spark plugs?

[mchargue]

Here’s a book on using LASERs to ionize gases. Something that would lend itself to designing a LASER-based spark-gap switch.

Theory of laser-induced gas ionization
http://www.springerlink.com/content/w85365618m238vlj/fulltext.pdf

Pat

[Breakable]

Before someone starts playing with lasers, could someone please explain to me what is the problem with Ignitrons?
http://en.wikipedia.org/wiki/Ignitron

[Henning]

Breakable wrote: Before someone starts playing with lasers, could someone please explain to me what is the problem with Ignitrons?
http://en.wikipedia.org/wiki/Ignitron

I think the difference here is, that you need to vaporize the mercury first before the current can flow. So it’s much slower, or you cannot orchestrate all ignitrons to switch simultanously enough.

[Breakable]

I wish there were some numbers.
Ok lets say vaporization is slow,
so maybe we can ionize gas some simpler way instead of using lasers?

[mchargue]

Henning wrote:

Before someone starts playing with lasers, could someone please explain to me what is the problem with Ignitrons?
http://en.wikipedia.org/wiki/Ignitron

I think the difference here is, that you need to vaporize the mercury first before the current can flow. So it’s much slower, or you cannot orchestrate all ignitrons to switch simultanously enough.

I agree with Henning on this.

I see a problem with speed, and the synchronization of multiple switches. The process used – a puff of vaporized mercury – is, I think, slower, and would be harder to coordinate among multiple switches. You can imagine this puff travelling inside the envelope of the switches to close the switch. It may be that travel time is hard to characterize switch-to-switch, and shot-to-shot

Ionizing gases, on the other hand, assumes no travel-time for the conductive media – just the time it takes to flash the gas into a conductive plasma along the LASER path. Something that I would think is easier to coordinate among multiple switches, and between shots.

Based on the recommendations that have been made, I have to think that this has been brought up in discussions within LPP. I’d appreciate it if someone from LPP could comment on suggestions that have been made. I’d like to hear the reasons for/against the approaches that have been suggested. (though I suspect it will come down to cost, simplicity, and unexpected complications with the switches)

And finally, there’s nothing wrong with playing with LASERs!

Pat

[mchargue]

Breakable wrote: I wish there were some numbers.
Ok lets say vaporization is slow,
so maybe we can ionize gas some simpler way instead of using lasers?

I think work has been done using plasma jets to close switches, and also microwave-generated plasmas to close switches. Based on my understanding of these processes, I considered them inferior to a LASER-mediated plasma switch. (though microwaves might work out, but I am not sure about the excitation frequencies that would have to be used)

Based on my understanding, you’d like a working gas with a low atomic weight, (H, or He) used at a high-pressure with a large distance between switch poles. I’m not sure that you can excite He with a microwave signal. Also, can microwaves be focussed along a line int he working gas? If not, I think that an ‘area-effect’ is going to be less consistent switch-to-switch, and shot-to-shot, than a consistent plasma ‘line’ drawn between the switch poles.

Pat

[Breakable]

mchargue wrote:
And finally, there’s nothing wrong with playing with LASERs!

http://www.mathematicianspictures.com/Images/375w_MATH_PG3020_WLAB_9000X.jpg
😛

[mchargue]

Breakable wrote:

And finally, there’s nothing wrong with playing with LASERs!

http://www.mathematicianspictures.com/Images/375w_MATH_PG3020_WLAB_9000X.jpg
😛

Oh, I see. (or at least I used to)

Pat

[zapkitty]

mchargue wrote:

And finally, there’s nothing wrong with playing with LASERs!

http://www.mathematicianspictures.com/Images/375w_MATH_PG3020_WLAB_9000X.jpg
😛

Oh, I see. (or at least I used to)

Pat

Actually, that’s what happened to me…:)

Of course the alternative was losing all remaining fragments of vision, so… diabetes takes no prisoners.

[mchargue]

zapkitty wrote:

And finally, there’s nothing wrong with playing with LASERs!

http://www.mathematicianspictures.com/Images/375w_MATH_PG3020_WLAB_9000X.jpg
😛

Oh, I see. (or at least I used to)

Pat

Actually, that’s what happened to me…:)

Of course the alternative was losing all remaining fragments of vision, so… diabetes takes no prisoners.

