Inconclusive Symmetry Test

14 08 2012

The symmetry test was an experiment we ran to determine if our potential well was symmetrical across its x-axis. It featured three langmuir probes, one in the center, on at the extreme left, and one at the extreme right. more details are in this post.

While there were a few interesting results few interesting results, on the whole the experiment was inconclusive, and totaled our electron gun assembly.

The first problem we encountered was the vacuum level. We barely got into the 10^-4 Torr range, and when we turned on the electron beam, the pressure went up into the 10^-3 Torr range. High pressures like these don’t render the experiment impossible, but they certainly don’t help. Ideally, the only particles in the chamber would be electrons, and so everything else just adds to the list of unknown factors.

The e-gun was running normally, giving us readings of about -50VDC on the oscilloscope.

The glow from the hot cathode

For the first shot we did a control. We hooked up one probe to the center langmuir, and one the the shunt resistor on the on power supply, and we got a small well.


So everything was working as expected, despite the unusually high pressure. This is good, but also strange in light of the last test results, in which the charge at charge at the center of the core became less negative when we fired the coils.

Then we switched the oscilloscope probe on the coil power supply to the left langmuir probe, and fired.

Top: Left langmuir probe
Bottom: center langmuir probe

Nothing on the left langmuir. We tried again with more power going into the coils.

Here’s what we got

The charge at the leftmost extreme of the well is about -3VDC, and the charge at the center is about -10VDC. Not surprisingly, electron density has some relationship to distance from the center of the core. We intend to eventually define this relationship precisely, but to do so would require much more data.

Notice how even before the coils were fired, the top line is a slightly below its zero point (indicated by the crosses at the left of the screen). This means that for some reason, the left langmuir is brought to a slightly negative potential by the electron gun, even though it’s not pointed anywhere near the probe

We then switched the oscilloscope probe on the left langmuir to the right langmuir, and fired the core again. The moment we did, we heard a metallic noise from inside the chamber, like a coin dropping onto a metal surface. Can’t be good. This was the readout on the oscilloscope

Not especially meaningful to us.

We rightly assumed that the noise meant our trial was over, so we opened up the chamber.

We found the accelerator anode laying in the bottom of the chamber. The heat from the filament melted the plastic enough for the screw mounted in it to come loose. Everything around it was coated in a thin film of blue plastic, and much of the wire insulation was burnt as well.


Obviously, ABS plastic and rubber insulated wires just aren’t right for this experiment. They can’t take the heat of the cathode, and they out-gas so much that they ruin the vacuum.

Back to the drawing board.

Domenick Bauer



7 responses

14 08 2012

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.


Maybe these will help.

14 08 2012

Sorry, this is the electron gun video:

15 08 2012

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

15 08 2012

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

15 08 2012

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: Hopefully, with more vacuum compatible materials, We can get near this level.

25 08 2012
Remy Dyer

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. :)

27 08 2012

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?

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