Electron Gun Success

24 06 2012

All photos

Today we tested the electron gun in the chamber, and we detected a negative potential on the Langmuir probe, which means it worked!

Negative nine volts on the Langmuir probe

Fuck yeah!

Here’s what we did

1) Added a faster-acting fuse to the power supply

the new 4A fuse is underneath the black shrink-wrap

We already have a .5 amp fuse to protect the light bulb filament, but this new one  takes less time to actually blow once its current rating is surpassed, so if the cathode arcs to the chamber wall and pulls a large current, this fuse will blow quickly, preventing damage to the chamber.

wide shot of the setup

2) Closed the electron gun assembly in the chamber, connected to feed throughs, and set up the Langmuir probe.

The Langmuir probe is a wire with one end in the path of the electron beam, and the other attached to a multimeter set to volts DC

3) Powered up the vacuum system.

Because there was so much stuff in the chamber, there was also (presumably) a lot of trapped air which leaked out slowly as we pumped down, so the vacuum wasn’t super deep, but it was deep enough for our purposes.

3) Powered up the e-gun.

The cathode immediately started to glow, amd as we turned up the voltage across the cathode, the Langmuir probe started to register a negative potential.

We could not get potential on the Langmuir probe unless we powered up both the cathode and the accelerator, so we concluded that it must be the result of a beam.

There were also a couple of other interesting things we noticed.

Changes in the voltage of the accelerator did not seem to affect the beam intensity. We brought the potential on the accelerator from +500 down to ~+250, and got similar readings on the Langmuir probe.

Changes in the voltage (and current) to the cathode do affect beam intensity. We found that the greatest value we could get on the probe was about -12 volts, using about 90 to 100 volts AC across the cathode. As we kept increasing the cathode voltage/current beyond that, the Langmuir probe started heading towards zero, until the fuse blew.

After this, the Langmuir voltage started to head toward zero.

A little hard to see, that’a 10.59 volts on the Langmuir, and 102.5 volts on the cathode.

We don’t know what is causing this.

Another cool thing we noticed was the effect the electron gun had on the vacuum. Leaving the beam at maximum intensity caused the vacuum meter to show increased pressure. We were literally filling vacuum space with electrons.

Weird to see the the materiality of electrons demonstrated in such a concrete way.

But all that aside, this is a big step for us. From here, getting that electron beam shining into the center of the Polywell shouldn’t be too hard. If we succeed in that and document our results, we will have performed real, original research on the Polywell design. If we can get the potential well deep enough, maybe even do Polywell fusion.

So let me reiterate, FUCK YEAH

Domenick Bauer

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17 responses

24 06 2012
K

Presumably the electrons are just affecting the ion gauge.

24 06 2012
jstults

We were literally filling vacuum space with electrons.

No you weren’t.

I think what you have done is really good progress worth bragging about though; keep up the good work.

24 06 2012
yonemoto

Could you explain how they were not filling vacuum space with electrons? Because AFAICT they were. You don’t have to fill your gas tank all the way to be filling your gas tank.

25 06 2012
kcdodd

I think there should be essentially zero pressure added by the electrons. Since ion gauges work on ion current, if there is extra ionization going on nearby and they get in there, then maybe that would register as higher pressure. How much increase are we talking here? Is there line of sight between the electron beam and ion gauge?

24 06 2012
saftwerk2

May be the heat radiation of the gun is the reason for releasing the rest of the trapped air faster than before.

24 06 2012
hyakugei

Wow, that is awesome! Congrats!

24 06 2012
Remy Dyer

Good work,

But was the apparent pressure increase correllated with the high tension current, or was it just correlated with the energy dissipation of the filament?

I wouldn’t expect you’d see any difference in langmuir probe voltage with accelerating potential, since the emitted current from the hot filament is going to be pegged regardless. It’ll look like a constant current above some threshold.

When you start shooting it into your Polywell, I suggest you set the electron gun filament to full power, then search through the accelerating potential vs magnetic field current space, without discounting “weak” magnetic fields. (this would be very original research, the Sydney experiment skipped over this and went straight to very strong fields, at least relative to the small current they were injecting).

I think you’ll find that you reach electron beta balance with quite a low magnetic field, since you’ll be limited by fairly small maximum electron gun current anyway. I think the Sydney guys ran theirs at much too high a field for the small injection current their gun could maintain, so they were effectively running their polywell far away from the wiffleball regime. When you’re in the WB balance, you should see a local maximum in potential well depth.

Recall that the wiffle ball beta doesn’t include the ion kinetic pressure, just the electrons. And also that the electron pressure is going to be depend on the electron beam current acheived, as well as the acceleration potential. Electrons shot into the PW are going to leak out very quickly, unless you can get enough to stick long enough to start pushing the applied magnetic field “back out”. BW alludes to this, when he remarks that these devices “have to be driven quite hard” to reach the WB regime. But he was using constant magnetic fields, and variable electron beam current. You could go the other route, and keep the electron beam constant, and very slowly increase the magnetic field.

