Inner Grid Sparking?

[Jeff Robertson]

Hi all,

So my fusor is basically done in terms of construction, now all that's left is testing and fiddling. The other day I attempted to do an atmospheric plasma run at around 20 mTorr, not sure exactly the voltage but it was anywhere from -8 to -12 kV. I was able to get some dim plasma, but around this point I started seeing sparks coming off of the inner grid. My power supply also began sparking a bit (a whole different problem, nothing a little insulation can't fix) so I didn't give myself much time to observe the pattern of the sparks. They occurred once every few seconds, and I couldn't tell if they were directed exclusively towards the outer grid or just randomly oriented.

I won't have access to the lab until monday so no pictures for now, but my grid consists of 3 circular pieces of tungsten (half millimeter diameter), held together by a tiny steel cylinder (I believe this method is credited to Jon Rosenstiel). There's no (visible) sharp edges which would make a hotspot for sparking. The outer grid is made of stainless steel, and is about 5 times larger than the inner grid (as I saw several sources on these forums recommend).

Is this sparking a problem? And if it is a problem, what can I do to stop it?

Cheers,
Jeff Robertson

[Chris Bradley]

Just asperities (too small to see) and perhaps other surface contaminations burning off... should be no issue once it's been run a while.

[Richard Hull]

All fusor demos see this grid flashing/sprakles, etc., as Chris mentioned.

This is a cleaning period. Most new grids undergo this process at first. All this will disappear as if by magic once the microscopic bits and imperfections from the grid's surface burn off.

External sparking is another matter.

Richard Hull

[Brian McDermott]

When you get the sparks, do you still have plasma in the inner grid or is the sparking the main mode of discharge with little to no poissor visible and a low current draw? If the former is the case, it's due to impurities and micro-sharps on the surface of the grid wires that are "burning off," as others have mentioned.

If the latter is the case, you likely have a connection or wire geometry that prefers sparking over glow discharge formation. For example, the cylinder I had used to attach my inner grid to the feedthrough stalk was slightly loose and exposed above the ceramic insulation. Instead of a poissor, I was getting a spark-like discharge between the feedthrough stalk and the grid itself. Tightening up this connection and improving the insulation on the stalk corrected this problem.

[Jeff Robertson]

Brian,

I still had a faint plasma. As far as I could tell, the sparking wasn't really influencing the plasma.

Thanks for the fast responses guys, it sounds like I can rest easy. Just wanted to double check that everything was working normally!

Jeff