Small ion trap

[Carl Willis]

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

[Mike Beauford]

As always, nice work Carl.

Mike

[Steven Sesselmann]

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

[Chris Bradley]

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.

[Wilfried Heil]

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.

[Richard Hull]

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

[Carl Willis]

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

[DaveC]

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

[Chris Bradley]

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.

[Wilfried Heil]

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.

[Richard Hull]

Carl,

What you had is what I thought you had. You have the standard biased extractor back at the gun, (on left). The ring in the viewing area is a target/accelerator electrode. You do have the sealed off tube as in the original work system you provided.

I wonder if the fusion you got is part from simple gas collisions in the right side gas leg coupled with some buried deuterium target fusion in the ring?

I especially like the idea of the turnaround magnetic leg on the right where you hope to recreate ionization in the gas for another pass at the central electrode. Nifty ideas.

Interesting investigatory setup.

Richard Hull

[Dustinit]

Very interesting Carl.
Looks like it may lend itself to a second ion source on the RHS?
Do you think you would benefit by having another electrode after the extraction electrode to provide some beam focussing?
Great to see some innovation and different ideas put into practice.
Dustin.

[Carl Willis]

Hi Chris,

If you're a critic of my explanation of the "passive" source on the right, I would refer you to this article on Steve Hansen's Belljar site (in particular the diagram on Page 6 and the citation of the Gow-Ruby article in Rev. Sci. Instruments) in order to demonstrate that this arrangement is an established practice:

http://www.belljar.net/634neut.pdf

Think of the passive source as an additional independent ion source like that in the above article. It will work by itself as an ion source to some degree (much inferior in relation to the opposite RF source), but it will certainly work better when its active crossed-field zone is supplied with some "seed" ions from the other source and that is what I am doing.

>they leave behind an unpaired electron. But that unpaired electron (and others, in bulk) form their own field and pull the ion back.

Think about this for a second. An ion and an electron are formed in a region that lies somewhere between a cathode (the ring electrode) and an anode (chamber and rest of the world). Where does the electron go? Does it stick around and contribute to space charge? Maybe a little bit as it curves around in any magnetic fields that may be present, but ultimately it runs into the wall because of the electric field. Measurable electric current flows in the HV power supply because of this electron and because of the ion that went the other way. Magnetic fields make electrons travel in more circuitous paths, increasing their ionizing efficiency (ions formed per electron removed by the electric field). Other than that aspect, this whole concept is nothing more in principle than a simple discharge tube.

>But I think you're changing too many variables here from the fusor configuration.

Fine, but this is a different animal altogether. In my own mind, the change in variables is from the solid graphite cathode that I used in this setup before, to the hollow cathode that is there now.

-Carl

[Carl Willis]

Hi Wilfried,

>You could use a NIB magnet and see how that beam is deflected

This is indeed how I know that central thread is due to electrons. The ion beam appears imperturbable by the magnets I have around. The thread however is "malleable" under the influence of a strong magnet waved around the sides of the apparatus.

I do hope I can accomplish some meaningful things with this test stand. Next on the agenda is probably a solid titanium cathode I just made. I do want to see for myself just how well this works. One thing that interests me is why we have so much trouble cracking the 10^7 n / s ceiling with fusors, even at 100 kV voltages, when this figure is routinely met in industry with tiny neutron tubes having titanium targets. I suspect the ion source (energy, beam fraction purity) is important, and I think I have that issue addressed. The target is another issue. Could be that in our fusors a lot of ions are disposed of by methods other than collision with deuterium, and that as unlikely as it sounds, an implanted titanium target can do just as well or better.

-Carl

[Carl Willis]

Hi Dustin,

I have that second ion source finished. What I don't have finished is the driver unit for the VHF amplifier that will allow it to supply more than about 50W, the power splitter I need to get power to a second ion source, or the gas feed fittings.

Thanks for your compliments. But along those lines, I will seize the opportunity to mount my soap box and reiterate the position that I do these and other projects sheerly as a hobby, to entertain Number One (with the most neutrons possible). I am not out to innovate for innovation's sake. I will of course innovate if I come across the right nut to crack, but I am resistant to any fanciful interpretation that I as a hobbyist am (or should be) doing much original research here, "sticking it to the man" who builds tokamaks for instance, solving the energy crisis, or any of that related chest-thumping fluffery that pervades the Internet discussions about fusion these days. I'd exhort anybody else who DOES harbor such fantasies to please gag on a big, stinging dose of reality! Yes, there may be a "lucky donkey" out there, but let's face it: chance favors the prepared, and the prepared in fusion energy research are those with the best equipment, lots of education and training and experience, and long-term commitment. Amateur scientists should not get delusional.

