There has been a lot of discussion the last few days about some fellow who printed a “plasma gun” and was shooting a bb at 450 ft/s. I’ve got some experience working with Dr. Mohamed Bourham back in 1992 – 1995. Plasma Engineering at NCSU. So I feel like responding to the usual threads.
The usual crap is:
- This must be fake.
- Regulate this before someone gets hurt.
- Really this isn’t very impressive.
- I don’t know much about 3D printing, ask Chad Ramey if you need to know. He runs (ran last year) Georgia Tech’s 3D lab. I’ve done some laser cutting and some milling, mostly plastic but a little steel. But, you can basically 3D print any shape. Usual limitations involve re-curvature and support of extended structures, same as any model building.
- Seriously, regulating 3D printing is as purposeless as regulating sex acts between consenting adults. Completely F*d up. Unless you decide to regulate the transfer of CAD files on the internet. You are outa luck.
- First off, 3D printing is cool, but milling is better. You can do a much better product with a 3D milling machine and a block of steel. Lexan makes a great ablative barrel material and a good bullet, but that’s it. This is a cute toy which could be sold next to the CO2 powered (paint ball) markers. No more dangerous (unless you shock yourself playing with wiring.)
So, what is a Plasma Gun?
Technical term – Electro-Thermal Chemical Plasma Device. Why? You use an electrical current to heat plastic so that it chemically changes into gas. The gas, mostly Hydrogen, is further heated to become a plasma. You get some Carbon deposition along the walls, which is good for encouraging all the current to flow through the gas. The Hydrogen gets hot, like 30 – 50,000 degrees Celsius. I normally divide by 11 k and call it electron-Volts, so around 3 eV.
The plasma expands at a much greater speed than speed of sound, so it won’t be limited to 1 km/s. Depending on how much energy you can get stuffed into that plasma. Well, a Dragon Con friend of mine asked for a plasma story, so I figure I can write that up today.
So, Here is the story. I was working in the lab and we had an idea to do an experiment, accelerating a 10 gram mass of Lexan with a plasma pulse. We were trying to determine how much momentum was coming down the barrel as a function of V applied. This mattered to some materials experiments we were working on. (Trying to separate out the thermal shock from the physical shock) So Eric and I decided to give it a try.
So, the ETCP device is set up in a wire Faraday cage about 10 feet on a side, lined with lead bricks. Hey, we worked in a reactor, there were lead bricks spare. We built three cubicles out of them, and lined them all with Faraday cages…we made a lot of EMP.
The device is solid stainless steel, bolted to the table, which is bolted to the floor. Very Immobile. The end was pointed directly at the wall, so if we pulled the rear seal off the experiment, the “bullet” er Lexan Mass would fly out of the back and hit a target in the center of the wall. We hooked up a pair of laser beams to measure the crossing speed of our 5 foot racetrack, and got to business.
I carved a bunch of 10 gram Lexan masses. They were about the size of your pinkie fingertip. (Little things) Eric put some bolts into the wall and suspended a chunk of Aircraft Aluminum, 3/4 inch thick, 4 inches wide, a foot long. We ran up a charge on our mass of capacitors. When we got to a good charge, I’d insert a bullet and we’d duck into our control room, turn on the warning light for 5 minutes, and he’d fire the gun. The Lexan converted to plasma on arrival on target, completely destroyed the bullet.
A few hours, a new charge, a new bullet, a new shot, a new velocity. Science! Each shot made a huge, single, noise. Like smacking a garbage can with a hammer. Scared the neighbors something awful.
So, at around 45,000 V, the shot made a sound like ringing a dinner bell, *ding a ling a ding a ling a ding a ling a ling a* It went on for a few seconds, louder, dimmer, louder, dimmer, louder… done.
We looked at each other, what the heck? Checked out the room, no damage, but the target was behind the control station folded in half. Marks on the walls, all the walls. Oh crap. The 10 gram mass of Lexan had folded the aluminum in half and bounced it off of every wall, probably twice. Dr. Doster decided that we didn’t need to be doing that experiment any more. But hey, we got to 3.54 km/s. Beat that.