome is there as it
is the hardest to evaporate metal we can electroplate.
The trigger electrode is a 10–32 screw with a sewing pin soldered
into a hole drilled in the end. The sharp point will make it possible to
trigger the gap with a relatively lower voltage. Sure is nice to have
a machine shop, this was an easy part to make! As to the general
machining, not that much was done. The caps had flats turned on the
ends, were shortened, and tapped 3/8 x 16 to match the standard bolts.
A flat was cut on one side to work with the washers for the trigger electrode.
Not required, but nice. Once this is ready to go into real service, there
will be a couple of holes drilled and tapped on the bottom, which will also
get a flat, for mounting with nylon screws. If past experience means anything,
this thing will try to come apart under the various forces involved. We
only want that to happen when we desire it for maintenance.
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Here is an example of a miniature rail gun. This is just very cheap
speaker wire, stripped back about 1/4", tinned, and with one
conductor folded to short the beginning of the rails. In use, a blob
of PETN in nitrocellulose–based glue will cover the end of the
rails. This in turn can detonate a larger charge in a normal sized
blasting cap. You might be wondering how one can make 2500 Volts go
down this cheap wire. If you think about it, the voltage doesn't happen
until after all the action is over, and any residual charge in the capacitor
is wasted anyway. A huge current pulse is sent down
the wire, and the resulting magnetic field operates the rail gun, usually
vaporizing the projectile, which just does not matter to the
application, it still has the momentum and velocity to do the job, no
matter the form of matter. In previous tests, nearly the whole thing is
more or less
destroyed, and interesting holes appear in a witness plate placed downrange
a little from the end. This is a safety–glasses and hearing protector
sort of thing
when done indoors, but of course actual use will not be indoors, nor will
the user be very close by! In use, the main cable of course, is the
good stuff; the rail gun is only a couple of inches at the end of it.
We use 4 conductors of #10 for this. Regular romex works
fine too.
The picture was taken through one eyepiece of our stereo microscope, at
about 10x magnification. I am surely going to have to get a whiter light
source for this, although it looks fine to the human eye as is. What
I am using now is a pair of compact fluorescent lamps, which are warm white.
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For initial use we will use an electronic ignitor such as is used in
propane grills to trigger the gap. This will have a large series resistance
so that the big energy can't get back into the little ignitor and fry it.
These put out a lot more voltage than is really needed, so probably down
the road there will be a smaller triggering power supply made up, likely
using parts from a xenon flash trigger. That is only to simplify things
so everything can run off the same battery. However, looking into the
future, it may be desirable to use a microprocessor to control triggering,
so that multiple shots can be fired with deterministic time delays. The
technique is used in mining to get a shaped charge effect when shock waves
from separate detonations meet in the rock, and to control the sonic
signature. Systems that do that are quite expensive, however, and have
a lot of features we do not need for our applications. Here all we do
with explosives (other than just have fun) is loosen stumps so we can
recover the interestingly twisted underground wood for woodworking, and
bury the occasional piece of water pipe. It is rocky here, and no digging
machine we've had out here can touch the bedrock we need to run pipe
through for about 30 feet. We want a little slot cut into this bedrock,
and to make things fun, its under a running creek. Sure we could call in
the pros, but the result would be a shallow crater 6 feet wide and the
overburden spread around the county. Here we can afford a little more
finesse than that. The result will finally be a reliable supply of
water from our artesian spring, which unfortunately is about a half
kilometer from the buildings here, and on the next hill over.