Anode layer ion source. Operation pictures

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Richard Hull
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Re: Anode layer ion source. Operation pictures (Second design + Data)

Post by Richard Hull »

Carl has asked a significant question that hasn't been dealt with yet. The current for an ion gun's actual ion beam current is usually measured with a faraday cup and bears little if any relationship to the ion gun's supply current, but is more a function of the gun design. In short, how well does your gun take the supply current and transform it to measurable ion current. A guess of 75% is just not right.

It also depends on where you intercept the beam for doing work in the vacuum environment as to how much of your actual "at extractor" beam current does what you want it to do on target.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Anode layer ion source. Operation pictures (Second design + Data)

Post by Andrew Seltzman »

Right of course with the requirement for current measurement.

The currents quoted above were currents feed to the anode not measured on the beam.
The 60-70% is just my best estimate based on papers written on anode layer sources, which seam to be pretty consistent in the percentage of total current that goes into ion beam current.

I don't have a Faraday cup yet, but when I get one build, I will post measured values.

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Re: Anode layer ion source. Operation pictures (Second design + Data)

Post by Chris Bradley »

I don't understand this concern with the longevity of the anode. Why would it be degraded at any sort of abnormal rate? Surely it only receives a relatively benign electron bombardment as they drift out of the ExB region (in circles [trochoids]), and do not even bombard it directly (i.e. not normal to the surface) but at a tangent as the exb orbits drift it across.
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Re: Anode layer ion source. Operation pictures (Second design + Data)

Post by Richard Hull »

Many components within ion guns suffer mightily. The more the materials suffer and the faster the gun degrades, the higher is the beam current as a general rule.

"Bright" guns need regular maintenance and replacement of damaged components.

The farnsworth systems had to have extractors replaced almost weekly according to Gene Meeks and Steve Blasing. Their guns were fairly compact and "bright". They were not externally cooled either.

Errosion, spallation, deposition and melting are the culprits in gun degradation.

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
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Re: Anode layer ion source. Operation pictures (Second design + Data)

Post by Chris Bradley »

But this is, as literally described, a virtual anode? So one of its beneficial features is that the physical anode doesn't experience the same flow of particles as a pure electric discharge device would, as you describe. The magnetic field provides the necessary polarisation of charge that make this quite a different beast to ones that you describe, that need regular electrode replacement.

I'm not saying you are wrong, but I like to understand a basic mechanism than have hand-wavyness towards dissimilar devices as a means to support a view on how it works.

I don't see how any electrons will head straight towards the anode at full field potential, as they would in these other devices. Instead the electrons would creep their way towards the anode, gyroradius by gyroradius, then glance against it once close enough. Intuitively speaking, I get the feeling that this would be a whole load less harmful than in a discharge device.
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Re: Anode layer ion source. Operation pictures

Post by John Futter »

Andrew
Nice work
your gut feeling for ion current could be right. It will be interesting to see a faraday cup downstream with a secondary electron suppression aperture in front biased -200 to -400 volts with respect to cup (to kill secondaries).

All our ion implanters use cold cathode penning ion sources and when tuned right, an arc current of 2mA in the ion source will give 1.85mA in the cup after the mass selection magnet. To add to Richards comments on wear we get around a 100 hours before the ion source has to be cleaned and ion exit aperture elements have to be replaced.

I was going to make the ion sources for my fusor similar to Carls RF driven one but this seems simpler and with the ions being produced in an annulus of significant diameter, eventual focus should result in more ions at the focus as until focus they are reasonably spread out so they shouldn't defocus due to similar charge effects.
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Re: Anode layer ion source. Operation pictures (Feedthrough flange, 30min operation, temperature data)

Post by Andrew Seltzman »

As of now the casings for 2 injectors complete, and an additional 3 are almost complete. This will provide a total of 5 injectors, 4 for the fusor, and 1 for testing.

The injector base plate is a 2.75" CF flange with a 5kv rated MHV feed through for the HV and a 1/4" swagelok VCR fitting for gas feed through. The gas will be feed into the injector casing with a 1/8" OD ceramic tube(not shown). Both are TIG welded to the CF flange. The flange has a tapped 10-32 thread for the injector to screw into (with vented screws).

The base plate seems to work fine. The injector was tested for 30min at about 5ma discharge current and 800v. The opposing window across from the injector became slightly warm to the touch, but showed no damage. After the 30min run the injector body/magnet were measured to be at 53*C (132*F), well within the NdFeB magnet temperature limits.

The anode ring showed slight darkening, but no damage.

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Re: Anode layer ion source. Parts for set of 5

Post by Andrew Seltzman »

Eye candy. Parts for set of 5 ion injectors.

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Re: Anode layer ion source. Parts for set of 5

Post by Mike Beauford »

Once again, beautiful craftsmanship. My hat's off to your skill at making these things.
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Re: Anode layer ion source. Parts for set of 5

Post by Chris Bradley »

Is this a set of ion sources for your grid-cooled fusor, or a set of positioning thrusters for the first amateur-launch satellite??

How's that cooled grid work going?
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Re: Anode layer ion source. Operation pictures (Feedthrough flange, 30min operation, temperature data)

Post by Jon Rosenstiel »

Very impressive work, Andrew. TIG welding that close to the knife edge must have been a little nerve-wracking, to say the least.

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Re: Anode layer ion source. Parts for set of 5

Post by Andrew Seltzman »

Cooled grid is complete and operational. Results and designs uploaded to my website.

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Final Design Modifications. Faraday Cup Measurements

Post by Andrew Seltzman »

Anode now has 0-80 threaded rods TIG welded into surface and sanded down to provide flat surface as opposed to screwing into nuts spot welded to back of washer.

