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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Carl Willis
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RF ion source sees first light

Post by Carl Willis » Sun Sep 14, 2008 9:09 am

I have finished up a little RF ion source, at least to the point that it's operable. The idea behind such sources dates back to the late '40s and a physicist named Thonemann. An electrodeless high-frequency discharge is induced in a dielectric "bottle" and ions are extracted from this discharge by DC electric fields. The design I use is closely related to the Bayly design that was improved by Kiss and Koltay, whose paper I provided in a prior thread. The difference between this embodiment and Thonemann's is that both the DC extraction electrodes are located at the downstream side of the bottle. This avoids numerous practical difficulties, and for fusor injection, has a major upside: particles injected via a negative-HV "puller" electrode rather than by a positive-HV probe at the upstream end of the bottle will not have too much energy to remain trapped in the fusor.

So here's my take on the Kiss-Koltay RF ion source. The puller electrode has a 1.5 mm x 7 mm axial hole in it and is supported from a commercial low-profile insulator by a piece of copper tubing. The grounded electrode fits just outside the puller on another flange that also carries a "quick connect" compression fitting to seal against the glass bottle. An axial magnetic field in the discharge region helps confine plasma to the source axis where it is a useful supply of ions. RF power (~30 W) at 200 MHz is coupled into the discharge via an L-match network and two turns of wire.

Photo #1: Schematic of ion source

Photo #2: The ion source in operation in air; diffusion pump system off to left.

Photo #3: Assembly of source showing installation of the negative puller electrode on its carrier flange.

Photo #4: Addition of the grounded electrode over the top of puller.

Photo #5: The target for this test system is a graphite cylinder.

Photo #6: Close-up of beam, probably about 1-2 mA (I cannot yet measure this accurately). Pardon the annoying soap scum on the front of my viewport. The puller is at -3 kV, the target at about -5 kV. Electrons backstreaming from the target cause the low-profile insulator carrying the puller to fluoresce.

Photo #7: Another pic of the beam.

Unfortunately my little diffusion pump stand lacks the speed to cope with this differentially-pumped system per my design. The diff pump is restricted to about 5 lps by the KF25 plumbing in that system. Therefore, I have to run the ion source discharge at reduced pressure for this startup test. I am working on a Varian V-80 turbopump to carry on the trials, which should allow me to put deuterium in the capillary feed. Stay tuned!

-Carl
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Carl Willis
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Steven Sesselmann
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Re: RF ion source sees first light

Post by Steven Sesselmann » Sun Sep 14, 2008 9:50 am

Nice work Carl, and well documented, great pictures too.

How many milliamp do you think this system is capable of?

I dont quite understand what you mean by differential pumping.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
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Re: RF ion source sees first light

Post by ScottC » Sun Sep 14, 2008 4:12 pm

Kinda looks like a HeNe laser tube.

Wilfried Heil
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Re: RF ion source sees first light

Post by Wilfried Heil » Sun Sep 14, 2008 9:02 pm

Carl,

interesting work. I wonder what difference such an ion source will make, at least it will allow you to adjust the voltage and pressure independently.

Is that Conflat cube your next fusor, or just part of a short linear accelerator?

Wilfried

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Carl Willis
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Re: RF ion source sees first light

Post by Carl Willis » Mon Sep 15, 2008 8:54 am

Hi Wilfried,

Thanks for your interest. I am hoping that two of these sources will allow a high-vacuum fusor with a lot of recirculation. My next fusor will have two diametrically-opposed ports for these sources. George Miley had a student who built an ion source that has some similarities, called "ILLIBS." It made a big improvement in fusor operation. That was my inspiration.

The CF cube is just a handy part for small vacuum experiments. Right now it hold a graphite cathode that is designed to handle ~15 kV and no real current, so I suppose you could call it a linear accelerator of sorts.

