Fusor Update - Liam David

[Liam David]

I found what was limiting me to 35kV. The centering washer slipped and let the grid become misaligned. Moreover, I think I overtightened the grid stalk, which cracked the ceramic shielding washer when everything got hot. I just got some more washers in and will be back in business soon. The colorful sputtering probably didn't help the arcing either.

Offset cathode

While everything was apart, I examined the cathode a little closer. There's a clear asymmetry caused by the stalk, certainly outside and probably inside the cylinder. The two bands on the ends thicken near the stalk and converge to it, and there's an interesting "cusp" on the inner wall. The patterns are more pronounced than last time because of the longer runtime and higher powers.

"Cusp" in pattern caused by stalk asymmetry...?

Ring where current flows in?

Asymmetric bands, caused by stalk.

This is how the moderator is positioned for activation:

Moderator blocks

Silver from Rio Grande, with two pieces of indium.

[Jim Kovalchick]

Liam,
Your grid pictures look like you are getting arcs secondary to the tube axis like I am. See my latest post for picture.

Jim K

[Liam David]

I haven't had any arcing, just the occasional sparking thanks to my grid shifting off-center. The spots on the side are from secondary beams that form once the voltage gets high enough, something like 5kV.

Deliberately off-axis, secondary beam downward

[Richard Hull]

I love that "cock-eyed" view of the interior of the cathode. Lots of interesting data in that view related to fields.

Richard Hull

[Jim Kovalchick]

Liam,
I meant to say beams not arcs. I can see them in your latest picture.

Jim K

[Liam David]

Arrived today, will be machined likely next week:

Both the chamber and endcaps will be fully water-cooled, and I have a few modifications that should help the field symmetry. I'm looking to push past 60kV, and with how I've arranged the water cooling, the angular neutron distribution should be minimally affected. Simulations show a max temperature of ~30C at the beam spots with 1500W input power, which is far above the 600W my supply can do at 70kV.


Getting better symmetry, and can push 45kV with the repaired feedthrough. There's static buildup somewhere and some sparking that's limiting the voltage.

droidcam-20210224-152949.jpg (5.09 KiB) Viewed 230 times

[Mark Rowley]

I always enjoy reading your updates Liam. Top notch work.

Very much looking forward to your cube build. I foresee good things on the horizon!

Mark Rowley

[Liam David]

Thanks, everyone, for the compliments. Been spending as much time on fusion as I do on school recently...

I found the actual source of the arcing and instabilities, which was only exacerbated by the offset grid. In moving the feedthrough quartz up and down, both for positioning in the chamber and servicing, vacuum grease from the o-ring and dust built up in a small ring. This got baked onto the quartz and limited the voltage to ~35kV. Now with some conditioning I can hit 50kV, limited primarily by x-rays. I have the lead shield top off for air cooling and scattering is an issue. Neutron rates are ~800kn/s @ 45kV, 8.56mA and can be held indefinitely, with brief excursions >1.3e6 (~100kcpm on the tube). Heating prevents me from exceeding the mega mark continuously.

[Richard Hull]

Good work Liam! 1.4 million n/s is about the best I ever did with fusor IV at 45kv. (my current limit on voltage.)

Richard Hull