Comments on: Inconclusive Symmetry Test

By: ap0r

ap0r — Mon, 27 Aug 2012 23:37:05 +0000

Just a tougth, but, may that slight negative ofsett you have in the langmuir probes come from the “cloud” of electrons that the e-gun produces?

By: Remy Dyer

Remy Dyer — Sat, 25 Aug 2012 11:21:58 +0000

Hmm, interesting results. I still maintain that going less negative ought not to be a surprise, given that your accelerating field is only towards the anode. Chris’ links above make sense. So too does wrapping the anode right the way around the core?

What was triggering your ‘cro? Can/are you able to synchronise the coil firing signal with the oscilloscope trigger?

Also, did you do a lot of shots in a relatively short time this time ’round? Working too hard this way was apparently what blew WB-6. In their case the coils themselves didn’t have time to cool from within their containers. In your case, it may be that the wires started to warm up, causing outgassing, which, although the gasses probably get removed pretty quickly with that turbopump, could perhaps have been enough for an arc to happen. This would seem to be consistent with the anode being burnt out..

Oh well – experience is proportional to equipment damage, but only if you don’t let it stop you. Personally, I prefer to measure my progress with experimental prototypes by the number of lessons learned while solving problems, not just by success with the planned milestones. This stuff is hard, and you’ll be appreciating Murphy’s Law more than ever the closer you get to success. So don’t feel too bad about not having a huge amount of rigorous planning – it’s always more of a hindrance than a help with this kind of work.

You actually might be ok with the materials you have, so long as you can keep everything from heating up too much. Perhaps an optical heatsink through the window might help? You could probably try pointing a non-contact thermometer in through the window between runs to decide whether you can hazard another shot yet.

An optical heatsink is a black surface (high emissivity/absorbtion coefficient) facing the insides of the vacuum chamber, through the window. Cool it somehow (the colder the better) and it should help cool things down through the window by providing somewhere for the ever-present IR to go.

Obviously, you can’t rely on any gasses within the high vacuum for cooling – much the opposite, they’ll likely always be far too hot. Conduction could also work, but rapidly becomes complicated. (say, passing cooling water through feed-throughs, or using feedthroughs which are themselves heatpipes. )

Good luck as always. :)

By: dbauer7890

dbauer7890 — Wed, 15 Aug 2012 16:59:45 +0000

Good call, I’ve already replaced some of the wires with Teflon coated wires, (they’re the blue and white striped ones) and for the next shot, I’m going to replace them all.

Our vacuum system is a roughing pump backed by a turbo-molecular pump. It gets down to the 10^-8 range with an empty chamber: http://prometheusfusionperfection.com/2011/02/18/deep-vacuum/. Hopefully, with more vacuum compatible materials, We can get near this level.

By: dbauer7890

dbauer7890 — Wed, 15 Aug 2012 16:32:07 +0000

Really cool video! I like how he uses an accelerator cathode rather than an anode, to “push” the electrons to the target.

By: Meanjean

Meanjean — Wed, 15 Aug 2012 11:37:01 +0000

Try Teflon coated wires. They hardly degas and resist temperatures up to 500F(260C). What type of vacuum pumping hardware are you using? If using only a rotary vane pump you might want to add a blower booster pump before your rotary vane. You could bring down your pressure even lower with a diffusion pump(10E-9 Torr).
Good luck

By: Chris

Chris — Wed, 15 Aug 2012 04:13:28 +0000

Sorry, this is the electron gun video:

By: Chris

Chris — Wed, 15 Aug 2012 03:59:14 +0000

This guy built a scanning electron microscope, and goes over the making of an electron gun. The first one is a more detailed overview of his system, and the second is a simple overview of how an electron gun works.

and

Maybe these will help.