Cube Fusor Cathode Comparisons

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Jon Rosenstiel
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Cube Fusor Cathode Comparisons

Post by Jon Rosenstiel »

My stainless-steel cylindrical cathode (OD= 0.75”, L= 0.75”) was running hotter than I cared for and I wanted to do something about it. While browsing the forum I ran across this post viewtopic.php?p=96875&hilit=cathode#p96875 in which Liam David explained that the cathode’s energy is lost mainly through radiation via surface area. Ok, so let’s try increasing the surface area and see what happens.

All runs were done at an input of 50 kV, 10 mA. The cube’s water-cooled base was connected to a lab chiller set to 5-degrees C. All cathodes were machined from 304 SS and all have a wall thickness of 0.038”

Run 1, original cathode: OD= 0.75”, L= 0.75”, surface area= 1.77 in^2.

Run 2, OD= 0.7” L= 1.25”, surface area= 2.75 in^2: This is a modified version (0.25-inches longer) of a previous cathode that performed well. I was surprised (and a little alarmed) that the chamber pressure was considerably higher. NPR (neutron production rate) was also up a little so let’s continue.

Run 3, OD= 0.7” L= 0.875”, surface area= 1.92 in^2: Run 2 cathode with its ends trimmed a little. Wow, 3/8” of an inch off its length and the chamber pressure dropped quite a bit. (And NPR was up yet again) At this point I decided to change course, forget overheated cathodes and try some other things I’ve been mulling over.

Run 4, OD= 0.625” L= 0.625”, surface area= 1.23 in^2: Chamber pressure dropped a little more and NPR, compared to my original cathode, was considerably higher. (Now we’re talking!)

Run 5, OD= 0.5” L= 0.5”, surface area= 0.79 in^2: Continuing with the “make it smaller and let’s see what happens trend”. Chamber pressure dropped yet again. NPR took a huge dive.

Ok, the run 4 cathode has the best performance and that’s what I’m going with. (Max neutrons above all else!) Compared to my original run 1 cathode, chamber pressure is 1.4μ lower and NPR is up nearly 20%. To counter the overheating issue I’ll modify my input parameters a little by running higher voltages and lower current. (Higher efficiency) If I’m still having overheating problems I may attempt to make a molybdenum cathode similar to the one Finn Hammer has so artfully constructed. viewtopic.php?p=93838&hilit=kathode#p93838

In the data I could see that cathode surface area and chamber pressure were related. To get a clearer picture I plotted their relationship in the below graph. It seems that increasing cathode surface area creates a denser “background plasma” (the plasma formed between the cathode’s outer surface and the chamber’s walls).

Jon Rosenstiel
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Richard Hull
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Re: Cube Fusor Cathode Comparisons

Post by Richard Hull »

Great work and report Jon! Cube owners take note. The data plot speaks for itself. There is a relationship to grid dimensions. The issue might be surface area or getting the chamber size to cathode size ratios correct. Interesting.

Richard Hull
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Jim Kovalchick
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Re: Cube Fusor Cathode Comparisons

Post by Jim Kovalchick »

Jon,
With all the different openings of the chamber to change out the grids, are you convinced that none of your numbers are influenced by the need to condition the chamber walls?

Jim K
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Richard Hull
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Re: Cube Fusor Cathode Comparisons

Post by Richard Hull »

Jim brings up a point. When doing an experiment where one change in a system is made. All associated parameters must be tightly controlled to compare apples to apples. I still wonder about there being some ideal grid size and form to chamber size ratio.

Has anyone noticed or reported wall loading differentials in aluminum of cube fusor setups?? This may not be a big issue in aluminum.

Still, Jon's work tends to tie down an ideal size in his case to some degree. Results speak!

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Liam David
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Re: Cube Fusor Cathode Comparisons

Post by Liam David »

Great work!

I'm with Jim on this one. The total variation in NPR is ~20% (which seems low for that variation in cathode dimensions) but could be within the noise of other factors. My suspicions are two-fold:

Opening and closing the chamber so many times makes contamination that much more of a concern. I'm not at all suggesting that you're not careful--from my experience, just venting and re-pumping the system, without even touching the geometry, can cause variations that large unless everything gets baked out and pumped to e-8 torr levels.

I'm pretty convinced that the cathode dimensions are really important only when the cathode/chamber symmetry/alignment is damn near perfect. Slight positional or angular inaccuracies seem to really mess things up, especially with smaller cathodes.
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Richard Hull
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Re: Cube Fusor Cathode Comparisons

Post by Richard Hull »

In small cramped volumes of most cubes the cathodes are indeed tiny and, as noted, alignment to a significant degree is the order of the day. In a 6" sphere, some slop in alignment is allowed with little effect on a one inch diameter wire grid located far from any sharp tubulations on the outside of a the sphere. On cubes of most sizes operated here, four sharpish edges are very near the grid.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jon Rosenstiel
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Re: Cube Fusor Cathode Comparisons

Post by Jon Rosenstiel »

Guys,

After changing out each cathode I did a pump & bake at 102 ~ 105 C, but due to conductance issues I’m only able to reach the e-6 Torr levels, not the e-8 levels Liam suggests.

With the aluminum cube there seems to be very little loading as after a pump & bake NPR will fairly quickly (sometimes under 5-minutes) reach a maximum level and then pretty-much stay there no matter how long the system is run.

I should note that to help prevent contamination I vent my system through a molecular sieve, and to make cathode changeouts go quickly I’ve constructed a fixture to help with cathode alignment.

But as always, fusor run data needs to be taken with a grain of salt.

Edit: To go with that grain of salt: My setup is old-school; everything is manual control with analog meters. Data collection is a pencil and a yellow pad.

Jon Rosenstiel
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Re: Cube Fusor Cathode Comparisons

Post by Frank Sanns »

Great job Jon!

I will attest to the outstanding techniques and procedures that Jon uses for his runs. Data that comes from Jon is golden. Every time I visit his lab, I am super impressed.

Jon thinks through things, checks, double checks but in the end his experience gives him the confidence in his own results long before he reports them. He knows when the conditions have stabilized and when there is a good reading to be had.

As for old school, I will take that any day as it requires thought as the results are being put down. Automated systems can gather the data but sometimes astute and important observations during the concentration of manually writing things down get missed. Keep doing what you are doing Jon, You have my confidence.
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Jon Rosenstiel
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Re: Cube Fusor Cathode Comparisons

Post by Jon Rosenstiel »

Frank,

Appreciate your thoughtful (and somewhat embarrassing) comments, but had I been a little more forthcoming with my test info like I had initially planned Jim and Liam most likely wouldn’t have had to question my test results.

JonR
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Richard Hull
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Re: Cube Fusor Cathode Comparisons

Post by Richard Hull »

Yes, When Jon reports on an effort, I tend to assume his work is well done and will always impart some new knowledge. I took away from his work that you can, in a small cube fusor, tune the cylinder grid to a point of improving performance. Well done.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Liam David
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Re: Cube Fusor Cathode Comparisons

Post by Liam David »

Jon, the additional details you provided give me much more confidence in the results. I know you're a superb experimenter and my comments were not meant to question that. Again, great work!

Given the relatively small variation in neutron rate (~20%) for such a wide range of cathode sizes, I would invite people to consider that maybe we're thinking about cathode optimization wrong.
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