Tungsten Cathode, no Welding Required (Rosenstiel grid)

[Jon Rosenstiel]

The photos below should give a pretty good idea of the process.

I used 0.026" diameter tungsten wire from Small Parts Inc. (Part# TW-026-60) This particular wire was amazingly ductile for tungsten wire. I also attempted to make a grid using 0.022" wire but it was much too brittle. As the saying goes, "your mileage may vary".

The tubing is 7 gauge thin wall 304 stainless steel hypodermic tubing from Small Parts also. (Part #HTX-07T) My HV feedthrough stalk has an OD of 0.156" and the 7 gauge tubing has an ID of 0.160". Produces a nice friction fit.

I cut the slots in the end of the ss tubing using a thin cutoff wheel in my Dremel tool.

There was no difference in performance (neutron count) compared to the more traditional 5 circle grids I had been using.

This grid will take some punishment. I've run my fusor at over 1kW input for 3 to 4 minutes at a time. And she ran exceptionally stable!

Jon Rosenstiel

Edit: As the photo here was lost, I have taken the liberty of substituting a diagram of the construction. If Jon has this image as a photo he can insert it to back up the diagram or contact me to edit it in.
I have done this to make the fine grid idea of Jon's retain a visual aspect for other to see how it is done.
Richard Hull 11/29/19

[attachment=0]Tungsten Cathode.jpg[/attachment]

[Q]

what a neat grid idea! and so simple too.

Q

[Starfire]

Thanks & congrats. Jon - miniaturisation indeed, this must be the smallest grid ever made - and punches a Kilowatt. These photos should be archieved. ( what are you like at watchmaking?

[Adam Szendrey]

A great idea! Thanks Jon!

Adam

[Richard Hull]

This is how some of the grids were made by Miley's group. (they have built many systems, by many grad students, over the last 20 years.

Some were also assembled by bundling the long stems, as well, instead of the tube held method.

The other grids, (cross ways) can be attached by binding with 1mil or similarly fine W wire.

Good work Jon. Tungsten will have the same cleanup issues as Ta, but have the edge on the melting point.

I might try my hand at this method. at some point and it might just be NOW as my current grid system is faulty. I don't know if I will have time to implement a lot of stuff for fusor IV before the bash, but I am moving on some of them.

Richard Hull

[Starfire]

I like the tie-down - photo 3

[Jon Rosenstiel]

John,

The grid isn't as small as it appears in the photos. I think what may have thrown you off are the 3, nested, alumina tubes in pic #5. The OD of the outermost tube is 3/4" (19mm).

The grid is about 1.3" in diameter. (33mm). That makes it about a 5:1 ratio to the 6" ID of the outer shell.

I should have included something in the photos to indicate perspective.

Jon Rosenstiel

[Brian McDermott]

Is that the same grid shown in your recent "supernova" post to Images du Jour?

Is there any advantage to having the crosswise rings on the grid as opposed to simply the setup shown in the photos?

[Richard Hull]

I wouldn't image the cross links are critical provided enough vertical circles are used to fool the e-field into thinking it is a perfect sphere. It is sad that the better the transparency is, the worse the field uniformity is. There is a point, however, where the field is virtually perfect and yet the grid is still relatively, ion transparent.

Richard Hull

[Jon Rosenstiel]

Brian,
It's not the exact same grid as in the supernova photo, but is of the same construction and size.

Jon Rosenstiel

[Richard Hull]

Adam, You cannot have any sharp edges anywhere in the central grid area, including those knife edges on the mechano piece.

Everything needs to have a large radius with no knife edges on sheet metal, no points on nuts, bolts or wire ends, No protrusion, no matter how small, outside of the grid sphere. Every thing that is not GRID should, ideally, be shielded or insulated against field emission.

Richard Hull

[Starfire]

Jon - what is material and source for ' HV feedthrough stalk has an OD of 0.156" ' Please.

[Jon Rosenstiel]

John,

Here's a link to the HV feedthrough I have. (eBay, what else?)

http://isi-seal.com/pdf/sec-4-4.pdf

Scroll down about 3/4's to model # 9442013. This should clear things up. (BTW, I'm refering to the center conductor as the feedthrough stalk).

Jon Rosenstiel

[Brian McDermott]

How did you actually make the rings with accuracy? I can't seem to get things right and things will not wind up fitting into the tubing. Could the problem be that the tubing is only 0.1" ID?

Anybody try a woven grid using finer tungsten wire? It seems like a possibility. If so, how was it done?

[Jon Rosenstiel]

Brian,

I imagine the tube having an ID of 0.1" is going to make things a bit more difficult, if not impossible.

To wind the wire I did a few test winds on various sized mandrels until I found a mandrel that produced loops close to the grid diameter I wanted to end up with. I then cut out the loops, flattened and "tweaked" them to size and then bent back the "ears".

FYI, the grid I show in the photos was probably my 8th attempt, so it does take some practice. (What grid doesn’t)?

Jon Rosenstiel

[Richard Hull]

I actually show this process on Fusor tape #2. I have also posted on the method before. It sounds like Jon took it to heart. This is the way jewelers make perfect circles and rings for jewelry repair and custom work.

1. Wrap multiple turns around a mandrel (12 or more)

2. Let the wire spring open of it own accord. You now have circles a bit larger than the mandrel.

3. Using diagonal cutters, cut the rings out leaving whatever overlap you need to complete the circle either by overlap welding or butt joint fusing or soldering.

In the above fashion, you have made 12, perfectly uniform rings in under 3 minutes that are ready to use. Nautrally, they can be slightly opened or closed to make slight adjustments for slightly oversized or undersized rings to fit together. Regardless, the hard work of forming good rings is done.

Richard Hull

[Jon Rosenstiel]

The original grid (as shown in my first post) finally died after two years in service. The outermost grid loop worked loose and had moved away from the other three loops 5 to 10 mm. This made the fusor very erratic; it became almost impossible to find a voltage / current combination that would allow stable operation.

New grid time: The new cathode is the same basic design as before, with a couple of improvements. (Hopefully they’re improvements)!
1. I managed to do a much better job of grinding the slots into the end of the SS tube by holding the tube in my lathe’s chuck (marked off in 45 degree increments) and then carefully grinding the slots with a tool-post mounted Dremel tool.
2. Before bending the grid retaining tabs over I gently tapped a snug fitting piece stainless steel (the threaded portion of a 4-40 screw) down into the end of the stalk to hold the tungsten grid loops tightly against the inside of the stalk. This made the final “tweaking” of the loops into alignment much easier and hopefully permanent. See attached pic.

Jon Rosenstiel

[Richard Hull]

A very fine piece of work, Jon. Let the inventiveness of man be consecrated to his continued advancement.

Richard Hull