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1kJ Electrothermal gun (ETG)

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Yandersen

Wed Feb 20 2013, 08:49PM

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

Well, cap? Isn't it what you are using right now? I mean, charge it from the HV source, then discharge it to the primary of your trans? Keep in mind that turn ratio between secondary and primary determine not only voltage ratio, but a current ratio also. Add more turns to the primary. Try 1:1 ratio. Actually, 30kV should penetrate >1cm of air (Wiki says 3kV per mm of air). What is the length between electrodes? Say, 5mm, so you need 15kV. Then make 2 times more turns on primary then on secondary. The benefit of ferrite core is that you can distance primary and secondary on different locations of the core and still have a good coupling factor.

About an extra inductance in series. Remember you reported some voltage leftovers? Inductor will adjust voltage level over the gap to keep current running until it cease to 0. I did some test with 600V discharge and a big coil (few mH of inductance) distancing electrodes by 0.5mm and dropping water on the junction to trigger the spark - I got 0V left after discharges (did a dozen of booms all with the same result).
Hope today I will be able to fix my blown converter to make it run at 800V and try to put couple of turns over the inductor and discharge a little cap to it - hope it will trigger the spark in a dry air gap.

I think, ETG is actually a gun of the future, as it CAN potentially do more than a conventional gunpowdered weapon. The ultimate reason is a temperature of propellant which limits the speed of a bullet - plasma can be heated to any temperature and dozen times higher than a gunpowder is not a problem - so the speed limit of projectile is higher too. The only problem I see is an electrodes lifetime and reloading. First one can be kinda solved by inductor (current limiting), the second one can be solved by Klugesmith ideas. :)

MrFlatox

Wed Feb 20 2013, 10:47PM

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

Klugesmith ?? What does that even mean ?

Ah, I did mention that the capacitor on the primary side is a homemade one, and it has crappy electrical properties, such as it does not keep its charge when power supply is removed.

In the actual ETG, the gap between the two electrodes is only about 1mm, so I have a wide choice for the transformer ratio and powering method, I think they will all work.

I am definitely tempted to try your current limiting inductor method, but I only have small ferrite toroids from computer power supply chokes, I doubt it would be enough.

Yandersen

Wed Feb 20 2013, 10:53PM

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

Haha! Succeed! Both fixing the charger and trying the air triggering (for <1mm air gap, 768V on main cap (17.6J) and the same applied on primary by 0.1uF little cap). I put 4 turns over the inductor (90 turns). Cap discharges to 0V (maybe it is ready to go negative polarity, but I have antiparallel diode to prevent this). Interesting thing I found: the shorter the gap, the less mentionable the spark is. The reason, as I think, is that the longer the plasma line, the more power it consumes to heat up itself to the temperature of appropriate conductance level. In other words, the bigger the distance between electrodes, the more plasma is involved in heat dissipation. So spark gap must be maximized.

WOW!!! I sticked a piece of paper inbetween electrodes - the blow was so loud that shocked me! Paper separated on two layers (even though it is a regular printer paper!).
So here is what I think: just a clean air becomes highly conductive shortly, so it makes sense to introduce some material inbetween electrodes as an interrupter of a spark.

Repeating paper/no_paper many times I mentioned that in the paper-blow case many shiny bubbles are jumping out of the discharge area. One is died on the table and I was able to see what left: a little metal-gloss ball. So those jumping drops are actually melted electrodes' parts. With clean air discharge there were no jumpers - only flash and a pop sound. With paper, blow was loud and electrodes partially melted. What I think is that electrodes should be surrounded by an air and actual heat dissipation should somehow happen away from them. Maybe with long discharge path it will be so...

Bad idea comes to my mind: if plasma's resistance lowers with the temperature and does not start to rise at any point, then air is not a good propellant at all...

Yandersen

Thu Feb 21 2013, 03:52AM

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

Another negative thought about using an air propellant... When air turns into plasma, the conductive channel is very thin, so volume change is neglectable and the major result of this expand is just a sound wave. In controversy, using an aluminum foil results in evaporating a solid matter, so the difference in volume is much higher. To prove this I put a little piece of paper on top of a gap between electrodes. 17J were discharged, spark was just under the paper, but it did not moved at all. Well, maybe without an inductor result would be different, but few hundreds of Amps were running through that gap surely. Looks like the major energy emission of an air spark is a light, actually. Anyway, say we got a plasmic arc building up. Why should it expand at all? As it consists of ions, those travel along the discharge line only. Yeah, they collide with each other, increasing the number of free charge carriers, but all of them are still travel the same path! In addition, the higher the current the higher the magnetic field around the path. As neutral air molecules and atoms are distracted by mag field, those are got pushed away from the ionic mess. As a result the arc isolates from the air, and consists of ions only. That means that all the energy dissipation is due to light emission only (electrons accepted/detached) - now the only places of heat dissipation are the tips of electrodes which act as a targets of ion shooting.
So IMHO, air propellant has no use for ETG. Sorry, MrFlatox, but do not expect anything good from this idea... Howhether, air gap may act just as triggering mechanism, while Al foil could still be used as a propellant being wrapped around one of the electrodes.

