1kJ Electrothermal gun (ETG) / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

1kJ Electrothermal gun (ETG)

first previous 6 7 8 9 next

Move Thread

LAN_403

MrFlatox

Fri Feb 22 2013, 08:20PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Nice one !

As my main capacitors are polarized, adding a transformer in series is not going to damage them from the resulting oscillating RLC circuit ?

Yandersen

Fri Feb 22 2013, 08:57PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Of course no. I mean, not that insignificant oscillations. Caps will be forced to repolarize because nothing will stop the huge current from running when 0 voltage will be reached at caps - parasitic inductances and trans will maintain required voltage over the gap to keep current running. It will be ceasing at the moment, yes, but without an antiparallel diode caps will be damaged. I use 10A07 currently. You may parallel dozen of those.

Yandersen

Sat Feb 23 2013, 07:36AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Another day of thinking has convinced me that using air as propellant has no use at all. As our goal is to heat the whole gas inside the chamber to few thousand K, then we can assume that each particle is an ion, actually. If we take volume into account, count the particles and their speed, and the area of electrode, we will get a current of google magnitude. The discharge time is opposite in magnitude - it is the time that takes ion to travel from one electrode to another achieving the desired speed, which is smg like a microsecond. The first number dictates huge losses in resistanses of circuit conductors and caps's ESR, the second value is not achievable due to cap's ESL and wires inductances. And if we will dream closer to reality, than we may take a light emission into account - every time opposite ions meet, the energy we spend to ionize them is get wasted as light emission. And this is purely waste of energy, because most of the light is getting absorbed by walls of a barrel, heating it up, which is not good - it lowers efficiency.

But suddenly I understood, that we occasionally invented a powerful and cheap switch for the coilguns! Two electrodes, distanced by a minimum air gap between them may act just as a thyristor: if we wrap a turn of wire over the coil, then connecting a little HV cap to that wire will temporarily rise voltage over the coil triggering the spark in an air junction turning it in a conduction state, just like a thyristor! Ideally, I think, the electrodes should be flat and wide, covered with palladium or something, encapsulated in a high-pressured neon-filled glass bulb, separated by some under-millimeter junction. This should ensure low voltage drop in a conductive state. Such device will be capable of handling huge currents and have small parasitic parameters, IMHO.

Smg similar can be done at home and replace expensive thyristors: two screws with sanded rounded tips can be used as electrodes, tightly screwed into some fiberglass with threaded hole. This way spark gap can be adjusted easily.

As for ETG, I would think of some not-perfectly conductive propellant with high temperatures of melting and evaporating. Carbon powder? Graphite pellets?
Triggering still can be done by air-sparker (but with minimum distance between electrodes!) - it will not waste much more energy than mechanical switch would.

Yandersen

Sun Feb 24 2013, 12:02PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Bingo! I got the whole air inside the barrel reacting! It was rather an intuitive solution than the result of thinking. So here is how it is done: thin plastic barrel; two copper rails along the barrel (from inside) as electrodes. And the whole barrel from outside winded over with a coil in series with one of the electrode. As a result I got the whole barrel filled with plasma during the discharge.
It even seems strange for me - why and how does it work? When I played with two rails as electrodes I expected to get a plasma-accelerating railgun. And yes, I got plasma blurring between those rails, but just a part of the distance was acting - my hope to see plasma ball flying out did not come true. So I made longer rails and put them inside plastic tube. But it worked even worse - I've seen flashes in random places in a tube every other shot. I got upset enough to start playing around with whatever was on my table. I sealed one end of the barrel with a piece of plastic glue and checked the blowing air with a hand - nah, it sucks. Then I mentioned a long coil seemed to be just like smg to insert the barrel into, so I did it and connected in place of inductor. And boom! I mentioned flame flash out of the barrel end! I put my hand in front and felt the same I felt from air guns shots - the same air blow. I put a piece of toilet paper on the end of barrel - it got ripped! I crumpled it and sticked inside the barrel and got it shot a meter away. So yeah, that setup deserves a cookie! I will work on it tomorrow - will wind a coil directly onto barrel. That seems promising...