-Carl
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Mike Beauford
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Re: RF ion source sees first light

Post by Mike Beauford » Mon Sep 15, 2008 2:19 pm

Hi Carl,

Very nice work. Are you sure your not an engineer at heart?

Mike
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Richard Hull
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Re: RF ion source sees first light

Post by Richard Hull » Mon Sep 15, 2008 4:10 pm

Engineering is as much a part of the scientist and science is a part of the engineer. The difference is just in what your job discription and pay is.

A formal engineer is a guy who designs and makes stuff

A formal scientist is someone who does experiments, collects data and arrives at conclusions.

There is not much room in today's world for the pure versions of either the engineer or the scientist. All tend to get their hands dirty and experiment with ideas within the limits of both science and engineering. Each occasionally pushes the limits of one or the other or both of the disciplines.

Employers often push hires in either discipline to be both. I have a friend who is a physicist with NRL and she is doing 90% engineering work only.

***********************************

As for Steven's question.........What is differential pumping............

Virtually all ion gunned systems are differentially pumped.

The ion gun may be considered or viewed as a slow leak in a very high vacuum system.

Ion guns have a flow of gas constantly added to them which is regulated. All ion guns have an extractor port where the ion stream pass freely through a focused hole into the reaction or vacuum chamber. Thus, a gas leak is a permenant load on the master pumps on the high vacuum chamber.

While the conductance of the ion gun or guns is extremely low, it is quite real, nonetheless, (leaks).

Thus, the pump on the main vacuum chamber has to have enough pumping capacity to evacuate the main reactor vessel or chamber to the its ultimate pressure against these real leaks. There are complex but very doable calcs extant that can compute the exact pump capacity needed to achieve the final bottom pressure desired in the main chamber against the gas leak load of the ion gun or guns. Most avoid the calcs and massively over pump a system and use a pinch off valve to set the final pressure to any desired level.

An example would be that the ion guns must be kept at 25-50 microns to allow for high current glow discharge while the main chamber is kept at 10e-5 torr to assure an MFP large enough so that the bulk of the ions produced will arrive on target in the vessel without gas collisons inducing major loses in the ion beam.

Some complex systems (rare) vaccum pump both the guns and the chamber.

The term differential is obvious from the above explanation. There are pressure differentials in the devices within the vacuum system.

Carl's extractor opening is huge by most standards. Thus his smaller pump can't handle the load. Traditionally, extractor openings are measured in the mm to sub mm range. As such extractors get very hot and their openings errode rapidly due to the velocity of ionic fluid flowing through them. Most ion guns are serviced regularly and must be designed for rapid component replacement.

Note: I am not saying Carl made his extractor hole too big. Large extractor holes usuall mean very bright ion sources and concomitant heroic differential pumping efforts. 1-2 ma ion sources would be considered fairly bright, especially for simple amateur efforts.

Carl, do you need or have access to an electrometer for faraday cup measurements?

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Larry Upjohn
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Re: RF ion source sees first light

Post by Larry Upjohn » Mon Sep 15, 2008 4:53 pm

Carl;
your copper components look as if you had etched them in acid. Is this from your cleaning process or is it a result of the operational environment within the generator? Thanks, Larry Upjohn
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Carl Willis
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Re: RF ion source sees first light

Post by Carl Willis » Mon Sep 15, 2008 5:45 pm

Hi Larry,

The photos showing copper parts were made prior to operation and right after assembly. The copper parts were very ugly after brazing due to the usual coating of flux and oxidation. To clean them up, I dunked them in "Tarnex", a polishing cleaner available at grocery stores containing thiourea. Hence the bright pink color. During operation, they develop a more usual reddish-brown copper coloration.

-Carl
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Larry Upjohn
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Re: RF ion source sees first light

Post by Larry Upjohn » Mon Sep 15, 2008 8:51 pm

Carl;
I think the wife has mentioned this for some other copper I need to clean. I will keep this in mind for future use. More later, Larry Upjohn.
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