Jim Kovalchick -My attempt at smaller fusor

[Jim Kovalchick]

I got rid of the alumina. Unfortunately, the problem I added the alumina for in the first place is back. At 38 kV I am hearing nasty arcing back at the connection of my feedthrough. I may just be at it's limit.

More bad news: Better symmetry has bought me a prolonged occurence of an electron beam orthogonal to the axis of my grid openings directly at the center of my view port. I have seen this before but it only showed at much lower voltages and higher pressures. In the attached picture there is a small dot where the electrons are fluorescencing the glass. Note there are two spots in the picture. There is a smaller, well defined spot and a bigger more ghostly one near it. I am more afraid of the small one. The glass was hot to the touch but because I could keep my finger on it, I am guessing below 115 F on the outside. I held a strong magnet near the dot, and the dot does not move. The only effect of the magnet was some flickering of the lighter fluorescence across the face of the glass.

In better news: At 35 kV and 6.6 mA, before I hit the arc wall at 38 kV, I measured neutron numbers that I wont report yet because they startled me. I pulled the tube from the moderator to make sure I wasn't seeing noise, and watched my indications go nearly to bottom of scale. Suffice to say, if these numbers are real, the tweaking of my grid positioning made a big difference in the fusion rate, as several have predicted.

[Jon Rosenstiel]

I keep confusing CF with KF, sorry. So the 2.75” CF cross has an ID of 1.44”… wow, that’s pretty darn tight, makes for a real “high-tension” environment.

With my 6" spherical fusor the alumina tubes seemed to be a positive, not so with the cube.

JonR

[Richard Hull]

Have you recently checked the x-rays at that dot on the window at that voltage?

Richard Hull

[Jim Kovalchick]

The xrays at the window are not trivial. I limit my exposure with remote camera pics, distance, and a lead flap that usually hangs over the view port. I'm not sure I want to publish a number. My ion chamber is not calibrated and the desiccant is pink right now so it will significantly over report.
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I'm not sure the xrays from the electron beam on the window will be as significant as those on the beam ends. The electron beam doesnt seem to be making near as much plasma because the beam is invisible other than the dot.
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This evening I did another run in the low to mid 30 kV range. I stayed away from higher voltages, but I also found that st 35 kV I had to keep current low otherwise grid to wall arcs start. I think I found this fusor's upper range.
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I also recorded a new personal high neutron count rate. Using rate indication only, not a scaler, the peak rate match the number from earlier today of 1200 cpm.
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Next steps will be to establish stable operation standards including applying final shielding installation, operation duration limits, and a workable neutron oven.

[Frank Sanns]

Looks like you are make good progress Jim.

You won't see an electron beam in a Fusor. The pressure is too low. The mean free path is multiple km so there is no significant interaction of electrons with the plasma. As you have noted though, you will see the fluorescence of the electron beam impinging upon surfaces.

All of the visible plasma in the fusor is excited by ions and fast neutrals alone.

[Jim Kovalchick]

I agree Frank. The electron beams can only be inferred from plasma which I don't see with this secondary beam.

Also as I understand it, we don't see plasma either. Only the photons released when it cools.

Jim K

[Frank Sanns]

Yes, and will all that is going on in there it has to be down to around 3 eV before we see it! Talk about energy loss.

[Jim Kovalchick]

Conditioning is really helping my fusor performance. Tonight I operated at 38 kV and 8 mA. My neutron numbers are up 20 percent.

Attached is a picture of why you shouldn't look into an unshielded view port of a serious fusor. This gadox coated xray intensifier screen at the viewport photographed in the dark with my cell phone. Some of the light is from the plasma, but the green color is from the gadox reaction to xrays.

[Mark Rowley]

Jim,
I know you don’t want to provide official numbers but could you provide a ballpark figure (on the low side) of where you think you’re at?

Mark Rowley

[John Futter]

Jim
just put a neodynium magnet against the fusor side of your viewport, this should pull the electron beam away from the glass

[Jim Kovalchick]

I have tried a magnet, but I will probably try some different configurations. I rushed my previous attempt for dose ALARA purposes because I don't like leaving my viewport shield off for very long.

[Maciek Szymanski]

I don’t think that a magnet can help. I suppose that the x-rays are produced not by the electrons impacting the glass but rather by the electrons impacting the metal shell on the opposite side. The viewport just passes them out as opposed to the metal chamber walls.

[Jim Kovalchick]

Maciek,
I absolutely agree that a magnet won't do anything for the xray problem through a viewport. Xrays are made copiously within my chamber, and they radiate everywhere. The glass provides very little attenuation.

The magnet discussion was more about trying to avoid the thermal damage the electron beam apparent by the dot fluorescence on the glass, may do to the glass. If the glass starts to crack, the port could fail catastrophically. The glass does get hot. I limit my run times both for glass heating and chamber heating.

Thanks,
Jim K

[Maciek Szymanski]

You can place a grounded metal mesh behind the viewport (vacuum side) to drain the eclectrons. It doesn’t have to be very dense, so it will not impair the camera image that much.

[Jim Kovalchick]

Yes. A mesh would work. I have also used a sacrificial piece of glass in a past fusor.

[Frank Sanns]

I have a round piece of leaded glass that is barely yellow and does the job of shielding from electron beams and x-rays. It is good up to around 40kv then x rays start leaking from feed throughs and the shell but it is nice to have in place when you need to see into the dragon's lair.

[Jim Kovalchick]

I studied an image taken last night during a run at 37 kV and 7.5 mA. I thought I saw a well defined ring of color in the middle of my grid. I cut the image to just the grid, removed the color and enhanced the contrast. Now the ring is very visible. In the black and white photo I am referring to the wide, dark band. I looked the port this morning and I can see that the grid has a faint band in the middle. The grid does have thinner edges but it is gradual, not a step change.

I looked back at images of Jon Rosenstiel's grid after he removed it, and there are blue rings of color on the edges. Maybe this is the start of the same thing.

I wonder what phenomenon is causing this.

[Jim Kovalchick]

Yesterday, I pulled the outer layer of lead off my fusor while I looked for a vacuum leak. Here is a photo of my cross with painted white lead strips on the cross's legs. The discolored paint leaves no doubt where the heat is found in a single axis fusor.

[Dennis P Brown]

Good pratice to paint over lead sheet.

My high voltage feed thru required a fan to keep it from over heating and I ended up needing water cooling of my chamber since after thirty minutes it was far too hot. My system was pushing 1200 watts thru it.

[Jim Kovalchick]

Dennis,
If I pushed 1200 W into my fusor for very long, something would fail. I usually run around 400 W.

Jim K

[Richard Hull]

The new fusor V I have already pushed to 300 watts in air with no real glow on the grid now. This amazes me. 5 microns, 24 kV, 13 ma. It is great to have no issues in this iteration. The old system would arc around 20 kV. I might expect the grid to glow at about 450 watts using deuterium. I am glad to be rid of the cylindrical, solid grid and back to a spherical system.

The x-rays are fearsome at the window and all day will be consumed with removing them from the picture. A long way to go before I rest

Richard Hull