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Anode layer ion source. Operation pictures

Posted: Fri Feb 13, 2009 11:47 pm
by Andrew Seltzman
Here are some preliminary tests of an anode layer ion source that I am working on.

Anode layer ion sources (http://www.plasmalab.ru/depos_ion.htm) are simular to hall effect thrusters, except they don't require the neutralizer source.

My source is 1" long, 1.25" OD with a 0.75" molybdenum wire ring as the anode.
It uses an N40 rare earth magnet for the magnetic field.

Some of these sources can produce up to 30mA of ion emission per linear cm of emission area. (http://www.veeco.com/Products/process_e ... rodGroup=0)

In the pictures below this source was being pumped down by a direct drive dual stage pump in good condition and was operated at about 1-2kV from a power supply with 5mA max output. More data will follow.

Andrew Seltzman
www.rtftechnologies.org

Re: Anode layer ion source. Operation pictures

Posted: Sat Feb 14, 2009 3:42 am
by Carl Willis
Hi Andrew,

This is an ion-source topology with (at first blush anyway) lots of interesting possibilities for fusors. Thanks for bringing it to attention with such fine craftsmanship and beautiful discharge photos. It would be one of the easier and more practically accessible ways of making radial "sheet" beams. Paired with the Jon Rosenstiel grid construction, sheet beams from anode-layer slot sources could really do a good, efficient job of putting lots of ions on unobstructed inbound trajectories with the highest possible energy.

Any idea yet what kind of lifetime this source gives you? I might imagine some heavy sputtering, and the small interelectrode spaces are vulnerable to whiskers. Just a thought.

-Carl

Re: Anode layer ion source. Operation pictures

Posted: Sun Feb 15, 2009 5:22 am
by Chris Bradley
This is a beautiful display of both craftsmanship and plasma manipulation.

I don't see any immediate issue for sputtering as I view this as essentially a plasma discharge across a small gap in which electrons can only get to the anode via a very sluggish exb drift (if they can manage it at all, magnetron condition depending) and ions mostly (?) head off in an alternate direction to the cathode anyway (else they'd have to stream against the exb drift direction to reach the cathode).

My query/comment would be over what the relative potentials here are. Presumably the plasma formed in the region is at an electric potential somewhere in the middle of the held voltage and, presuming the cathode is being held to ground, ions are accelerated by some fraction of that voltage above ground and would therefore accelerate to ground by fields extending away from the anode and off to 'the nearest' ground potential.

So in a fusor, ions emerging from this would have already energetic ions that would accelerate through the central grid, out the other side and then on, straight into a ground potential (as they have enough energy to do so). Plus electrons would happily stream into the device towards the anode and get caught up in the exb mêlée.

This is not to denigrate the application in any way, I am sure the relative merits and means to usefully operate this in a fusor setting can be established. I'm just wondering what the best way to set up the relative potentials are, to achieve the desired effect.

best regards,

Chris MB.

Re: Anode layer ion source. Operation pictures

Posted: Sun Feb 15, 2009 9:38 am
by Hector
It looks just like a small home made Hall Thruster. Very nice.

Re: Anode layer ion source. Operation pictures

Posted: Mon Feb 16, 2009 3:11 pm
by FogerRox
Thanks Andrew for the update. Always a pleasure.

Re: Anode layer ion source. Operation pictures

Posted: Mon Feb 16, 2009 4:14 pm
by Richard Hull
Nice stuff! This near shell ion sourcing is what the simple fusor has needed to advance a bit and, finally, some folks are getting on the job in an attempt to make it happen.

Richard Hull

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

Posted: Tue Mar 03, 2009 9:21 pm
by Andrew Seltzman
Improved anode layer ion source design.

Dimensions of casing and magnet type remain the same.
The casing is now fabricated out of alloy 410 stainless steel (magnetic alloy) instead of 1038 steel. It is a little harder to machine but retains the rust/corrosion resistant properties of stainless steel with the magnetic properties of carbon steel.

