RF ion source sees first light

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Doug Coulter
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Re: RF ion source sees first light

Post by Doug Coulter »

Truly beautiful Carl!

Finally someone is publishing numbers close to what I've been getting on the microwave version of this design, not needing ridiculous RF power inputs to light it off, and with good output.

So I feel a bit vindicated myself ;~) Thanks!

I've found that the fusor really likes a good source of monatomic ions, which I guess no one yet has measured the ratios of, but the RF sources are known to excel at. I guess one of us is just going to have to put a weak magnet in the ion beam and a ZnS screen back there to see the rough ratios of monatomic vs diatomic ions produced.

I find it especially cool that you can run the fusor at pressures below normal, where it doesn't "take off" and become its own best (or at least most prolific) ion source, and in that mode you can dial the main fusor current up and down easily via changing the ion extraction voltage. My particular version has a too-long extractor electrode, so above a certain extraction potential, the electrostatic lens it makes (like it or not) has too short a focal length and bashes the nice ions into its own walls after they pass the focus. At the peak output I am getting fusor currents of about 2.6 ma at 50kv no matter how low the operating pressure (down to the mentioned 1.6e-6 mbar), which goes to zero at low pressures if I turn the ion extractor off -- not as slick as a faraday probe, but indicative of good operation.

I will posit the following idea that may make yours work even better -- I found with mine that getting a uniform H field wasn't trivial (and I never quite managed that, in my case about 20 gauss out of ~980 over the length of the glow was best I could do), but that it's moderately critical around the electron cyclotron resonance area. If you really nail that -- you'll be able to run it at lower enough pressures with good performance to not need differential pumping, or at least mine does, and will stay lit down to 1.6 e-6 mbar or so....nicely low gas flow rates at that point and no need for better pumping.

I did that on mine feedforward via a hall effect magnetometer and some shims to space the magnets (or more properly, hold the spacing I set). This was however confirmed to be the best setting by also just empirically tuning that while in operation for best ion current and lowest possible operating pressure -- pretty much got the same answer by theory or experiment, which is always gratifying.

Good show all around! This helped our fusor effort probably more than any other single thing we've tried so far. Hopefully you will find the same on your gear.

I intend to fool with the geometry a bit more on mine to correct that issue with over-focus and see if that helps. In my case I'd used too many thick adapter flanges which is why the puller had to be too long, but that can be fixed up. I'm pretty sure that there's a lot more output possible with that and by better control of what the H field does after the glow area (so it doesn't also bash ions into the tubing walls). In my case, I have the RF normal to the tubing axis, and the H field along it.

Tyler -- the polarity of the H field shouldn't matter -- just the orientation vs the RF, you want E and H fields crossed at about 90 degrees to get the cyclotron resonance, and the field strength correct for the RF frequency in use. I doubt it matters whether the electrons are going clockwise or counter clockwise, as long as they are going around. The fields required shouldn't affect the actual ions too much, due to their much higher weight.


Edit:
On cleaning copper: The firearm business has this as one of its more diverse and profitable lines, as bullet gilding metal (Cu-Ni alloy) ruins accuracy when it gets inside a rifle bore by being rubbed off the bullets. The stronger cleaners tend to have a high component of ammonia (eye watering levels), which used alone works fantastically here. Most of the gun supplies also have some other stuff in there which itself can be hard to get back off -- various oily stuff. But 28% ammonia (diazo machine grade) really whips up on Copper fast and is easy to clean off once you are done -- simple water rinse, which I do in an ultrasonic cleaner, but mainly because I have one and that makes it easy -- plain old water spray would do as well.

In a pinch I've found that plumbing flux (ammonium chloride + zinc chloride + HCl in liquid form) also really gets the crud off quick, and cleans up easily as well. That last would almost have to win the "cheap" award, as it's about a buck for a large bottle. It's also easier on the eyes and throat than the concentrated ammonium hydroxide by quite a large factor, but more difficult to really rinse off fully.

Any of these that etch copper tend to get into the inter-grain space and continue their action for awhile, so no matter what you use, a really thorough rinse is a very good idea. That effect makes the pure ammonia a little better in real life use as it's easier to get rid of.
Why guess when you can know? Measure!
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John Taylor
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Re: RF ion source sees first light

Post by John Taylor »

Carl,
I know this is an older thread but I wondered how this particular ion gun performed as far as sputtering goes. Has the copper given you any problems?
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Carl Willis
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Re: RF ion source sees first light

Post by Carl Willis »

Hi John,

Yeah, copper sputters like crazy. It's also less than ideal for hydrogen plasmas, being established as a particularly good catalyst for molecular ion formation.

Copper eventually (10+ hours of operation or so) coats the downstream interior of the glass discharge tube to the point of opacity. It probably has a lot to do with degradation in performance over time. For this reason, just last month I built a new version of this source that, among other improvements, gets rid of the copper entirely and replaces it with stainless steel. I also eliminated some braze joints. Sometime "soon" (maybe this weekend, maybe next month, who knows) I will post an update here about the new source. I have not operated it yet.

-Carl
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Chris Bradley
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Re: RF ion source sees first light

Post by Chris Bradley »

Would it not be better to pick aluminium - the lowest sputtering of all common materials?
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Carl Willis
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Re: RF ion source sees first light

Post by Carl Willis »

Chris,

I can't think of how you would practically follow this blueprint with aluminum. It calls for a number of braze or solder joints using off-the-shelf insulators, feedthroughs, etc. and uses standard stainless CF hardware. That doesn't mean aluminum is beyond consideration if a more custom approach is feasible. The original Kiss-Koltay ion source upon which mine is based does in fact use extensive aluminum.

-Carl
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John Taylor
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Re: RF ion source sees first light

Post by John Taylor »

I would like to see how a stainless version would perform. will you TIG weld the places that are soldered on the current version?

I would like to replicate your design in stainless as well. Would you be willing to share your dimensions so I could attempt to machine the parts to your already proven design?
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