-Carl

[Jon Rosenstiel]

As always, very nice Carl. With the loop electrode at -30 kv what's the current draw?

Your solid Ti cathode sounds quite interesting, please keep us posted.

Jon R

[Carl Willis]

I took off the passive ion source and replaced it with a blank flange for this run. I do accept the logic that it is a complication. Everything else is the same as previously described.

In this series of photos, the ring electrode is grounded and the voltage on the ion source extraction electrode ranges from -250V (top image) to -5000V (bottom image). Clearly, extraction voltage is an important parameter for source operation. At -5kV, the current drawn from the extraction supply is about 1 mA. This does not mean that the beam current is 1 mA; on the contrary, beam current is probably less than half of that. This setup lacks a method of measuring beam current.

1: -250V
2: -500V
3: -1kV
4: -2kV
5: -5kV

-Carl

[Carl Willis]

Here is a series of photos showing the visible effects of changing the voltage on the ring electrode. Extraction voltage is -5 kV. The ring electrode voltages range from -10kV in the top image, to -35kV in the bottom image.

1. -10kV
2. -20kV
3. -30kV
4. -35kV

The most obvious visual effect is the fluorescence of the ceramic extraction electrode insulator, which becomes brighter as the ring electrode is brought to higher potentials. The fluorescence is because of electrons heading back against the ion current.

-Carl

[Carl Willis]

Hi Jon,

The ring-electrode current can be as low as 50 microamps if the vacuum valve is all the way open. Under these circumstances the ring voltage makes little difference in current. At higher gas pressures, the effects of self-sustaining hollow-cathode discharge start to become apparent both visually and on the electrometer measuring current.

-Carl

[Steven Sesselmann]

Carl,

Nice pictures, hope you don't stand too close with your camera at 35 kv. I have been thinking about your ion source and looking at your diagram here;

getfile.php?bn=fusor_ionguns&att_id=5032

The D2+ ions in your fusor are entering the chamber hollow at ~ +2.5 kev, which means they will traverse through the hollow cathode and collide with a wall on the opposite side at rougly 2.5 Kev, the result is not much recirculation.

Consider floating the ion gun below ground, ie leave the conflat cube at ground potential, but float then ion gun at say -5 Kv, the extract the ions at -10 kv, so the ions entering the chamber have -2.5 Kev.

They will still see the -35 kv cathode as -32.5 kv and will be extracted, but the ions will not have enough energy to reach the other side of the cube.

My spinning thoughts for what it is worth.

Steven

[Chris Bradley]

Carl Willis wrote:
> An ion and an electron are formed in a region that lies somewhere between a cathode (the ring electrode) and an anode (chamber and rest of the world).

This was the question I was trying to raise. Are they formed in such a region? It looks to me like they are formed outside the main shell, so the e-field strength will be small. The ion and electron move so that they try to neutralise the field. If the field is small then they won't go very far, very fast, especially if there is a prevailing magnetic field. My question is simple - how strong is the e-field in that region?

If your suggestion is that it is strong enough to project ions into the centre, then what is the evidence for this? If the fields are oriented in this way, then the ions would also reciprocate back to the centre. I am merely highlighting the point that they do not appear to be reflected back towards the central electrode as the optical edge of the beam appears to diverge on one side [only] which leads me to question the mechanism described.

Does the glow at the end of the pyrex part move with a magnet as you'd expect if it was caused by electrons alone?

best regards,

Chris MB.

[Carl Willis]

Hi Steven,

>The D2+ ions in your fusor are entering the chamber hollow at ~ +2.5 kev, which means they will traverse through the hollow cathode and collide with a wall on the opposite side at rougly 2.5 Kev

As I described to Wilfried, the whole point of using an extraction region containing a grounded anode and a negatively-biased cathode is that the ions will NOT have enough energy to collide with the grounded opposite wall. To use the marble-in-a-bowl analogy for potential and kinetic energy, the marble in this arrangement starts at most at the rim of the bowl--ground potential. It can also start somewhere on the side of the bowl, up to -2.5 keV below the rim if -2.5 kV is the ion source extraction voltage. If a marble is let go below the rim of the bowl, how can it possibly get over the opposite rim?