This thinking bring me to an another idea... What if voltage across electrodes inverts with a very high frequency? Then all ions inbetween will oscillate and will not be able to reach electrodes at all. This should result in a growing plasma ball which should expand according to the temperature rise - just what we need! Hmm... Am I talking about microwave?.. +_+

P.S.: Maximizing the length of an air gap seems to be the only way to shoot well with an air propellant.

MrFlatox

Thu Feb 21 2013, 09:32AM

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

Interesting thoughts.

Now that the thing is partially built, I will however finish it, and see what results I could get. Maybe you are not using enough energy ? I truly see your point in there, but I will give it a try anyway. If the air propellant does not work, I will still be able to add a small piece of Al paper like the old design, but now results must be better because there is less dead volume, and a greater barrel lenght in this design.

First testing fires this afternoon.

Steve Conner

Thu Feb 21 2013, 11:00AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yandersen, you are right, a good ETG is basically a steam engine. :)

Ash Small

Thu Feb 21 2013, 11:06AM

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

I think I have to partially disagree with you, Yan.

If you compare this to an arc welder, a lot of heat is produced (many, many thousands of degrees C). You could argue that with an arc welder, the electrode is vapourized, but in TIG welding, the electrode doesn't even melt, yet very high temperatures are achieved with only a few tens of amps (sufficient to melt the workpiece).

More amps will result in more heat, and expansion of the air. Also, you don't want the 'dead space' to be too small, you want the energy to go into expanding the air in the dead space, not to heat up the chamber walls instead.

I also think an ETG could be much more efficient than a CG, as the efficiency of a CG is limited by saturation of the projectile. You could dump huge amounts of power into an ETG.

Steve Conner

Thu Feb 21 2013, 11:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The efficiency of an ETG is limited by the Second Law of thermodynamics since it is a heat engine. It converts electricity to heat, and heat back into mechanical work.

However, a coilgun is an electric motor, it converts electricity directly to work, so in theory it could be made 100% efficient.

In practice the ETG will probably work better, but in theory it is wrong to confuse efficiency with power output.

Ash Small

Thu Feb 21 2013, 12:03PM

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

Steve Conner wrote ...

The efficiency of an ETG is limited by the Second Law of thermodynamics since it is a heat engine. It converts electricity to heat, and heat back into mechanical work.

However, a coilgun is an electric motor, it converts electricity directly to work, so in theory it could be made 100% efficient.

In practice the ETG will probably work better, but in theory it is wrong to confuse efficiency with power output.

OK, Steve, I guess I'll have to re-phrase the efficiency bit. For a given projectile size, the amount of energy you can efficiently use in a CG is limited by saturation, whereas this is not the case in an ETG.

(EDIT: In practice, all electric motors have losses.

)

Yandersen

Thu Feb 21 2013, 09:55PM

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

I was thinking about air-discharge: why increasing the gap between electrodes gave me higher boom? I come to the following explanation: ions are accelerated by electric field between electrodes, so the bigger the distance between them, the higher the speed ions achieve before hitting the target electrode. And this is actually bad. The desired effect is to hit neutral molecules, not electrodes.

So what is the desired operation of air-propellant operated ETG? We have an air inside the chamber, we got energy inside the cap, and what we want is to use that energy for heating up all the air inside the chamber. In other words, we need to increase the speed of air molecules inside the chamber (preferably all of them). In order to do that we ionize them and accelerate by electric field.
Ionization is a process that consumes energy - cathode takes electrons from air molecules and negatively charged particles, anode gives electrons to positively charged particles and neutral atoms (light is emitted as result) and this process leads to voltage decrease over cap - this is purely a waste of energy, but this is unavoidable unless we put electrodes inside a prepared plasma. Still, it is desirable to minimize the number of ions reaching the electrodes as actual energy consumed for what we need is used when ions are accelerated between electrodes. During the trip from one electrode to another, ions collide with each other and neutral atoms, thus distributing achieved kinetic energy. The number of such collisions must be maximized in order to make more mess inside the chamber with less ions meeting the electrodes.
So the question is: how to maximize the trip length of accelerated ions to prolong their collision adventures inside the chamber? Just distancing electrodes and increasing triggering pulse voltage is good, but still we got a thin plasma wire just between the electrodes. So I'm thinking about magnetic field, with flux lines perpendicular to the spark line - this should force the arc to bend increasing it's length.
The actual setup I have in mind is a few wide turns of wire in series with electrodes. Spark line should be inside this "coil" and be aligned perpendicular to it's turns. As I think, as current will rise, magnetic field will rise too, forcing the discharge arc to bend more and more increasing it's length. If this will work, this may stabilize the current, and increase the amount of air involved in a mess. And I hope my dream about plasma gun will come true - if discharge arc will bend too high, it may form spiral knots, and if those happen to close, it may form closed loop of circulating ions which is ball lighting, actually. I'll keep my fingers crossed. :)

BTW, I made my own plasma boomer (but with just 17J of energy) - I took a little piece of copper tube (first electrode), sticked rubber cylinder in one end penetrated it with a screw right in the center (second electrode). The gap between round screw head and the tube walls is around 2mm. Shoots incredibly loud - sound wave percievable by hand half meter away. Looking from the side I can see some plasma blown out, but if paper sticked inside, it just slightly falls out when shot. Anyway, I tried to hold neodym around the exit - it has no effect at all. Obviously, as outer plasma is electrically neutral in average, and current doesn't run here.

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