Ion Source Aiming

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Tyler Christensen
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Ion Source Aiming

Post by Tyler Christensen » Sun Aug 22, 2010 7:56 pm

I started testing a new ion source yesterday which consists of an electromagnet around a flange and an electrode in the flange, a fairly basic DC magnetron. But the thing is, it seems like the ions aren't coming out aimed in the correct direction. They're shooting at the stalk and heating that up and don't appear to be headed straight into the grid for the most part. You can see the attached camera view where the stalk is glowing bright but there isn't even enough plasma to make a star visible or make the grid glow. This just screams inefficiency to me.

Is there any way to aim this type of ion gun? It seems like the electrode is pointed right at the grid.

Does the fact that the coil is wound haphazardly potentially explain this?
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Linda Haile
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Re: Ion Source Aiming

Post by Linda Haile » Sun Aug 22, 2010 8:11 pm

It would seem at first glance that this is a focusing issue Tyler.

The haphazard coil probably plays a part.

Do you have an extraction electrode as such?

Tyler Christensen
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Re: Ion Source Aiming

Post by Tyler Christensen » Sun Aug 22, 2010 8:13 pm

No, there is no extraction electrode since the ions are made right at the edge of the reaction chamber in a large pipe, there didn't seem to be much need or even practical way to make an extraction electrode.

I will try to work on the coil and improve its quality

dbrown
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Re: Ion Source Aiming

Post by dbrown » Sun Aug 22, 2010 11:55 pm

First, the winding is only an issue for the uniformity of the field within the coil but this is not causing your real problem - the main issue is that any electric coil (even perfectly wound) will act as a defocusing lens (divergence) once any ions leave it. A linear coil cannot be used to focus or even just hold an electron beam in a straight line once an ion leaves the coil (I have learned this from my ES Linear Accel. project.) In fact, it causes the beam to diverge - any irregularities in the coil design will, of course, at that point causes the beam to go in a number of possible directions. An Enzil lens will work or a magnetic quadruple or an electric quadruple but a simple coil will not (unless it goes all the way to the inner grid ... that design, just might create a shorting issues.)

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Carl Willis
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Re: Ion Source Aiming

Post by Carl Willis » Mon Aug 23, 2010 3:17 am

Hi Tyler,

Any idea what the discharge in this ion source looks like? It might be spatially quite diffuse and / or non-uniform. Also, what polarity is the central electrode in the source? And if you change or turn off the solenoid current, what happens to the spot you observe? Is any part of the grid conductor stalk magnetic?

When I played with permanent-magnet magnetron sources I pretty much got beams that went right into the grid.

-Carl
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Tyler Christensen
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Re: Ion Source Aiming

Post by Tyler Christensen » Mon Aug 23, 2010 6:17 am

>Any idea what the discharge in this ion source looks like?
Unfortunately there is no flange configuration that would allow me to view the discharge.

>Also, what polarity is the central electrode in the source?
The electrode has been tested both non-rectified AC off an NST and also positive rectified. The last time I tried negative, the source did not work.

>And if you change or turn off the solenoid current, what happens to the spot you observe?
I can get it running with the ion gun and then slowly turn the gun off and it keeps running, but at that point the spot is already glowing bright and does not cool down. If I run the fusor without the ion gun at all at the sweet-spot pressure of free-running mode, the stalk does not get that hot at the same power input level.

>Is any part of the grid conductor stalk magnetic?
No


I'm guessing the issue is that the gun is shooting from within a 2" pipe and it can see much of the stalk as well as the grid from that perspective. Perhaps I will try rebuilding the ion gun in a smaller KF16 flange.

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Steven Sesselmann
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Re: Ion Source Aiming

Post by Steven Sesselmann » Mon Aug 23, 2010 7:08 am

Tyler,

Why don't you try something like this.

viewtopic.php?f=12&t=5013#p32321

Basically you need a two wire feed-through and a filament of some description, with a floating PSU, negatively biased. The filament becomes an electron emitter when hot, and will ionize the gas flowing through the nipple. A ring magnet around the nipple will improve ionization.

A narrower opening between your ion source and the fusor will create a differential pressure, thereby increasing the ion source efficiency, so providing the distance and potential between your cathode grid and the ion source is right, the grid should be able to extract the ions. You may need to play with this to get it right.

If your filament is glowing orange, and biased at -1Kv you should be in the ballpark. Note, your nipple will get very hot, and may need cooling.

Be careful with floating power supplies, these should best be enclosed in a wire cage.

Observe safety and have fun longer...

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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