Sorry to hear that, Zap. Glad you’re here. Good moniker, BTW.
Pat

[DerekShannon]

This is a great discussion about future spark plug options, but it looks like there is a very real way some technically inclined folks could help out right now. Murali has been wanting to resolve thermal and electrical breakdown issues of our evolving spark plug designs with finite element analysis in COMSOL or similar, but there just hasn’t been time. So rather than continuing to put it off and ending up with a “make and break” approach, we’d like to reach out and see if we could put together a team of volunteers that could work together with Murali for spark plug simulations, similar to the team supporting plasmoid simulations. After some feedback here we’ll post a dedicated thread or other official call for volunteers–Fusion wants *you*!

[Rezwan]

Any particular reason you want to wait for feedback here? We can post it fresh right now. Fresh topic, also website post. And tweet.

[DerekShannon]

Yeah, cause I was gonna wait and ask you to do it! ;-P

[Rezwan]

😆 Great demonstration of delegation by example.

You got it. Folks, to join the sparkplug simulation team, switch to this thread.

To keep talking about future spark plug options, remain here.

[mchargue]

Folks;

Initially, I had made the suggestion of a LASER-mediated spark-gap switch in order that the experiments could continue. From reports, it seemed like the current implementation was a gating item to experimental progress. That said, if you can’t use it now, I hope that it helps in any future design.

FWIW, I’ll start looking at alternatives that would dove-tail well with your current design.

A few questions, then…

As I understand the design, you’re using a spark-plug that has had it’s negative return arm removed, and only the center electrode is used. In addition to having one pole of the capacitor bank connected to the center electrode of the spark-plug, you have also wired an additional, higher-voltage, initiator line to the center terminal. (through a diode of some kind) The other pole of the capacitor bank, and the return for the higher-voltage initiator, are connected to the second terminal of the spark-gap (not the spark-plug) through the FF chamber.

When the capacitor bank has charged to an appropriate level, a higher-voltage pulse is passed across the spark-gap to initiate a conductive plasma that then allows the capacitor bank to discharge through the plasma, and FF.

The problems that you’re having include: insufficient heat dissipation, (melting terminals) insulator breakage, (from heat expansion) and terminal wear. (too high current density)

How about…

If you’re having difficulty with the ceramic/plastic/peek/whatever insulator breaking down, why not dispense with the insulator entirely? As you’re not attaching both poles of the capacitor bank to the same spark-plug, the the insulator serves no purpose.

If that can be done, you could replace the spark-plug with just a metal rod. (threaded, maybe?) The metal rod would be the (former) center electrode of the (erstwhile) spark-plug. All other connections would stay the same.

Using this, you’d be able to machine a rod from raw stock with a larger diameter that should have better characteristics: lower current density; less thermal expansion; better heat dissipation; (especially if the rods are threaded into a solid, non-conductive plate) and no more insulator to break down.

If you like, you could thread the rod – all along the rod, or only along a portion – to mate with the threads that already accept the current spark-plugs. Another nut over the top of the rod with a compliant gasket between the nut and the shoulder of the threaded bore may be enough to insure a gas-tight seal. (or plumber’s tape?)

Now you can generate electrodes from raw stock, and a lathe. The machining should be simple, and you can play with the electrode tip design to help insure it’s life-time, and shot-to-shot characteristics. Using the threads along the electrode, you can control the gap by winding the rod in & out, and better thermal characteristics will get you more life-time after you tune the gaps.

The only problem I can see with this is that I didn’t understand your original design.

How’s that sound?

Other ideas include using more switches, and rotating them between shots to minimize thermal breakdown. That means, 12 switches fire 100/sec, or two banks of 12 fire at 50/sec, or 4 banks at 25/sec… You only need to use different initiators to control which set of switches is used. Assuming that you can place all the positive electrodes on a single, non-conductive plate, you need only drill/thread more holes, and set more rods in them.

Other improvements might include a better shaped initiator pulse. I would think that something as close to a square-wave is what you need, It may be that running the transformer that provides the initiator pulse from it’s own spark-gap switch may provide a better current pulse to the transformer than an electronic version. (you may be doing this already)

Anyway, there’s some thoughts.

Pat

[Author]

Posts