Hull-Raney Ion Gun Efforts

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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raneyt
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Joined: Fri Jun 29, 2001 8:40 am
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Hull-Raney Ion Gun Efforts

Post by raneyt »

Hi Folks,

This is the first or a series of updates on our ion gun project. As an overview, Richard and I have opted for a 25-mm diameter Pyrex envelope connected with stainless steel flanges. I hope to post a jpeg diagram before too long. To kill two birds with one stone, I'm posting the following information extracted from my lab notebook. Good luck with your efforts.

Regards,

TIM

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5 February 2003. Ion Source Design Notes. This is the first entry for the ion source project. The purpose is to design and build an ion source suitable for producing hydrogen (protons) or deuterium nuclei beam. I plan to make two “master flanges” and give you one to Richard Hull on 9 February. This way, we’ll have two identical templates for making any number of flanges. Details follow:

• Flange Outside Diameter: 1.75825” (actual). Allows more space for mounting bolts and 25-mm Pyrex tubing. Note: Bar stock O.D. is left “as is;” mill finish is adequate. No need to take a fee mills off so it looks pretty.

• Flange Thickness: 0.315” for the cathode flange. This is thick enough to engage 8.5 threads of the 1/8-27 Swagelock fitting with only a few threads exposed on the outside. Other flanges would be thicker, 0.370” or so, depending on flange type, i.e., cathode, intermediate electrode or anode flange.

• Material: #304 Stainless Steel. I felt the larger diameter (compared to the 1.5” O.D. aluminum) was better. Note: #304 machines fine using carbide tools, I use light feeds at 1,000 RPM for a good quality (shiny) finish.

• Bolthole Circle Diameter: 1.35825”. This equates to individual bolthole centers being 0.20” from flange outside edge. This leaves enough space between a bolthole and flange edge so the bolt head does not protrude beyond the flange edge. NOTE: I have not drilled a flange yet, but I believe the 0.20” is a good number from measuring an existing, similar flange. (NOTE: 0.020" from edge is adequate).

• Bolt Hole Separation: Six (6) boltholes equally spaced at 60-degree intervals.

• Bolt Hole Diameter: 0.1875”. This is a “close fit” for a 10-32 machine or socket head cap screw. A #12 hole (0.1879”) would also work. One mating flange could be threaded, unless using nuts is adequate.

12 February 2003. Ion Source Design Notes: Flanges and Molybdenum Anode Insert Details. Excluding the two “master flanges,” I’ve machined six other flange blanks. Next step is to mill the annular groves for the Viton o-rings and the central through-holes. With the 3-jaw chuck and rotary milling table on the milling machine, I can drill all six-bolt holes in a couple of minutes. the blank flanges now take about 10-minutes each to machine and includes cutting the bar stock, facing to length and drilling the holes.

I machined a “practice molybdenum insert" last night for the ion gun’s anode. The insert resembles a wire-drawing die insert. It is short cylinder with a conical hole on one side and a 0.4-mm through hole or orifice. It turned out so well, that it’s not a "practice insert" now. I did a little research and found that molybdenum machines much like cast iron, but with a few notable differences. Molybdenum’s machining properties depend on the ambient temperature and the original molybdenum processing. It can be more brittle than cast iron and is more abrasive; tool life is understandably reduced. Additionally, cutting lubricants are strongly recommended. These are the notable differences when compared to cast iron. Feeds and speeds are very similar.

Considering all this, I faced the 0.375” OD molybdenum rod at 1,000 RPM, using a wax lubricant and a sharp, carbide tool. I used the wax lubricant since it sticks to the work piece at high speeds. I took off up to 0.003” at a time. This was conservative, but I had never before machined molybdenum. The resulting finish was very good. I then tried 400 RPM when facing the other side and there was no noticeable difference in the surface finish quality. Based on this limited experience, the process seems to have a large speed range that will yield satisfactory results.

The next step was to machine the 90-degree conical cavity. I based the 90-degree dimension on ion gun design literature. To machine the cavity, I used a 90-degree center drill. This was not a normal center drill used on a lathe to create a center hole for the tailstock’s center (when turning on centers). This center drill was expressly designed to start a hole for a follow-on drill. As such, these center drills are short with a short flute length and are very rigid. Lathe speed was 400 RPM. I coated the drill and work piece with lubricant wax and a liquid cutting lubricant, since the speed was much lower. I used a 0.250” and then a 0.375” center drill. The first drill created a pilot cavity. I used the larger center drill to cut the conical cavity to its final dimension.

Next step is to drill the 0.4-mm orifice. I left about a 0.020” thickness between the apex of the cavity and the faced, outside surface. This will improve chances of successfully drilling a 0.4-mm hole in the molybdenum vs. drilling a deep hole. I’m not sure how much erosion this orifice will experience in service, but this is a prototype and we’ll see what happens. If I need to leave more material on one side of the orifice and drill a deeper hole, then I’ll try that approach. However, at the ion current densities we’re thinking of, this may be a non-issue.
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