Small ion trap

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Carl Willis
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Small ion trap

Post by Carl Willis » Mon Jan 26, 2009 6:10 am

This weekend I swapped around a bunch of Conflat hardware on my ion source "test stand" and removed the graphite target. Toward the goal of making a small fusor-like electrostatic ion trap, I mounted a copper loop electrode in the center of the CF cube with axis aligned to the ion beam. Across the chamber from the ion source is a simple passive magnetic electron trap, made out of 19mm Pyrex for heightened visibility. My thinking is that ions from the source, having entered this region to decelerate and turn around, will produce some electrons that should be put to work causing as much local ionization as possible.

The photos show operation with deuterium at a background pressure of around 0.1 mtorr, and a potential of -30 kV on the loop electrode. The ion source extraction voltage is -4 kV, RF power supplied is ~50W.

Neutron production is apparent at 15 kV, and seems to be around 10,000 / sec. at 30 kV according to a sensitive He-3 detector. Not particularly interesting from a neutron perspective just yet. The feedthrough on this is good to at least 60 kV, and I WILL be pushing it up there.

-Carl
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Mike Beauford
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Re: Small ion trap

Post by Mike Beauford » Mon Jan 26, 2009 6:32 am

As always, nice work Carl.

Mike
Mike Beauford

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Steven Sesselmann
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Re: Small ion trap

Post by Steven Sesselmann » Mon Jan 26, 2009 10:27 am

Carl,

Thanks for posting these nice pictures of your latest experiment.

Interesting idea with the magnets and the pyrex, somehow I think you should have a grounded reflector somewhere at the passive end. I had some success with an inverted cone reflector (concave side towards beam).

Are X-rays a problem for you yet?

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Chris Bradley
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Re: Small ion trap

Post by Chris Bradley » Mon Jan 26, 2009 10:40 am

Fine experimental skill on display to your ususal high standard, Carl.

Can you explain a little more on what you are aiming to do? I'm unsure why generating electrons just where the ions slow down and turn around is a good thing. Surely you're just encouraging neutralisation, rather than promoting ionisation? Plus, the ions seem to be diverging directly towards the grounded shell, the population likely to make a clean transit across the device would seem to be on the low side to me.

If ions do make it across to the magnetic region, what e-fields would they experience in this magnetic field region, it seems to be on the outside of the ground shell? Also, if the efield there is small and their velocities have backed off then their own small gyroradius may keep them there and/or cause them to head off to the ground shell transversely, exB style.

I resist saying it, but it looks to me like you have a 'single pass' system operating here, displayed by the asymmetry of the ion beam either side of the electrode. If so, your neutron numbers are excellent and reworking it to get a higher popluation of ions reciprocating could prove to be 'miraculous', in neutron rate terms!

best regards,

Chris MB.

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Re: Small ion trap

Post by Wilfried Heil » Mon Jan 26, 2009 1:31 pm

Carl,

how much fusion will you get when you turn the ion source off?

The way it looks, you have one beam from the ion source on the left which passes through the chamber once. You would have to find a way to capture these ions, as they are off axis and too fast by ~4keV to oscillate in the hollow cathode discharge. So they end up on the glass wall of the tube on the right.

In addition, there appears to be a small bunch of ions recirculating through the hollow cathode, as they would normally do in a fusor.

It would be interesting to know what the relative contributions to fusion of these two beams are.

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Richard Hull
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Re: Small ion trap

Post by Richard Hull » Mon Jan 26, 2009 2:38 pm

Great images, Carl! Nice effort to increase ion production.

I am trying to figure out the system you show. It looks as if the ions are created on the left and move through a central ring in the viewing chamber and then go off to the right into what appears to be a closed off "test tube" glass end at the extreme right. I assume the central ring is a form of extractor or the accelerating "target electrode" of some sort.

I'm just trying to get a grip on things. Sounds as if others are too.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Carl Willis
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Re: Small ion trap

Post by Carl Willis » Mon Jan 26, 2009 6:21 pm

Hi Richard,

I attached a schematic drawing that hopefully will clear up how this is configured.

The ion source itself was previously described here:
viewtopic.php?f=12&t=5008#p32316

What is meant by "extraction voltage" is the voltage on the ion source puller electrode, in this case -4 kV. The ion source anode is grounded. The central ring electrode could be interpreted as another extraction electrode with a higher potential than the first (-30 kV).

