Finn Hammer, fusor update.

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Matt_Gibson
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Re: Finn Hammer, fusor update.

Post by Matt_Gibson »

What are some of your mR/hr values that you’re seeing each 1/2 second? I’m seeing values that average 4mR/hr, with a few as high as 6.8mR/hr. Multiplying by 350 seems too good to be true for my setup and run parameters (best performance was at 40kV at 9mA, 14microns). This is with the hammer 1in from chamber body.

-Matt
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Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Back in October I posted this picture of the Molybdenum kathode equipped feedthrough:

viewtopic.php?f=18&t=13936&p=92977#p92977

And since I have moved further with it, here is the description I promised.

First a couple of words about why I want to cool the kathode:
Running the kathode hot seems like a good idea. If you look at the 2 following pictures, the first one has a blue hue to it in a muddy atmosphere, and the kathode is still relatively cold:

Yeah yeah, I know I could chase leaks to the end of the universe, or perhaps diff pump each entry to the chamber.....<br />Come to think of it, I just might do that in the next iteration.
Yeah yeah, I know I could chase leaks to the end of the universe, or perhaps diff pump each entry to the chamber.....
Come to think of it, I just might do that in the next iteration.


On the other hand, in the following picture, the kathode has been red hot for a bit, and now the chamber appears to be clean:

20211218_110943.jpg

However, when the power level rises beyond some 2kW, the kathode gets so hot that the fusor gets almost impossible to control. I assume that the kathode starts to emit electrons which recombine with the deuterons, and this accounts for the wild fluctuations in pressure, and the associated current draw, whatever, this situation has to be adressed, and cooling the kathode seems like a, -well, I just wanted to try it out.

Most of the feedthrough is, although flashy to the casual eye, just simple compression fittings disguised as field control toroids.
It is the internals that matter.

Here is a closeup of the first iteration:

feedthrough1.JPG


As you can see, the cooling airflow in the center tube, is brought up close to the kathode, and reverses in the "sponge" like endplug, where it returns to the outside through the bigger pipe, both copper. The kathode is screwed into the "sponge" with a 6mm thread, and all this works well.

However, this feedthrough, with these dimensions is limited to 40kV by arching from the 12mm copper tube to the 19mm internal diametre of the quartz tube.

This led me to rev. Mkll, which looks like this, the one I am using now:

feedthrough2.JPG

As you see, by adding length to the molybdenum stalk between the kathode cylinder and the sponge, I gain an increase in clearance by almost 100%, from 3.5mm to 6.5mm.
This has increased the stand off voltage to 75kV.

But it has also decreased the ability to cool the cylinder of the kathode, which has become even more desirable, since the increased voltage has also enabled a drastic increase in input power.

There appears to be 2 possible paths away from this blind alley, one is to increase the diametre of the glass tube to, say, 32mm internal. this would allow me to return to the previous kathode/stalk configuration, with now 10mm clearance, and this should then extend the voltage standoff towards, or even slightly beyond the magic 100kV.
Another option is to ditch the glass tubing, and switch to Macor. A suitable Macor slug is up and around 300$, so for now, I will stick to glass.

Trying to determine the voltage standoff properties from a theoretical point of view has not been an easy task. I see mention of the works of a certain gentleman named Mr. Paschen in this context, and even smirky comments like: "Now, there is Paschens law to you, my friend"

Is that really so? Can we use Paschens law at micron levels? I find that hard, with pd products between 0.01 to 0.03 torrcentimetres, from my view we are way off the chart to the left:

Paschen_curves.svg.png

I know there is radiation, heat and all sort of stuff going on to confuse matters, but:

Perhaps Maciek Szymanski found the right passage in the right publication, when,

in this thread:

viewtopic.php?f=25&t=13111#p85254


he wrote:
Maciek Szymanski wrote: Sat Dec 14, 2019 5:03 pm Snip.


For the other hand the statement of L. Loeb Form almost 100 years ago seems to be still true, as the glow discharge is no longer a fashionable research subject:

About this region which lies in general below the pressure of the minimum sparking potential, ranging from 10e-5 up to 10e-1 mm of pressure, very little is known. Leonard B. Loeb “Fundamental Processes of Electrical Discharge in Gases” John Wiley & Sons, New York 1939, p. 476.
Cheers, Finn Hammer
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Richard Hull
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

You mention once the cathode gets really hot around 2KW it become impossible to control. This is the famous electron runaway. Your cathode is becoming a heavy electron source that can runaway as the current wants to go through the roof. Thermal based electron emission coupled with high field emission will be a runaway condition. Solution: Instantly back off gas pressure and or the voltage. This will reduce the current and allow the cathode to drop back to a thermal level of comfortable electron emission and reduce the field. We are always at the edge of arc break down in out gas glow lamp (fusor). While not the classic hot arc, electron runaway precedes the arc condition. To take this to true arc you would need a power supply capable of many amps. Long before arcing occurs in our fusors the cathode would melt or the typically weak high impedance fusor supply would fail.

Wall or target loading can under electron runaway feed this condition with a sudden rise in gas pressure as the electrons emitted by the cathode slam into the target/walls releasing significant amounts of deuterium/deuterons which add to the current load.

I have discussed this state rather constantly in posts and Some FAQs since 2000.

We all find that peak of operation where this happens. You have found yours. So much of this runaway is related to grid construction and the chamber design. High field emission points are to be avoided. Sharp right angles facing the grid anywhere within the chamber or sharp angles in the cylinder grid ends or very fine wires in a spherical grid will set the limits in most situations where power applied forces the high fields to run away.

Even with no sharp or field emission points in a well designed fusor, thermal grid electron emission and wall unloading will still set a limit.

As for muddy vs. clean in the view port. when I see no glow in a clean, clear field with the grid at some high temperature and beaming flawlessly clean, I know I am at or very near an optimum operational point and am very close to possible runaway. I tread very softly to boost operation from this point. I never add more gas at this point just slow increases of voltage while watching the current like hawk! Increasing the voltage here increases the current by as little as .5ma. I make sure the current remains constant or falls back a bit. I can only then inch the voltage up. If the current starts to rise I see it rise very slowly and then back off. If it does not, but increases ever rapidly, I am at the runaway operating point.

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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