Yandersen

Sun Feb 24 2013, 05:57PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Okay, seems like I (re)invented sonic cannon - the thing shoots compressed air which can be felt a meter away, blowing my ears also. The thing is actually plasma-accelerating railgun - the plasma works as a plunger pushing air out of the barrel. Here how the first model looks like:

As a railgun the thing has two rails, encapsulated inside the plastic tube (one end closed). Triggering is done by transformer, as previously (but this time I increased turn ratio to get higher voltage). Outside tube is wound around with the coil, just like a coilgun. What is the purpose of this coil I will tell later. It is not visible on picture as it is insulated over by a tape. The visible juicy wire is a magnetization coil - it increases the field accelerating ions forward (in addition to rails' field).
Without all those coils, plasma will hardly move: with two rails some of the ions will fly forward ionizing the whole air between the rails and by the time ionization wave will reach the end of rails current will cease, carried mostly by the back part of plasma mess with minimum resistance and energy dissipation, which is not what we need. So the purpose of the coilgun-like coil here is to shut down the current at the back of ionization wave: coil's field lines go along with the rails, so ions traveling forward has no trouble in forward movement, while those ions traveling from one electrode to another are forced to deflect vertically and circulate. In other words, the strange coil is a magnetic lens filtering out only forward-flying ions. Both coils are wound perpendicularly to each other so they do not interact.
Looks like plastic doesn't melt, everything cold, electrodes are fine so far...
Checked energy consumption - the whole 16J are used. The question is what for? As nothing got mentionable hot, probably the device emits light and sound only. Mention a little dark red flame thrown out - is it plasma from discharge or compressed to plasma temperatures air? Doesn't feel like any hot air coming out. Funny to see the hole got ripped in a paper laying upfront. :) Yeah, that is a funny toy - will scare the shit out of anybody so loud it is!

MrFlatox

Sun Feb 24 2013, 08:02PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Your creativity is limitless as we can see

A video of the last one ?

Yandersen

Sun Feb 24 2013, 09:25PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Later.

I tried long pipe (>30cm) without magnetizing coil and doubled energy - works worse. Either coil is uber necessary, or just that is wrong idea to make a long pipe.

I think this device just generates supersonic sound wave. Any use for this? Maybe at least it will be able to accelerate plastic balls from airsoft guns...

Just got to another explanation why coilgun-like coil is important: it prevents ions to hit barrel walls. The closer ion get to the wall and the higher it's speed, the higher the deflecting force turning it around. As a result I got cold plastic barrel sustained few dozen shots even though plasma flame is getting thrown out of the exit.
I also understood why the device generates sound wave ripping toilet paper apart but does not significantly propel any solid objects. The actual shot increases pressure inside the barrel so rapidly, that air coming out of the opened end goes with the supersonic speed. But as soon as current ceases, coils get deenergized and plasma gets in contact with barrel. So cooling happens rapidly also, and as result pressure inside the barrel drops down dramatically. This generates two fronts of air - high-pressured followed by low-pressured, which is a single sound wave with high amplitude. Thin paper oscillate together with wave in resonant and resulting vibration rips it over. The solid object experiences acceleration and decelleration resulting in nearlu zero average force, so it does not propel.
The solution I see is to increase chamber volume and decrease it's surface area - i.e. make it short and wide.

MrFlatox

Mon Feb 25 2013, 10:14AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

What about using higher energies ? like 10 times what you are curently using ?

Yandersen

Mon Feb 25 2013, 06:16PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

I think this setup will work just like you need with higher energies - the fact that mine only generates sound wave is because plasma cools down rapidly after expanding. Higher cap's capacitance will prolong the pulse - if projectile will leave the barrel by the time pulse is over, it will not suffer from sucking after-shot vacuum inside the tube.
The problem for high energies is that this setup require not-magnetic material for the pipe able to sustain high pressures. Fishing rode pipes seem to be perfect, I think.

Yandersen

Wed Feb 27 2013, 11:01PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

As promised, here is it, at last:

first previous 6 7 8 9 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob