Tungsten needle cathode

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Jon Rosenstiel
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Tungsten needle cathode

Post by Jon Rosenstiel » Mon Feb 18, 2008 9:43 pm

In the recent thread “One-inch diameter 316 SS solid cathode test” Richard Hull posited that sharpened tungsten needle may produce fairly impressive neutron numbers at low input voltages.

The “One-inch diameter 316 SS solid cathode test” thread is located here:
http://www.fusor.net/board/viewtopic.ph ... 796#p12306

And Richard’s post is located here:
http://www.fusor.net/board/viewtopic.ph ... 796#p16956

Pic #1, data: Richard was correct, at 10 kV, 10 mA the neutron output for the sharpened tungsten needle equaled that of my normal 4-loop tungsten wire grid.

Pic #2: 0.062” sharpened tungsten needle before fusor runs.

Pic #3: After ~77 minutes of fusor operation, maximum applied power was 200 W (20 kV, 10 mA) for a period of seven minutes. No "wear" whatsoever on the needle... it could have easily taken more power.

Pic #4: Shot off of the video monitor… “Look ma, no rays”! The white spot near the 4-o’clock position is a reflection; it wasn’t visible with the naked eye.

Jon Rosenstiel
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Carl Willis
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Re: Tungsten needle cathode

Post by Carl Willis » Mon Feb 18, 2008 10:31 pm

Very nice experiment, Jon.

It looks like at low voltages, the needle is the way to go, equaling (maybe even besting) the hollow cathode. At high voltages, the needle falls to second place, still much better than the solid ball cathode. I'll have to think about this trend a bit and see if there is any good explanation.

The needle likes pressures that are intermediate between the solid and hollow spherical cathodes. Field effect may come into play to make the needle a more efficient ionizer than the solid cathode.

-Carl
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Re: Tungsten needle cathode

Post by Starfire » Mon Feb 18, 2008 11:00 pm

I also note the 7640v 10ma @ 64mT - there's hope for MOT's yet - great work Jon and the photo is beautiful.

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Steven Sesselmann
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Re: Tungsten needle cathode

Post by Steven Sesselmann » Tue Feb 19, 2008 6:05 am

Nice work Jon,

Operating at higher pressures than the wire grid, neutron numbers are going to be better.

Are you going to try higher voltages?

Steven
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Re: Tungsten needle cathode

Post by DaveC » Tue Feb 19, 2008 7:21 am

Jon -

Very interesting indeed!

About the needle, do you have an idea approximately what the tip size is?

A needle electrode brings some new things to the fusion table. One of the most important is the ability to be in a field emission mode, at rather low voltages. If the tip radius was around 0.05 mm or less, then you could be on the edge of field emission with voltages around 10 kV or more.

Most of the time, a wire grid cathode will not be a field emitter, unless the voltages are very high. Electron emission is either when the grid goes into thermionic emission, which will requires temps above 2200 - 2500 K for significant current densities, or from secondary emission by ion bombardment..

In contrast, the needle could produce a significant space charge out in front where the ions arrive. Because of the very steep voltage gradients, even after the space charge sets up, the acceleration of the incoming ions will tend to be quite close to the point where they enter the electron cloud.

Since overall neutron production rates are about as high as with the wire grid, yet the reaction volume would seem to be very much smaller, the neutron volume density must be much higher.

I wonder how the neutron production would go, with lower pressures and higher voltages?


Dave Cooper

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Re: Tungsten needle cathode

Post by John Futter » Tue Feb 19, 2008 8:14 am

Jon
Many years ago I had to make electrodes for my prof out of tungsten, he required that the tip be smaller than quarter wavelength of blue light.
This was accomplised by electropolishing the wire by reciprocating the wire in a bath of potassium hydroxide watching until the wire at the surface layer became so thin that you then pulsed the current until the end fell off.

one in ten were within spec each taking about two hours to make. The tungsten cathode was then inserted in a gridded chamber filled with liquid xeon at room temp and field emission tests conducted.

alot of time has gone by since the late seventies and now I also play with carbon nanotubes with field emission specs of 0.25volts per micron spacing.

