Triggered Spark gap:

Coulter's Smithing Home.

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.

Assembled Spark Gap 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.
Disassembled Spark Gap 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.
Spark Gap components 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.

Miniature rail gun 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.

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.