Polywell Assembly Overhaul

26 08 2012

It’s clear from the inconclusive results of the symmetry test that our experimental apparatus isn’t durable enough. It needs to for a long period of time over the course of many trials, without breaking. Every time it breaks, we need to open up the chamber and fix it. In doing so, we inevitably make some slight change to the alignment of the components, the material makeup of the assembly, etc. Each of these changes introduces unknowns. This makes it difficult to compare the results of one set of trials to another, and thus difficult to accumulate the date we need to actually demonstrate something conclusively. To make matters worse, we aren’t getting deep vacuums because the much of the plastic and rubber components of the assembly are out-gassing.

It’s as though instead of running one experiment a thousand times, we run a hundred slightly different experiments ten times each.

In a word, the three main problems with the old polywell assembly are alignment, structural integrity, and vacuum compatibility. I designed a new assembly which should remedy these problems. instead of multiple separate components, it will be one solid piece of 3D printed ceramic which include a core, acclerator anode, hot cathode, and langmuir probe, all bolted to the 8” conflat flange:

The two feet (bottom left) will be bolted to the conflat flange.

Close-up of the hot cathode holder:

The cathode holder is actually two pieces which sandwich two 2o mm lengths of 10-gauge solid copper wire between them into the inside grooves (A). The grooves on the outside of the cathode holder accept zip ties which will hold the copper wires firmly in place (B). Once the wires are secured, the whole thing is put into place against the left column, and another zip tie is slipped through the hole (C) on the extreme left of the cathode holder and looped around the column. This connection will be strong  enough to prevent the cathode holder from moving during setup/normal operation.  A light bulb filament, serving as the cathode itself, is soldered to the ends of the copper wires.

Close-up of the anode holder:

The anode, a copper cylinder,  is put into the crescent moon shaped space (A), and a zip tie goes around it and through the hole (B), and secures it to the assembly.

Langmuir probe holder:

The langmuir probe fits int the groove in the cylinder (A), and attached with a zip tie or perhaps teflon tape. It extends into the center of the core, indicated by the blue sphere (B). The cylinder is oriented and positioned such that the langmuir probe will extend into the center of the core.

Here’s the new langmuir probe:

The new langmuir probe is a strand of wire inside a very thin ceramic tube. The 9-volt battery on the right is for scale.


The coils fit into the cavities in the core, and then covers go over them. The covers will be secured with zip ties or perhaps hose clamps.

Other than the zip ties and wires, all of this will be made of ceramic. The zip ties will be made of tefzel, a strong, heat tolerant, and highly vacuum compatible material similar to teflon. All wire insulation will be teflon.  This assembly will be heat resistant, electrically insulating, and much more rugged than previous designs. Ideally, we will be able to put it together, put it into the chamber, pump down to much deeper vacuum, and do hundreds of trails without anything breaking. moving, or changing shape.

In order to work at all, this design has to be compatible with the vaccum chamber and conflats that we already had. If one dimension is even slightly off, then the whole thing fails. To prevent that, I first took measnurements of the chamber and flange, and maodeled exact copies of them in OpenSCAD, and built this assembly inside the chamber :

Here you can see the conflat flange (left) and the chamber. Notice that the blue sphere, which indicate the center of the polywell is not centered in the chamber. By offsetting the core slightly, I was able to get more clearance between the walls of the chamber and the core, which in turn allowed for a larger coil radius.

Another pic of the whole thing:

While I have uploaded these models to shapeways, they will probebly not be the ones we actually have printed. This is more of a first draft.

The source code is here

Domenick Bauer



15 responses

26 08 2012

What vacuum is your target? I’m used to mass spectrometers that run anywhere from 10^-5 to below 10^-10 Torr (and this with a hole in the front introducing ions and a whole lot of air) and every time I see one of your vacuum gauge photos I’m vaguely surprised that a turbopump is happy running at such high pressure on a sustained basis, though I suppose it’s not really super high.

27 08 2012
Dave Price

That’s an interesting setup, using an anode instead of the Magrid. It will be interesting to see how deep a well you can get that way.

If you could electroplate the ceramic casing surfaces, you could have a Magrid too. Instant WB-6!

Bet Bussard would have loved having a 3-D printer, might have gotten from SCIF through HEPS to WB-6 in a year or two instead of a decade and a half.

1 09 2012

Electroplating the casings is an interesting idea. I wonder if there might be a glaze conductive enough to serve that purpose. It seems unlikely because glazes are usually mostly glass, but I’ll look into it.

27 08 2012

Looks neat! Finally finidhed reading ^^ lol

29 08 2012

I’d think that with a conductive magrid shell at proper potential, needed for recirculation, you wouldn’t need a separate anode for electron injection. Just emit electrons on one of the appropriate cusp symmetry lines.

1 09 2012

That’s probably true. Many people have suggested using the magrid, and it’s definitely something we’re looking into. Right now, we’re trying to reproduce the Sydney Experiment, which used a separate anode for electron acceleration and no magrid. This approach circumvents the problem of insulation the magrid shells from the coils

30 08 2012
Matthew I

I’m excited to see how the printed ceramic comes out – it really does seem perfect for this. Given the tolerances, I assume you’re prototyping in ABS first?

1 09 2012

We thought about it, but because ceramic is such a finicky material, we decided that an ABS prototype wouldn’t have enough relevance to the ceramic version to really be helpful.

31 08 2012

Have you printed any ceramics with Shapeways? I have. If you have not, I think you might be disappointed. The dimensions are not likely to be as precise as you hope for. A, worse problem will be that your 3D model will be slathered with thick and sloppily applied glaze.

1 09 2012

Yeah, we have. Due to the problems you mention, getting this right is going to be a real challenge. To try to mitigate such problems, we’ve had some test pieces printed. Simple shapes (holes, cylinders, cubes) of various sizes which will help us get a handle on the relationship between the digital design and the physical product. They should be in the mail, and I’ll post pics when they come.

1 09 2012

Have you checked out the
ULTEM* 9085

From what I understand it’s high heat tolerant and vacuum friendly. Here’s the datasheet.

I doubt you’re going to have problems with the print quality with this material on a Stratasys 900mc for example.

Stratasys is working with another company to combine 3d printed electronics. So you maybe you could combine the two and have your conductive shell.

1 09 2012

Oops, double post. Please delete the one with my email in the name form. *here come the spambots

13 12 2012
Vidya Wasi

Based on the talk “Mark Suppes: Full talk from Wired 2012” I got the impression you updated more often. Checking github and this blog made me disappointed.

I’m an impatient guy so i must ask:
How is it going ?
Is the project stalling ?

To a guy that have not read about fusion or the whole blog it looks like you are repeating the same experiment over and over. Rebuilding and improving the toys as you go on. Yet no new results as in progress forward to the goal.

What i’m wondering is:
A: When will you be able to power up a light-bulb with this thing ?
I’m guessing you have learned enough to make some sort of prediction “guestimate”.
B: How will you do that ?
Fission use steam with turbines. Will this use that too ?

9 01 2013
Mark Suppes

Hey Vidya, Sorry to disappoint. I’m disappointed with the project myself sometimes.

Over the years this project has progressed in fits and starts. At the moment I’m busy making a living.

But one thing this project requires is patience. So exhale that breath you are holding and enjoy the show!

2 02 2013
Ben Hynes

Excellent! I love watching the progress. Fusion is the future.

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