Armature Upgrade and More Trials

24 07 2012

All photos(1/2)
All Photos(2/2)

We weren’t satisfied with the layout of the inside of the chamber last time. The alignment between the center of the polywell core, the electron gun and the lagmuir probe wasn’t very good, so I upgraded it.

The main advantage of this version is simplicity. It puts the filament, accelerator, and probe, and polywell core all on one flange, rather than two, which means the whole thing can be assembled on a desk and then put into the chamber Much easier than trying to make electrical connections and get alignment right with the thing half way in — something which, rest assured, is a royal pan in the ass.

Another advantege of this one is strength.

Dosn’t look like it, but that’s actually a very strong connection

A big problem last time was that we were unable to get the whole assembly into the chamber without accidentally bumping it a little, and ruining the alignment. This version is more securely attached to the flange, making it easier to keep all the components in the right place and pointing the right way.

Another flaw in the earlier version: The screw which attached the accelerator to the armature extended pretty far inside the copper sleeve which we were using as the accelerator anode, so it was partially blocking the beam.

In this version, that connection has a much lower profile.  I also switched out the old copper sleeve — which was too big and full of holes — for a new smaller one.

While trivial things like holes in the accelerator probably don’t matter, I’m trying to correct them because at this stage of the game, our goal is to eliminate as many variables as possible before we really start collecting data on a large scale. The cleaner the setup, the better.

We also took Remy Dyer’s advice and grounded the positive side of the DC output going to the hot cathode, in order to maximize the potential difference between the accelerator anode and the cathode.

Then, I pumped down the chamber and tested the electron gun.

If you look closely, it’s clear that all the components are aligned pretty well

The lower line is the voltage picked up by the Langmuir probe. The little cross shaped marker on the left side indicates the zero point. Every box in the y direction is equal to fifty volts, so our electron bean is delivering -50 volts to the Langmuir probe, with almost no AC disturbance! Not bad, and this is with the voltage across the hot cathode at about 60 volts out of a possible 120, so it could get even higher.

With this bigger, badder electron gun, we ran another set of trials.

We pumped down.

Hooked everything up.

And took our first shot.

As before, the lower line indicates the voltage on the Langmuir probe at the center of the Polywell core, where the potential well should be. Instead of a well, we have a hill! The magnetic fields generated by the Polywell are supposed to compress all the electrons within the core into its center, so the voltage detected by the Langmuir probe should go even lower. Instead, it goes up, from about -50 to -25. Very confusing.

We rant it a few more times, increasing the current sent through the coils each time, which translates to stronger containment fields. The results were similar.

Here’s a strange one where the center of the core seemed to become very positive. We suspected an arc.

Sure enough, we couldn’t get the core to discharge after this, so we worried that we fried the coils. When it was safe to do so, we opened the chamber and saw that there was an arc, but thankfully not on the core.

Evidently, there was a bad connection between this din rail connector and the core feed throughs on the inside of the chamber, and so an arc occurred, and broke the connection entirely.

So that was that for our trial.

It’s left me confused. What on earth could be making our well positive instead of negative, whereas in the last trial, we got good, negative wells?

Domenick Bauer

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6 responses

25 07 2012
Remy Dyer

Interesting!

I think what may be happening is that the coils are deflecting about half the electron before they enter the coil. So the Langmuir probe voltage isn’t really “positive”, really just “not as negative as it was without the magnetic field”.

My next suggestion would be to wind a fine wire loosely and “toroidally” about each of the coil holders, (as if the coil holders were a toroid transformer core, this new winding will be everywhere perpendicular to the main coil windings, and so shouldn’t cop any current from the magnetic field, because it will run “with” the field and not across it.)

I guess you could instead make a spherical shell anode to go around the magrid to the same effect – just punch out holes for each of the cusp lines – on each face and in each corner. Would really look like a “wiffleball” then!
Could be in several parts, and gaps are ok.

Then wire each of these “cage electrodes” together – and use them all as the high positive voltage anode. They need not actually form a circuit, or even each be connected at both ends, just use them as a loose cage insulated from the coils, so you can keep running the coils separately, yet still make the whole magrid assembly into, well, a grid.

This way it won’t matter if the external part of the magnetic field deflects the electrons – they’ll still be pulled inwards if they’re anywhere outside the Magrid. Don’t need to worry much about “focusing” the beam either, since this will do that anyway. So long as the electron source has line of sight on the centre of the grid it’ll work.

But once those electrons get inside, then they won’t see the anode electric field at all. Because of Faraday shielding they’ll see just each other, and on their way out, the inside-facing magnetic field…

You might need a couple of Langmuir probes to see the shape of the potential well, rather than just one in the middle. All the interesting physics happens close to the inside edge of the WB, just within the magrid’s ID and near to the coils. To get the WB mode to light off, you’ll probably need a lot of electron gun drive current to get the electron number density up high enough that Lenz’s Law en masse starts to counter the magnetic field within the magrid.

You could then use the existing anode near the filament as a “grid” connection to control the current from the filament – a sense resistor in the earth connection i suggested there will let you measure what the electron beam current is.

PS: I’m perfectly happy to get no credit: Just seeing your next post is reward enough! Still, it was nice, thanks! But you needn’t again ;)

25 07 2012
Remy Dyer

Actually, on second thoughts, you could just try moving the anode up into the nearest coil.

This would do nearly the same thing, in terms of keeping force on the electron beam until it’s through the tightest part of the magnetic flux. (which would be close to the center of the that nearest coil.) This should stop the Langmuir probe going “more positive” when the coils fire. (What exactly it’ll do I have no idea, I hope go more negative again.)

It should be just entirely within the magrid there. But cut a slot along the anode tube though, so it doesn’t accidentally form a one-turn winding in the middle of your coil! (probably a good idea anyway, I think)

But having the anode “cage” about the whole magrid would be much better – this way any electrons leaking out through the cusps would be encouraged to leak back in almost immediately, and being outside the magnetic field, the electrons won’t be able to hit the anode and leave so easily, allowing for multiple pass trajectories.

Anyway, keep up the good work! ^_^

25 07 2012
erblo

I’m not an expert but I think it’s important to consider that in your setup the electron beam should hit the Langmuir probe while the Sydney experiment (and your previous attempt?) had it aimed at an angle away from the probe. This means that the zero reading corresponding to no confinement is the full potential of the beam (-50V) instead of ~0V. The maximum field resulting in -25V then means:

– The full interior of the device is now at a negative potential due to trapped electrons. The trapping is effective enough to result in a central negative cathode at half the beam potential. :)
Beam: -50V -> 0V, Cathode: 0V -> -25V

Or:

– The magnetic field deflects/disperses/interferes with the beam so that only enough hit the probe to give a reading of -25V instead of -50V, unfortunately telling us very little about the confinement. :(
Beam: -50V -> -25V, Cathode: 0V -> 0V

Or a combination thereof.

27 07 2012
Symmetry Test « Prometheus Fusion Perfection

[...] more pressing  issue is measurement. In light of last week’s murky trial results, It’s apparent that a single langmuir probe in the center of our Polywell does not [...]

3 08 2012
crossedfields

You’re trying to use a Langmuir probe in a magnetised plasma … Very tricky physics there. It could mean anything!

14 08 2012
Inconclusive Symmetry Test « Prometheus Fusion Perfection

[...] 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 [...]

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