Shorting from sputtering: No good outcome for 'electric fusion'.

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Chris Bradley
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Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Chris Bradley » Wed Mar 24, 2010 11:46 pm

I've done no work on my gadget for the last month or two. A couple of the electrode supports became sputtered-on by, presumably, copper and this was a hurdle I haven't been able to attend to as other things needed attention.

I will endeavour to rerun, in due course, with some of the materials Doug has very helpfully pointed at with the help of the research papers he has collected.

However - I am running uA and yet all this metallic 'stuff' is floating around in there. I am also pretty sure that most all of us have faced conductance across feedthroughs, &c., after some given time of operation.

Which brings me to a sad contemplation; that no good [net power wise] will ever come from using electric fields to accelerate fusible ions, for the simple reason that a 'working' reactor will run dozens of amps of ion beam currents to get any pay-back, but I just can't see how such a reactor will ever avoid heavily sputtered and shorting feedthroughs.

...is there a solution? Can you, for example, shield a feedthrough from sputtering particles (inc. particles with MeV energy, viz. fusion products) by some clever magnetic/electric structures?

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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Dustinit » Thu Mar 25, 2010 2:39 am

Just place it in an sacrificial insulator "shadow" ie an insulator in front of your insulator.

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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by John Futter » Thu Mar 25, 2010 5:16 am

Chris
all you have to do is shield the insulators with low sputtering cups one at each end of the insulator.
arranged so that the insulator is shielded with a gap of say 2-3mm of the cup wall from the insulator and the cup extends up the insulator about one third -- the same from the top of the insulator so only the centre third of the insulator gets sputtered.

I've got some here that camr out of a 1000 l/s ion pump and they ran for 20 years using up around 1.5kg of Ti so plenty of sputtered Ti around and the insulators were still good @10kV

can't find the digital camera at the moment but I'll take pics when I find it and reply to this post. Scaling the cups and insulators will take you to ant voltage you like. The same principle is used in our acceleration tubes @ work on the accelerators the oldest tubes being 1958 vintage and the younger machine 1967 --- kg of ions have flowed down these over the years.

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Chris Bradley
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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Chris Bradley » Thu Mar 25, 2010 8:05 am

I can see how you can stop the sputtering with shadowing from the centre of a fast ion source, but the issue I appear to be suffering with is that the-then sputtered atoms from metal parts that have to be exposed are roaming the chamber as neutral vapour looking for somewhere to settle.

I can't say that this is the exact mechanism I'm looking at, but the structures [which are PEEK, and are not in any immediately apparent direct line-of-ions] seem to have developed a very fine surface layer of 'stuff'.

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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by John Futter » Thu Mar 25, 2010 8:30 am

Chris
the electrodes do not have to be in direct line of sight --if they are supporting a structure of the opposite polarity then they will attract ions if they are negatively charged --the reverse of course if they are negative ions ---remember no insulator is perfect so consider them a conductor for attraction -- then with the more ions attracted, the surface resistivity falls from G ohms to M ohms and the process speeds up.

start from fundamentals if the insulator is perfect it has a dielectric constant from 1 to whatever so charge builds up the ions want to cancel this charge --so impossibly thin support insulators are the best ----but then you have to take into account field emission from sharp edges --the gotchas are everywhere---- a reason why semiconductor process equip is so expensive ---people have sweated blood minimising these probs
FWIW

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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Linda Haile » Thu Mar 25, 2010 6:34 pm

Try aluminium instead of copper Chris. It is much more difficult to sputter in the first place. Copper sputters very easily.

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Chris Bradley
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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Chris Bradley » Thu Mar 25, 2010 6:41 pm

I'm sure I can try all manner of things and get improvements.... and I shall...

My point was that I was just pondering the wider issue (with a little bit of my own experimental observation thrown in).

It seems to me that what will really need doing for useful fusion power is to have a fully closed container whose interior surface can be sputtered ad-infinitum without electrical effect, which tends to infer no feedthroughs for electrode driving. But it also seems to exclude the current arrangements of tokamak, given *they* need feedthroughs aswell even though they aren't electric fusion devices (for RF heating/control elements).

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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by DaveC » Fri Mar 26, 2010 12:03 am

Chris -

If we are considering REAL power production from fusion, as in MegaWatts or GigaWatts electrical, then there are a "few" other issues that loom very large on the reality horizon.


There's actually no reason whatsoever to scale the present fusors to large sizes, since the efficiency is sooooo low. If and when something can be done to improve this, then these other next tier issues will become important.

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Chris Bradley
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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Chris Bradley » Fri Mar 26, 2010 8:42 am

Dave Cooper wrote:
> If we are considering REAL power production from fusion, as in MegaWatts or GigaWatts electrical, then there are a "few" other issues that loom very large on the reality horizon.

Of course. And I am exploring what those issues are, rather than just rolling over and thinking the way I recall Homer Simpson saying to Bart ["Son, if a job's too difficult, then it's not worth doing."!]

Thing is, it has got me thinking that the current tokamak approach (or similar) that run RF power into the chamber may suffer similarly once they actually start running MW. Sure, there are other problems to be dealt with, but if you are running a project and you can see 10 years of work and a show-stopper, why wouldn't you deal with the show-stopper first?

If you come to recognise that there are mitigations for the show-stopper, then you can set that running as another activity and 'worry less' about it.

My view is that fusion power is totally essential for the future but I see very few folks really cogitating over it effectively. We have the 'main-stream' who are hell-for-leather going for a big plasma and to hell with whether it is practical, to the other end of the equation like Mr Lerner who are so totally focussed on selling the practicalities of the thing that they might be getting over-optimistic over the limitations of the physics. There are the mainstream who think you can nibble away at it slowly and it'll eat the problem up. There are those that think some new approach might do it in one single go, all problems solved. Both *might* get you to a solution, but surely the best way has to be by looking at each possible solution and then lining up your ducks on that particular shooting range before getting to work on it? If you're a few ducks short of a row, why start shooting (with a 50 year, 30 billion effort)? I say; get to know what the ducks look like and how many there are, before going for a 100% score....

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Chris Bradley
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Re: Shorting from sputtering: No good outcome for 'electric fusion'.

Post by Chris Bradley » Fri Mar 26, 2010 8:45 am

Thanks for the food-for-thought, John.

I have [aimed to] design mine so that the feedthroughs are fully screened right up to the magnetic head (I use kapton over silver wire) but of course they must emerge "somewhere". It is around the electrodes that this phenomena is going on.

I have a few ideas now, will have to go play some more....

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