Steven: Yes, there are plenty of low-energy x-rays. Most come from the end of the glass tube on the "passive" side, which can be seen glowing green from electron bombardment. Electrons born close to the central electrode are not easily arrested by the magnetic field that surrounds the glass tube and most appear to just sail on towards the tube end.

Chris: I'm trying to exploit the ionization that occurs in the turnaround region to the right of the chamber to produce more ionization there, by magnetically confining low energy electrons born in that region with the magnet. If more ions are made there, they will be accelerated toward the central electrode.

Wilfried: When the ion source is turned off, or equivalently when its extraction voltage is turned off, the luminous beam disappears and so too do the neutron counts. The pressure can be raised into the 20 millitorr range and a hollow-cathode discharge will form in the chamber with or without ion source assistance. It is too unstable to make a meaningful check for neutrons, but there probably are a few. I agree with the observation that there is mostly one beam in evidence here, passing from left to right. I disagree that the beam is too fast by the extraction potential when it reaches the opposite wall--the extraction is done with a puller electrode that is negatively charged with respect to ground. While REBOUNDING ions coming back right-to-left may well run into the exposed end of the puller electrode and its insulator (a problem I intend to remedy with a little grounded shield), they will not be so fast on the first pass to hit the wall. The ion source was specifically designed, in its use of a grounded anode, to avoid this issue. What I think you might interpret as a small bit of recirculation is probably an electron beam. That little thread of enhanced glow disappears at lower pressures.

Thanks for your comments.

-Carl
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Re: Small ion trap

Post by DaveC » Mon Jan 26, 2009 7:09 pm

Carl -

Let me add my appreciation for this interesting work, to the others.

Re: To Steven's comment about the extraction voltage.... the amount of extraction voltage one needs depends on the size of the opening in the ion generator. The smaller that opening the more extraction potential is necessary to get the ions out into the main acceleration region.

With simulations, you can see that the 30 kV potential will not reach very far into the ion generator through a small opening. The extraction voltage could be lower with a larger opening, but I would expect that to affect the RF tuning of the chamber. (I am presuming the ion generator is working as a resonant cavity...).

FWIW...

Dave Cooper

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Chris Bradley
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Re: Small ion trap

Post by Chris Bradley » Mon Jan 26, 2009 7:22 pm

Carl Willis wrote:
> I'm trying to exploit the ionization that occurs in the turnaround region to the right of the chamber to produce more ionization there, by magnetically confining low energy electrons born in that region with the magnet. If more ions are made there, they will be accelerated toward the central electrode.

I'm afraid I don't follow this logic. *If* you form an ion with these low energy electrons (and I don't see why that is a strong line to follow - what is the ratio of ions from the ion source that will recombine, compared with those that will ionise afresh?) then for them to head off towards the centre would mean that they leave behind an unpaired electron. But that unpaired electron (and others, in bulk) form their own field and pull the ion back.

I would go with Steven's interpretation, some form of reflection grid extending beyond that magnetic region, maybe at earth potential but I think preferably at a positive bias above might be better because you would want the e-field to be consistently directed *through* that electron region and on to the central electrode. I don't think you'll get this configuration with only earth potentials beyond the 'passive ionisation region' as you are intentionally trying to get ions to flood inwards, not to hang around that ionisation region.

At the moment, those electrons have a small glimpse of the inner electrode and try to get away from it, heading off to the end of the tube (albeit under the damping effects of the mag field) towards the 'universal earth potential' that pervades the space around it. They ionise the end of the tube creating a negative charge region that pulls more ions out (the 'wrong way') and/or contaminant ions back in.

I would be inclined to focus on repeating the fusor configuration first - a contiguous inward-directed e-field in all the free spaces of the vessel. The magnetic field may potentially help reduce electron conductance losses so I think that is fine as a single delta. But I think you're changing too many variables here from the fusor configuration. I am not suggesting you are following the wrong line here, just that I don't see it. It doesn't look right with that odd region sitting beyond the earthed outer shell, but only because I can't get an intuitive feel for what is going on there.

Nonetheless, let me not detract from my enjoyment of seeing new experimentation being posted up. I have hope, and little doubt, that you will iterate and will create success from it.

best regards,

Chris MB.

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Re: Small ion trap

Post by Wilfried Heil » Mon Jan 26, 2009 7:59 pm

Carl, thanks for the explanation. Maybe you are capturing some of the ions in the central beam.
You could use a NIB magnet and see how that beam is deflected, to find out what it is.

Anyway, another nice setup. Let us learn what it can do.

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