First Demo build... something isn't quite right.

[Josh Miller]

Hi everyone! I am new to the fusor community, and have been working on my first demo fusor. I seem to be having a difficult time getting to star mode, and only get plasma encasing 1/2 to 3/4 of the inner cage. I've tried various cage sizes and constructions, and I don't seem to have any vacuum leaks. The vacuum guage I have isn't the most accurate, but I am using a 2-stage rotary vane pump that claims to have enough power to get me where I need to be. My power supply is a NST claiming a 10kv 300ma output. my gut tells me it's probably vacuum related, but I was hoping someone here could give me some insight before buying a bunch of stuff I don't need. Any help is greatly appreciated!
Josh

[John Futter]

pics of your setup please

otherwise we are talking to a vacuum (excuse the pun)

[Josh Miller]

Yeah, sorry... I tried to upload a few pictures with the post but it kept giving me an error. Hopefully this helps, but let me know if you need something more specific.

https://ibb.co/NTt1ryd
https://ibb.co/fS9pmP7

[Liam David]

The pressure is far too high, probably on the order of a few torr. Star mode occurs at some couple dozen millitorr. That gauge will tell you next to nothing - you need a thermocouple gauge or the like.

[Gunnar Mein]

I really like this gauge, provided that you have the means to fashion a serial cable with a VGA jack on this side and USB on the other: https://www.ebay.com/itm/114332188209?epid=1438064168. You might also be able to find a model with display built in.

[Josh Miller]

Thanks! I had a feeling it was vacuum related, but hadn't been able to get a sensitive enough gauge to confirm. I'll get right on that now.... The vacuum pump I have is 1/2HP 5CFM and claims it is capable of ~2.25mtorr. Right now it's just hooked up with braided vacuum tube and hose clamps. Any thoughts on whether I should try to hard line connect it, or if the pump I have is just not quite powerful enough?

[Liam David]

Presumably you have a direct-drive HVAC pump (you haven't given us much to go on). In a perfectly sealed, non-outgassing system you might expect 15-50mtorr, but it will never reach the "2.25mtorr" pressure it claims. From what I can see your vacuum hose is pretty long and narrow, which doesn't help. But long ago I used an identical hose and got in the 15mtorr range with an HVAC pump, so you must have a leak or really bad outgassing. Or the pump is bad, but you can't really tell without a better gauge.

[Maciek Szymanski]

For this kind of leak you can easily do without any gauge, just by looking at the plasma. Here you are a very useful illustrated reference:

viewtopic.php?f=24&t=2795

If you use a flat electrode instead of the grid you can even determine the pressure by the width of the faraday dark space. It was once a standard method for vacuum measurement: