The "wire" ion source

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
DaveC
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Re: The "wire" ion source - link

Post by DaveC » Thu Dec 01, 2011 12:09 am

Good comments, Carl.

The additional information adds to fill in the picture. Do you have the pressure in the Druyvesteyn-Penning paper? The tests I ran were able to move out to right some distance past the Paschen minimum, until the DC supply ran out of voltage... (around 2kv).

The point regrding curve shapes, that you make is most important. These classic curves come with several important assumptions, that often are not realized in actual systems.

Our ionizer geometry was roughly cylinder-square. The grounded screen had a several mm open mesh, and it and the solid chamber walls were several cm away from the fine tungsten wire. So this was only somewhat like a coaxial cylinder setup, with a D/d ratio of maybe 100.

The ions travelled about 0.25 meter to the main electron beam channel, entering at about 45 deg angle, where the strong local cathode field drew them in on axis.

I was able to model (approximately) the paths of both ions and electrons using Simion 7.xx treating both as non interacting. (I also tried unsuccessfully to use Vector Fields for a more refined analysis, but the mesher choked repeatedly on the relatively complicated geometry of the ionizer screen. Simion's transparent screen is a very, very useful feature. David Dahl knew his stuff, when putting this software together.

One interesting result I remember getting was from an attempt to measure incoming ion energy. By adjusting the bias on a collector plate in the electron channel (cathode off) until current went to below 1 nA, a rough ion energy spectrum was developed. I'd have to go dig up the test results, now, but I recall ion energies (He) were pretty large, perhaps up to half the voltages in the ionizer.

This suggested the ions at 10-4 Torr pressures were fairly long lived, and could travel significant distances ( and around corners). This probably is not what happens at the higher pressures in the fusor. Again, as you say, it's mean free path.

We always ran with a positive wire ionizer, because the ions were for the SEE (secondary electron emission) gun application.

Our pressures had to be quite low, in order that main e gun did not itself go into glow discharge... (which did happen from time to time).


So much for the anecdotes....

Dave Cooper

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Chris Bradley
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Re: The "wire" ion source - link

Post by Chris Bradley » Thu Dec 01, 2011 12:12 am

In the limited sample diameters of electrodes I have tried out, there also seems to be a 'cut-off' point when the electrode diameter gets bigger than some value between 1 to 2.5 mm. Up to then, electrode diameter appears to play only a very small part in the voltage (600 to 900V - with respect to an intermediate electrode positioned at the ends of the volume) at which the thing lights up. I moved to 0.2mm wire (from the picture I show above, with 1mm diameter electrodes) and was surprised to find that it made very little difference to the voltage at which a corona was initiated. At ~2.5mm diameter, it just wouldn't light up at all, at seemingly any voltage (at least, below 10kV).

My preliminary conclusion is that you need a certain voltage gradient to get the process underway, but if the diameter gets too large then the voltage gradient does not drop off (with radius) quick enough, such that ions are repelled from the region before they have a chance to sustain a discharge through further ionisations. This is just a 'working hypothesis'.

DaveC
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Re: The "wire" ion source - link

Post by DaveC » Fri Dec 02, 2011 11:32 pm

Chris-

Quite likely you moved to a value on the Paschen curve whose breakdown voltage (or field) exceeded yoursupply capability for the pd product of the gas you were using.

Away from the Paschen minimum, the voltages climb with either a higher or lower pressure for a given electrode geometry.

The natural oscillations that I've measured in long ( 1meter long) plasma discharges were in the 60 kHz range. They occurred towards the low end of an unassisted mechanical pump's pressure, (guessing now at perhaps a few tens of microns maximum).

They coincided with the appearance of plasma bunchings that were approximately spherical. I've called them "ion acoustic" oscillations in past references, simply because the plasma bunches coincided with the nodes of the measured oscillation acoustic frequencies. I used a shielded piezo-electric membrane to detect the acoustical) signal.

I need to set up this apparatus once again, and this time take and post photos along with scope traces of the acoustical signal. It was an interesting experiment.... sometime soon, I hope, when the shop-rebuild project plateaus.

Dave Cooper

Dustin
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Re: The "wire" ion source - link

Post by Dustin » Sun Dec 04, 2011 11:14 pm

Sorry Dave, I think I have been unclear,
In very low pressures ie:the mean free path being greater than the device diameter,
I was hoping to illustrate that an irregular outer shell may extend electron oscillation lifetime.
As a circular outer shell, all fields focus the electrons on the wire, so an electron has a relatively high probability of hitting the wire.
By changing the shell shape, you can reduce this probability by defocussing and promoting eliptical orbits.
Steve.

ab0032
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Re: The "wire" ion source

Post by ab0032 » Wed Feb 15, 2012 6:18 pm

Would one also get a positive corona discharge if the center wire was at zero potential or perhaps even negative and the outside more negative, if one wanted ions that start their existence at a certain negative potential?

Could one use a grid or mesh on the outside if one is not interested in sending the ions only in one mayor direction?

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Carl Willis
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Re: The "wire" ion source

Post by Carl Willis » Wed Feb 15, 2012 7:38 pm

For the purpose of establishing the wire discharge, all that matters is that the wire is positively charged relative to the outer cylindrical surface. The potential relative to any external point of reference is irrelevant.

You can certainly extract ions over the entire 360-degree azimuthal range, provided you provide the necessary mechanical support for the extraction grid (or slits) and you keep any external fields from penetrating too far into the trapping region.

-Carl
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