To reiterate: the current the gun shoots is going to depend on its perveance. (see wikipedia), which will depend on the effective emission area of the filament, as well as on it’s work function, temperature, geometry etc. The gun will probably have reached space-charged saturation the way you were running it, hence why increasing voltage didn’t pull any more electrons out. It’s like a mosfet driven “hard on”, it looks like a current source.

Those electrons would have been “lost” the moment they hit anything metal within the chamber, so it’s very unlikely that they were responsible for your increased pressure measurement directly.

Adding light to a vacuum chamber is known to increase the rate at which light gasses such as hydrogen desorb from the metal walls. This trick is commonly used to “bake out” high vacuum systems over many hours. The pressure increase was therefore probably just due to an increase in the “virtual leak” caused by hydrogen leaving the walls of the chamber. UV lights are common for this trick – your bare filament would likely have been producing some UV in its thermal spectrum.

It might be worth trying some different filaments to see whether you get differing currents. I’d expect the current to depend on the “effective surface”, ie the tubular surface traced out by the helically wound filament. (this is done to reduce heat loss due to radiation – the helix makes the filament seem “less sharp” to the surrounding electric field, and this slows the rate of heat emission, since part of the filament is ‘facing itself”, rather than facing the colder environment. ).

You could try with a needle as well – a sharp point with a little negative potential should give you a little field emission, although heating it will give a lot more – hence the design of your EBM cathode.

Keep the good work up!

25 06 2012
Remy Dyer

Actually – on second thoughts, could be what K said about the vacuum gauge – if it’s got line of sight on the gun.

Anyway, it’s really a side issue: Just because it’s measurable doesn’t mean it matters!

The fact that you’re detecting space charge on your langmuir probe is still a milestone!

Congratulations!

25 06 2012
kcdodd

One way to test the wall pumping notion is to see if it’s reversible. If the pressure goes back down instantly when the gun is turned off, then I would guess its interference and not wall pumping.

27 06 2012
Remy Dyer

I like this.

Anyhow – what’s causing this is a small issue compared to the accomplishment!

If it is this, and there is line-of-sight, then you should be able to prevent interference by cutting the line-of-sight with a thin conductive shield connected to the vacuum chamber. This might be important when you come to adding fuel – you want to have an accurate idea of the pressure, and not get a measurement skewed by ionization.

25 06 2012
Remy Dyer

Actually, looking back over the circuit design you’ve got, it occurs to me that a parameter you’re going to want to measure is the current delivered through to the accelerator anode. Although this might be a bit difficult to measure, it’ll be very helpful to have, since it’s very much related to how much power you’re actually inputting into the device. (since HV power dominates the losses if you’re using a superconductive coil magrid.).
Maybe use a reasonably large series resistor and a fairly high voltage opamp with very good common-mode rejection? Or perhaps a little coil with good insulation in series with the anode and a linear hall effect sensor nearby might be easier?

You might be able to find some good hall-effect current measurement modules if you go looking. They do get used on the outputs of three phase inverters quite often. Just make sure to get one that can measure DC.

What you’re really trying to do is achieve the deepest possible well depth with the least possible electron gun drive power. So with constant electron current from the cathode, you’d want to watch the power delivered to the magrid, whilst you vary the magnetic field strength from zero upwards. This might be best to do without a langmuir probe in the magrid for the electrons to hit.

Then if you see a point that looks interesting, check it out with the langmuir probe. (I’m not sure if your probe might “poison the well” so to speak, by providing a surface for the confined electrons to hit. ).

Anyhow, exciting stuff!

25 06 2012
o_O

“We were literally filling vacuum space with electrons. Weird to see the the materiality of electrons demonstrated in such a concrete way.”

Are you sure about that? I’m skeptical.

27 06 2012
pontifor

Getting a working ion gun is some awesome progress, great work!

28 06 2012
JoeP

Congratulations!

Question: what happened to the phosphor shard you guys were going to use to see the end of the beam? Out would be reassuring to see that working.

5 07 2012
Ben Wright

Reading history in the making.

Congrats and thank you for the blog!

10 07 2012
AJ

I love love LOVE the fact that if this project works as intended, “FUCK YEAH!” may take its place in the history books alongside such utterances as “Eureka!” and “The Eagle has landed.”

Anyway, keep up the great work!

9 11 2012
you dare not say

The pressure increase was therefore probably just due to an increase in the “virtual leak” caused by hydrogen leaving the walls of the chamber. UV lights are common for this trick – your bare filament would likely have been producing some UV in its thermal spectrum.

Untrue.

The decrease in pressure is coming from the gettering action of the alkali metals on the cathode of the electron Gun assembly.

Barium carbonate, strontium carbonate and some calcium carbonate. is heated to about 900 degrees Celsius yielding the oXides of the said carbonates. These oxides aid the electron emission of the gun. they are however highly unstable and the fragile coating of oxides is damaged when exposed to “air”

you should read in on some fundamentals of vacuum tube/CRT production. this might help.

cheers.

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