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Theoretical improvements for ETG?

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Ash Small

Sat Mar 02 2013, 07:57AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I woke up this morning with an idea forming in my head.

I also found that Maxwell had posted this:

Maxwell wrote ...

Ash Small wrote ...

I've also been thinking, as E=1/2CV^2, does it make more sense to connect capacitors in series, rather than in parallel? (more voltage, but less charge, but energy is a factor of V^2)

You typically get more bang for your buck as you increase your voltage rating.... but the capacitance quickly drops off. In CG's, the higher voltage, lower capacitance, faster pulses. In these ETG's, if a longer pulse is beneficial, greater capacitance is needed.

Now, Yanderson has recently experimented with adding coils to ETG's to get bigger/longer sparks. How do we put all this together?

What we have is a capacitor, spark gap, and coil....where have I seen that before?....the primary circuit of an SGTC.

If you get the circuit to oscillate, you get a longer, high voltage spark, you can also use the coil to get the spark to spiral, or whatever, (Z-pinch) lengthening arc length. If the capacitor is charged to a sufficiently high voltage, the circuit should oscillate for quite a while. (long enough to significantly lengthen the pulse, anyway)

I think I need to build some more saltwater capacitors and experiment.

Any commemnts will be appreciated.

Yandersen

Sat Mar 02 2013, 11:18AM

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

Agreed, efficiency of air-operating ETG will be higher if ions will not be able to reach electrodes, so the period of voltage inversion should be shorter than the time it takes for ion to travel the distance between electrodes. The problem is that the speed of ions is different and mass too. And after all, the microwave does the same. :)

Let's think about the process of electric discharge through the ionized air. Once ion formed near one electrode, it goes toward another. Electric field accelerates it - that is what we need. But when ion hits another electrode, all it's kinetic energy either heats up electrode, vaporizing it, or breaks neutralized ion back. And this is unavoidable because we can't discharge cap faster than it takes ions to travel between electrodes.

So as for air discharge, I think it makes more sense to make a laser as ion recombination process is dominant here. If we concentrate on this goal, then air gap should be minimal, voltage is minimal, capacitance - maximum, pressure is maximum. With this setup we minimize increase of kinetic energy achieved by ion traveling and maximize the number of ions.

Ash Small

Sat Mar 02 2013, 12:47PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Suppose, for example, that tungsten electrodes are used, vapourization of electrodes will be minimal.

Suppose also, that a higher voltage is used, spark gap can be much bigger, increasing 'time of flight' of ions.

Suppose also, that spark oscillates several; times before dying away, more energy goes into the chamber, over a longer period.

Suppose, also, that spark length is increased using the coil to make the spark spiral.

Suppose, also, that water is added to the chamber.

If a higher voltage spark has more energy, but spark duration is too short, why not use bi-polar capacitors, and make the spark oscillate backwards and forwards a few times? (you already have all the components of a 'spark tank')

Yandersen

Sat Mar 02 2013, 01:32PM

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

In practice it doesn't! After the shot voltage reversed from 760 to -220V if inductor used, but without I got 30-70V Left.

Well, I have a solution if you got so interested. Chamber is a copper cylinder - first electrode. It has cylindrical magnet placed over that pipe. Inside the chamber as a central axis, goes another cylindrical electrode (like pipe inside the pipe). This one has stick magnet hidden inside. Both magnets repel each other, so flux lines go along the chamber through the space between two pipes. Any ions traveling between those pipes will be forced to circulate around the central electrode. As a result a spark will blur into ring.
Now assume central pipe is thicker in the one end. So first spark will happen here and blur into ring. As current will rise, the plasma ring will be forced to move along the chamber (like in a circular railgun) kinda pushing onto the air in front of it. As a result, moving plasma ring will disturb all the air inside the chamber (like a laser net in Resident Evil).

Ash Small

Sat Mar 02 2013, 06:03PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yandersen wrote ...

In practice it doesn't! After the shot voltage reversed from 760 to -220V if inductor used, but without I got 30-70V Left.