If the ion source used a grounded cathode and a positively-biased anode, as many designs do, the marble would probably be starting above the rim of the bowl, having enough energy to roll out the opposite side--the scenario that you're worried about.

-Carl

[Carl Willis]

Hi Chris,

The ionizing region for the passive source extends up into the metal and glass fittings, and it is logical that the field there is smaller than if this region existed in free space since it is partially shielded by the conductors. Electric fields do penetrate holes in conductors. As to precisely how strong in the field is at various positions inside the quick-connect stem, I don't know. It never occurred to me that that this fundamentally mattered to the concept.

I wholly concur with the observation that there aren't visible indications of recirculation. The intent of the hollow-cathode arrangement is to improve recirculation, but I don't claim it accomplishes this task effectively. On the basis of the evidence, it doesn't. However, as my new pictures show, the visible indications of recirculation are absent also when that confusing passive source is replaced with a blank metal flange. I'd be inclined to blame beam focusing and chamber-ring electrode geometry for poor recirculation, and I really don't see any avenues for attacking the supposed principle of the passive source on the basis of the beam shape in the photos. What can be said, and what is absolutely no surprise at all, is that if the passive source generates some ions, they are small in number compared to what the active source makes.

A magnet moves the glow around at the end of the glass tube. However, I can't say whether the glow at the end of the tube is due to "electrons alone." There could be a few ions and a lot of electrons, or a lot of ions and a few electrons, or anything in between. Do ions make glass (or other materials) glow as well as electrons of similar energy? No. Does the magnet experiment really say anything about the relative numbers of ions vs. electrons? Not really. You could easily suspect some of each arriving down there--decelerating ions, and accelerating electrons. To me, the magnet trick is a mere qualitative indication for the presence of some electrons, regardless of whether any ions are there or not.

-Carl

[Steven Sesselmann]

Carl, I am thinking the same way as you here, but I can't quite get my head around how your negative extraction tube affects the ion when the ion does not even touch the tube.

To my thinking, the ion needs to be created in a negative potential well, and not just accelerated through it, and your ionization chamber is from what I can see at ground potential.

At best I can see your ions with ground potential energy, or to use your analogy, marbles just rolling up to the edge of the bowl.

If your RF chamber where your ions are created was at negative potential that would be a different matter.

By all means, correct me if I have understood this wrong.

Steven

[Richard Hull]

Carl is having fun and doing a hands on here. He is noodling out what he is doing on the fly and repeatedly noting that glows and his musings are qualitative in nature, which is about all one can do here without a lot more effort.

It has been stated in any number of places here and in the past that you can't go to the bank with all that you see within a glow in vaccuo as a true indication of what you might attempt to posit from it.

The one thing we know for sure is that he has a cool ion gun system here, for sure. For grins and googlers he appears to be sharing his wonderings and thoughts about other aspects of this test rig. It is tough not to imagine fusion occuring within this device based on the obvious ion beam of deuterons and the acceleratory potentials in place. Carl noted that the fusion is not great, but it is there. Remember, fusion is easy.

It is obvious from Carl's posts, he is not working on a new aspect of IECF or to hit break even. He has indicated that, like myself, we are in this for the neutrons and not the fusion, though they go hand in hand here.

Frankly, if I could get 10e7 or 10e8 neuts per second and needed 10,000 watts out of my 240 volt lab electrical mains, I would be doing hand springs as the little wheel on my outside electrical meter base spins like a toy motor. Efficiency be damned! I have money for the electric bill, but I don't have a lot of neutrons. I want the neuts! Fusion is an also ran for me, in this quest.

The key to this effort is to get decent neutron numbers under 50kv applied. We see the limits of the simple fusor here as we have bumped against its glass ceiling already.

It appears that we need a concentrated source of deuterons (ion guns) or some novel source of ionization at the shell of the fusor to get the true influx ion currents up at our reduced voltages.

The last thing we need is a linear target machine working at 155KV where the X rays are lethal in nature, taking the simplicity, safety and, therefore the fun, out of the effort.

Richard Hull