Anode is secured by nuts on back of alumina standoff.

Spring loaded connection between HV feedthrough and threaded rod leading up to anode.

Alumina gas feedthrough added.

This is the final design, components for all 5 injectors have been machined, only 1 has been assembled.

Faraday cup data:
-------------------------------------------------
No optimization after assembly, just tested as assembled.
Faraday cup was an injector casing, 7/8" depth, 7/8" ID, current measured as voltage across 1.1k resistor, no cup biasing, no secondary supression.
Data as follows:
Icup Vsource Isource
1mA 700v 2mA
2mA 1000V 4mA

50% of source current goes into beam current.

Construction details will be uploaded to wwebsite presently.

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Re: Final Design Modifications. Faraday Cup Measurements

Post by Steven Sesselmann »

Andrew,

This is top quality engineering work, with a fair amount of innovation, real eye candy. Just revisited your web site again, and saw the excellent star mode images from your revent tests.

I look forward to see how the ion guns perform.

What is the maximum voltage of your current PSU, your experiments seem to be around the 15 kv mark ?

Steven
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Re: Final Design Modifications. Faraday Cup Measurements

Post by Andrew Seltzman »

Maximum PSU voltage is -40kV. Voltage was limited in tests since the lack of thermionicaly emitted electrons fron the grid prevented sustained plasma below 10mTorr without ion sources. At 10mTorr, 15kV was the max voltage that could be used without drawing too much current from the power supply.

Once these ion sources are used on the fusor, pressure will be decreaces and voltage will be increaced.

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Re: Final Design Modifications. Faraday Cup Measurements

Post by Steven Sesselmann »

Andrew,

Can't wait to see your results..

I like your setup, the grid has two advantages, a) it is cooled, and b) it has a thick radius, thereby reducing the voltage gradient around the grid wires (less likely electron emission from the surface.)

Hope to get the stats for my Q list soon..

Don't forget to count the power to the ion guns

Steven
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Re: Anode layer ion source. Operation pictures

Post by Chris Bradley »

There's no mistaking this is serious, well crafted kit.

I'm still unsure about what the 'wiring diagram' is going to look like. In the fusor application, are you intending to hold the anode at ground potential, or the cathode at ground (chamber) potential? Presumably if you hold the cathode to ground then all your ions would be attracted through the centre EHT cathode and on to the ground on the opposite side of the chamber? That's why they are accelerated away from the anode in these examples, because they see a nice, lower ground potential and desire to head towards it. But if it were the anode that is grounded then you'd need to isolate the flange from the chamber to float it a little negative, upto the ion supply potential.
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Re: Final Design Modifications. Faraday Cup Measurements

Post by Carl Willis »

Hi Andrew,

Your skill and speed at turning these out are enviable. Nice photos, decent data so far. You'll probably get quite a bit more current out of these with deuterium than with air, all other factors remaining the same. There probably are secondary electrons coming out of the faraday cup arrangement, but the geometry should limit their number some (it is the bored-out housing of one of the ion sources if I read you right).

Nice post.

-Carl
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Re: Anode layer ion source. Operation pictures

Post by Steven Sesselmann »

Andrew,

Looking at your Anode Layer Ion source design and the way you use the powerful magnet, would it not have been better if you used a ferromagnetic material for the casing, rather than stainless steel?

Looking at your pictures, the casing appears to be ground, so I assume you intend to have a positive voltage on the anode ring when you use it on your fusor. Can you confirm this?

Steven
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Re: Anode layer ion source. Operation pictures

Post by Andrew Seltzman »

The casing is made out of 410 stainless, which is ferromagnetic, with roughly the same properties as carbon steel. It is slightly less ferromagnetic, but probably no more then 5-10% or so. If you pull a magnet off of it, you can't tell the diffrence in force between 410 stainless and carbon steel.

The case is grounded, the electrode is held positive.

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Anode layer ion source. Set of 5 complete and tested

Post by Andrew Seltzman »

All 5 ion sources have been completed and tested to produce an ion beam for 30min continuously. All 5 have been helium leak checked to the 10^-8 range, initialy 2 of the welds were bad, however those have been repaired.

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Re: Anode layer ion source. Set of 5 complete and tested

Post by Richard Hull »

Needless to say, we are all looking forward to the final operation and reports related to all this hard work you are doing. I wish you all the very best in this first effort at a fully gunned amateur fusor..

Richard Hull
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Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Anode layer ion source. Set of 5 complete and tested

Post by Dustinit »

I also am looking forward to seeing this in operation.
Fantastic effort andrew.
As a possible extension on the idea,
I view the design as an assymetric penning trap which traps electrons
and pushes out the ions.
Therefore I see no reason why it needs to be a circular source,
So if you used bar magnets instead, interspaced with electrodes it may be possible
to build a source around the circumference of the fusor, or possibly all the fusor wall space if magnets are placed in a grid pattern.
This would make a more uniform source but I'm unsure it would have any benefits over Andrews superb effort.
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Re: Anode layer ion source. Set of 5 complete and tested

Post by Steven Sesselmann »

Dustin,

A few more good lateral ideas there, good one!

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Re: Anode layer ion source. 4 mounted on core

Post by Andrew Seltzman »

4 ion sources are now mounted on the core.

In this picture 1 of the ion sources is in use at about 800v on the source anode and about -2kv on the grid. The plasma will form a stable discharge all the way down to very low voltage on the grid with no observable pulsing or flickering even with 0.5mA collected by the grid.

When all 4 sources are put into operation, they should allow very stable operation at high vacuums and low grid currents.

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