I wonder if substituting the tungsten needle for a carbon nanotube emitter might give even better yields at lower voltages????????

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Richard Hull
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Re: Tungsten needle cathode

Post by Richard Hull » Tue Feb 19, 2008 3:07 pm

Jon, Great work!

I am so happy you took to my suggestion. I wish I had time to do this stuff and was retired like you.

I have had this idea for some years now, but just never moved on it.

I just knew that this would boost low end fusion, but, like Carl, also figured it would not beat the hollow cathode at higher voltages due to emissive runaway. AS I have harped before, there is always a gotcha and I figured this gotcha would occur at some median voltage level where the wonderment of low end fusion would nose dive into power hogging. I feel field emission takes over from fusion and actually defocuses the act a bit. I do not believe for one minute that a hollow cathode will focus to a micro pin point the way the needle point gradient would. The stalk to point diameter and even the taper form factor might alter results, but I bet not by much. A long cone might even be the best.

I was sort of hoping the temp of the tip would form a micro ball of sorts. Sort of a self assembling, optimized sphere. Perhaps SS or a copper needle might do this.

I am with Dave on the explanation. I just knew in my gut that this would focus the ions better, up to a point due to the intense field gradient.

John Futter's comment related to nano tubes is sage and might be an interesting experiment. We are trying to focus all ions to ideally the nuclear diameter range!

Jon, if you would like to try something, try going sub micron in pressure with this needle tip senario. It might be real tricky. This will hold the current hogging back a bit and allow for a longer mfp for the ions. The electrons are virtually all doomed to be wall slappers. You might see an increase in fusion at lower voltages and far less current. This needs to be fondled a bit more before moving on.

While the yield might be low at 5kv, the efficiency might be very high. More ideas and no time. I wish I could go home and fart about on these ideas.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Tungsten needle cathode

Post by UG! » Tue Feb 19, 2008 3:28 pm

could a picture of the profile of the plasma be taken? i'm assuming the plasma pic is end on, so were just seeing a hot ball, it would be interesting to see to what extent the rest of the needle is surrounded by plasma, and weather it is a ball at the tip, beyond the tip, in sheath covering the whole needle, in cone shape at the end of the tip etc

Oliver

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Richard Hull
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Re: Tungsten needle cathode

Post by Richard Hull » Tue Feb 19, 2008 3:33 pm

Interesting thought, but before Jon answers, I would imagine that the intense gradient at the tip would hog all the plasma action, the rod is a smooth radius and I could not imagine much of a glow there.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Jon Rosenstiel
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Re: Tungsten needle cathode

Post by Jon Rosenstiel » Tue Feb 19, 2008 4:48 pm

Steven-
At some point I may try higher (and lower) voltage and current values, but for the moment I’m burned out on fusor testing. Every time I change the grid or open my fusor the neutron output drops… but that’s a topic for another post.

Dave-
I really have no idea of the size of the tip…. I sharpened it on a 60-grit grinding wheel, just as I would sharpen a tungsten electrode for my TIG welder. (FWIW, it easily drew blood)!

John F.-
Interesting process… and ideas, thanks for sharing.

Richard-
I’ll play around with this more in the future, but right now I’m stressing over my fusor’s neutron output. (About one-half of what it was before I started on these cathode tests).

I want to modify the needle’s attachment method, try to do away with the set-screw holder so the majority of the length of the needle can be shielded with an alumina tube. I also need to make some changes to my power supply to make control (and measurement) at these low voltages and currents a little easier. Right now it’s kind of like driving a Ferrari in the Mobil economy run! I’ll keep your suggestions in mind.

Oliver-
I had the same thought. Next time I’ll attach the viewport to my unused ion source port; instead of head-on the camera will be “looking” at the needle from an angle of 45 degrees. But I’m afraid that the intense brightness at the tip will overwhelm the camera and as Richard mentioned “hog all the plasma action”.

Jon Rosenstiel

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