It is noticeably more granular when machined, creating chips and short strings with rough edges as compared to the long smooth strings when machining 304/304L stainless and is difficult to get a shining surface finish even when machined with sharp carbide tooling. It is possible to smooth/polish the surface with sandpaper/scotchbrite.

The anode is now constructed out of an 18-8 stainless shim (non-magnetic) with 0-80 nuts spot welded to the back. 0-80 screws insert through 1/8” OD 1/16” ID alumina rods to hold it in place.
The ring is held in a more consistant position resulting in a more even ion beam distribution.

Operation data:
Pressure unknown, estimated in the mTorr range about 5-10mTorr
Voltage Current
500 1.6mA
700 3.6mA
900 5.7mA
1000 7.5mA

In the mtorr pressure range, about 60-70% of total current goes into ion beam current.
Attached is a paper describing anode layer ion source operation


Andrew Seltzman
www.rtftechnologies.org

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

Posted: Tue Mar 03, 2009 9:48 pm
by Carl Willis
Hi Andrew,

Thanks for the update on this interesting ion source.

I'm curious, do you feed gas in during operation? Your drawing has what might be a Swagelok-type fitting, though it's hard to tell.

Your numbers are for air ions, correct? Do you plan on running it with a fusion gas?

What do you use to measure current?

-Carl

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

Posted: Tue Mar 03, 2009 9:50 pm
by Mike Beauford
Very nice work Andrew. Question, how long do you think the anode will last and how long have you been running it for?

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

Posted: Tue Mar 03, 2009 10:12 pm
by Andrew Seltzman
That cad is of the final design, the sorce is running without active gas feed at ambient chamber pressure.

In the final design HV will be supplied to each injector through an MHV feedthrough, and gas through a swagelok fitting through one of the ceramic tubes through the base of the injector.

Anode life is unknown, however after several minutes the source is barely warm.
It will arc above 1kv with the current anode (the shim), most of the pitting is from that. From what I have read, they will operate reliably for a long time.

Andrew Seltzman
www.rtftechnologies.org

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

Posted: Wed Mar 04, 2009 11:09 am
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

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

Posted: Wed Mar 04, 2009 1:59 pm
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.

Andrew Seltzman
www.rtftechnologies.org

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

Posted: Thu Mar 05, 2009 6:32 am
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.

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

Posted: Thu Mar 05, 2009 9:59 am
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

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

Posted: Thu Mar 05, 2009 10:28 am
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.

Re: Anode layer ion source. Operation pictures

Posted: Fri Mar 06, 2009 9:42 pm
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.

Re: Anode layer ion source. Operation pictures (Feedthrough flange, 30min operation, temperature data)

Posted: Fri Mar 06, 2009 11:39 pm
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.

Andrew Seltzman
www.rtftechnologies.org

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

Posted: Wed Mar 11, 2009 2:23 am
by Andrew Seltzman
Eye candy. Parts for set of 5 ion injectors.

Andrew Seltzman
www.rtftechnologies.org

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

Posted: Wed Mar 11, 2009 2:40 am
by Mike Beauford
Once again, beautiful craftsmanship. My hat's off to your skill at making these things.

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

Posted: Wed Mar 11, 2009 3:12 pm
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?

Re: Anode layer ion source. Operation pictures (Feedthrough flange, 30min operation, temperature data)

Posted: Wed Mar 11, 2009 7:45 pm
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.

Jon R

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

Posted: Fri Mar 13, 2009 9:37 pm
by Andrew Seltzman
Cooled grid is complete and operational. Results and designs uploaded to my website.

Andrew Seltzman
www.rtftechnologies.org

Final Design Modifications. Faraday Cup Measurements

Posted: Fri Mar 13, 2009 9:50 pm
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.

Andrew Seltzman
www.rtftechnologies.org

Re: Final Design Modifications. Faraday Cup Measurements

Posted: Sat Mar 14, 2009 5:13 am
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

Re: Final Design Modifications. Faraday Cup Measurements

Posted: Sat Mar 14, 2009 3:11 pm
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.

Andrew Seltzman
www.rtftechnologies.org