Small demo fusor for educational purposes

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Maciek Szymanski
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Small demo fusor for educational purposes

Post by Maciek Szymanski » Fri Nov 22, 2019 9:35 pm

As I’ve mentioned in my introduction post I’m employed in Plasma Physics and Laser Microfusion Institute in Warsaw and recently build a small demo fusor for our stand at Scientific Picnic. I’s an outdoor event open to public at all ages and promoting science. The requirements for the fusor were:
  • The device must be portable.
  • must be safe to operate, even by spectators.
  • It must be reliable enough to be operated almost continuously for 12 hours.
  • The system must be straightforward so it can be easily explained to a layperson.
To meet the safety requirements the HV power must be restricted in voltage and current. There is of course only low vacuum rotary vane pump, and the fusor will be running on air.

The chamber is cylindrical with 100mm diameter and about 100mm depth. There are 5 DN25 KF flanges and one DN40. To the DN25 there are connected: pumping line cut-off valve, HV feedthrough, micrometric needle valve for pressure control, and TC vacuum transducer. On the DN40 there is a small viewport which may be used for the camera or for the optical spectrometer.

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The HV feedthrough is adapted from an old glow discharge vacuum transducer. Not a perfect design, but usable. The inner grid ist TIG welded 0,4mm stainless steel wire. The stalk is aluminum with glass tube shielding. There is no outer grid.

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First test run. The power supply is and old 4kV 5mA Polon ZWN-41 unit used for powering radiation detectors. It was possible to get the discharge, but the supply turned out to be unreliable. Because of the decade type voltage adjustment it was not possible to go beyond 1999V and at every overload the supply must be reset.

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Because of those problems I’ve picked up another supply - this time an real oldtimer: 1971 Vacuum tube ZWN-2,5 rated 2.5kV and about 20mA. After brief overhaul it turned out to be built like a tank and fully functional, including capacitors.

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The second supply is reliable, and much nicer to operate. The voltage can be adjusted with 200V increments plus additional knobs for 20 and 2V steps. This time a stable and controllable plasmoid was formed.

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Today I’ve added a 10Mohm shunt to allow current measurement with a digital voltmeter and mounted the fusor on the stable stand.

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For the moment I think that the inner grid is a bit too big for the chamber size and that maybe installation of the spherical outer grid may improve focusing. But generally I think this setup should give some fun to the visitors (they will be allowed to adjust the voltage and pressure themselves).
Last edited by Maciek Szymanski on Sat Nov 23, 2019 8:24 am, edited 1 time in total.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Mark Rowley
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Re: Small demo fusor for educational purposes

Post by Mark Rowley » Sat Nov 23, 2019 3:34 am

For a demo it looks great! With just a little more work it'd be fusion capable in short order.

Mark Rowley

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Richard Hull
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Re: Small demo fusor for educational purposes

Post by Richard Hull » Sat Nov 23, 2019 3:50 am

Fabulous work! If you have an optical spectrometer that is highly accurate, look at the H beta line for hydrogen and figure out the doppler broadening of that spectral line. From this you can get and idea of the velocity spread of the deuterons. I worked with Dr. Robert Bussard in 1999 doing just this in my Fusor II and fuso III as they had a specially adapted Ocean Optics spectrometer to look specifically at this line and critically measure its broadening.

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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Maciek Szymanski
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Re: Small demo fusor for educational purposes

Post by Maciek Szymanski » Sat Nov 23, 2019 12:39 pm

Fabulous work!
Than you very much! It wasn't difficult taking in account that I've access to lots of old, no longer used vacuum and physical laboratory equipment. Plus maybe a bit old fashioned but completely equipped mechanical workshop. Kind of an amateur experimenter dream :-)
I worked with Dr. Robert Bussard in 1999 doing just this in my Fusor II and fusor III as they had a specially adapted Ocean Optics spectrometer to look specifically at this line and critically measure its broadening.
That's nice to hear. I was thinking about this idea, and as you say it has been done. What kind of accuracy you were able to achieve? We have OO spectrometer in the laser laboratory, but I'm not sure if I will be allowed to take it to the event. For educational purposes it would be nicer to have a purely opto-mechanical instrument. The "black box" electronic spectrometer it's not a good thing to explain the measurement principle.

The way to doing fusion with this device is indeed quite obvious. On the whole system photo, behind the furor stand there is 300l/s oil diffusion pump with all the control valves, the ion vacuum probe and the cooled oil baffle with a flange fitting directly to the fusor chamber. And the big gray unit on the upper self of the rack is the pumping system control unit (for the demo fusor serving merely as a vacuum gauge and rotary pump switch). So the real upgrades needed are in HV - a real feedthrough and proper high current capable supply. But for the portability and safety reasons this will be not allowed at the Scientific Picnic. Nevertheless in the future I hope to produce some neutrons in my spare time :-)
Last edited by Maciek Szymanski on Sat Nov 23, 2019 6:19 pm, edited 1 time in total.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Richard Hull
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Re: Small demo fusor for educational purposes

Post by Richard Hull » Sat Nov 23, 2019 6:17 pm

Ocean Optics made a great USB connected spectrometer that they could hone with various slit sizes and narrowing the band width such that only a tiny part of the spectrum could even be seen of scanned. While this limited its capability to a very narrow bandwidth, it increased significantly the accuracy over that narrow, custom tuned portion of the spectrum ordered by the customer. Bussard and his engineer ordered their spectrometer custom tuned to the narrow H beta line region of the optical spectrum. The accuracy was very good and allowed us to see that with 5kv applied to the fusor, the bright central region in the grid area had the typical Maxwellian distribution in the doppler broadening with the fastest deuterons on the tail at about 4 keV in both directions with a peak around 2keV.

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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