Well, I have a solution if you got so interested. Chamber is a copper cylinder - first electrode. It has cylindrical magnet placed over that pipe. Inside the chamber as a central axis, goes another cylindrical electrode (like pipe inside the pipe). This one has stick magnet hidden inside. Both magnets repel each other, so flux lines go along the chamber through the space between two pipes. Any ions traveling between those pipes will be forced to circulate around the central electrode. As a result a spark will blur into ring.
Now assume central pipe is thicker in the one end. So first spark will happen here and blur into ring. As current will rise, the plasma ring will be forced to move along the chamber (like in a circular railgun) kinda pushing onto the air in front of it. As a result, moving plasma ring will disturb all the air inside the chamber (like a laser net in Resident Evil).

I was thinking something more like a conventional ETG chamber, with the projectile at the chamber end of the barrel, with some water in the chamber.

The idea would be to concentrate the heat in the chamber, vapourizing the water, and superheating it.

I'm thinking several thousand volts, and not much capacitance, as energy is a function of V^2.

A smaller capacitance and inductance will give a fast oscillation period (higher frequency) in the spark tank.

I've obtained some huge sparks from 200nF saltwater capacitors charged with a DC flyback in the past, I think adding an inductor to increase spark length and to try to get it to oscillate must be worth a try. Adding some neodymiums might be worth trying as well (You should get a tighter helix that way).

I'm going to have to give this a try sometime.

Yandersen

Tue Mar 05 2013, 11:26AM

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

Some people on russian forums say that aluminum foil used in ETG works like a burning materrial - being heated and evaporized, Al reacts with oxygen inside the chamber producing the tremendous amount of heat that actually does the majority of the job of bullet propelling. In other words, the electrical energy required only for vaporization of aluminum - the rest is done by conventional combustion.

MrFlatox

Tue Mar 05 2013, 01:25PM

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

interesting.

So how someone on this forum succed with a air fueled ETG.

Ash Small

Tue Mar 05 2013, 01:30PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yes, aluminium is a common rocket fuel, powdered and mixed with a suitable oxidizer.

The main advantage of not using foil in an ETG is it's simpler to re-load. It's much simpler to inject some water than to connect another piece of foil between the electrodes.

I'm getting my new workshop sorted at the moment, and hope to get my lathe installed at Easter (if all goes to plan).

I do have a few other projects underway which are waiting until the workshop is fitted out, but I will try this when I get a chance.

I was obviously mistaken about energy being a function of voltage squared, but I think there may be some advantages to using higher voltage, (ie the main discharge will jump the gap without superimposing a high voltage on it), and, by using a coil and bi-polar capacitors, it could be made to oscillate to lengthen discharge time. Using a metal chamber will presumably mean the coil will need to be inside the chamber, which does pose quite a challenge, though. (Some form of ceramic coating may be the solution here, it could have several advantages, as both electrical and heat insulation). Current will be a lot less, which has some advantages (resistance will be less of an issue). The same amount of energy should give the same amount of heat.

If it doesn't work, I can just use the HV to start the discharge, and use lower voltage capacitors for the main discharge, as you have done.

I think it's worth more research, as it will make re-loading much simpler than using foil.

I'll give it some more thought in the meantime.

2Spoons

Tue Mar 05 2013, 09:31PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

No reason why you couldn't use a liquid fuel instead of Al or water. Methanol comes to mind - easy to get, easy to handle, only nasty if you are stupid enough to drink it. Easier to vaporize than water or aluminium (less energy needed). Plenty of other candidates too.

Yandersen

Tue Mar 05 2013, 09:39PM

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

-Honey, are you drunk?! You all smell like...
-Nommm, mum, I was j'ssst shootin' my eletramagun...

Well, Al foil, huh? 6.7 KJ per 1 gram of aluminum foil burned in oxigen - how about that "addition" to ETG "efficiency"?
Ethanol is 30KJ per 1g though. Methanol is slightly less and it is toxic.

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