Electrokinetic Acceleration with Pancake Coils
Recently at the Dallas Makerspace I had a bit of a crowd when retesting and calibrating my high voltage pulse system. We went through a few different applications, and the most entertaining is always the Pancake Coil Gun.
Unlike a traditional coil gun, the projectile is coupled to a flat pancake coil of wire. Eddy currents induced in the projectile repel the coil via a lorentz force, and it moves forward. A toy disk launcher comes to mind, except the large flat area of the disk is moving forward instead of the thin edge and hefty 6 inch aluminum washer
Pictures and videos below; some may recognize this setup as something from Information Unlimited and, it most certainly is. I found after going through several banks over the past 15+ years doing this that it made more sense to have a design where parts and suppliers were at the ready, versus forging something just to find the specific capacitor as no longer available for my purpose, or that electrodes of choice became harder to source, etc. This has allowed me to focus more on designing the applications and target systems, and have to worry about the pulse system itself.
For calibrating the spark gap and testing the overall energy of the system, I dug up the original components the system came with rather than design a new contraption (after all, better to do tests with as few variables as possible). Overall it’s operating fine; I had to run the tungsten electrodes under a grinder to de-burr them after a large amount of plasma pitting and thermal erosion caused premature discharges too easily from point emissions, and I have also since wired in a remote trigger that is not inside the charger chassis (complete with interlock and nice, big red button).
The theory of operation is very simple: A charge controller brings a capacitor up to the desired voltage (and thus chosen energy). The only item preventing discharge into the target is a physical spark gap created using two tungsten electrodes, combined with a high voltage pulse transformer. When the pulse transformer and a 3rd, traditional brass sphere, electrode activate, a small spark forms across the primary gap. This drops the effective resistance to a negligible value, allowing the capacitor to discharge into the final target.
The pancake coil is merely a large number of turns of copper enamel wire around a form on some ABS sheets; it induces an opposing eddy current in the aluminum disk, and as previously explained this causes acceleration.
I use this pulse system for a fair number of things, ranging from testing materials ability to withstand plasma discharges, to coating the backs of my business cards with Blast Art by explosive deposition.