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





Electron Gun + Polywell Results

8 07 2012

All Photos

Today, we tested the electron gun in tandem with the Polywell core. Right now, our goal is to simply understand effect the e-gun has on the potential well, if it has any effect at all.

We had the pump running since 3AM the night before, and so by about 2PM today, we had a vacuum in the 5 x 10^-5 range.

Not super amazing, but certainly good enough for out purposes.

First, we mounted the power supplies in the rack.

Hooked them up to their proper feed-through pins.

Attached the langmuir probe and the shunt resistor to to the oscilloscope, allowing us to monitor the potential well depth and current going through the coils simultaneously and in real time.

Then we began testing. The coils in the core worked great.

The spike on the top line is the power supply’s capacitor bank discharging. The smaller spike on the lower line is a current induced in the langmuir probe by the sudden appearence and dissappearence of a magnetic field generated by the coils in the core. So the Polywell works.

Then we tried the electron gun, and it didn’t work at all. the filament was glowing, and we were getting high voltage on the accelerator’s feedthrough pins, but no reading on the langmuir probe.

After extensive thinking, speculating, and white-board writing, we decided we had to open the chamber up to see if the connections to the accelerator were right.

Here’s what we found

That’s your problem right there, ma’am

It’s a little hard to see, but a gray plastic piece which connects the core to the feed-through pins was right in front of the accelerator, totally blocking the beam.

Funny how sometimes the causes of problems are so obvious that you don’t even think of them.

Anyway, I got that fixed, tried it again, and got a beam. The readings on the langmuir probe attached to the multimeter were more or less the same as those from the last test. When we attached it to the oscilloscope however, things were a little more complicated.

The beam intensity was not static, but periodic. It fluctuated with a frequency of 60 Hz, pointing to the AC current which powers the hot cathode.

We expected something like this, because the availability of electrons to accelerate fluctuates with the AC powering the hot cathode.

Ideally, our electron beam would be have a perfectly even intensity, because then we could eliminate it as a variable.

Fixing that would involve rectifying the AC, a major upgrade to the power supply, so we decided to leave that for another day and run the experiment.

Here are some of the results

A really good one. The downward spike on the lower line signifies a a potential well. Nice!

Here we see a well, but it’s at the wrong time, it seems to have appeared just after we pulsed the core.

Here’s a strange one in which the Polywell core pulse seems to cause some change in the voltage on the langmuir, but not a well.

This one really demonstrates why the periodic electron beam is such a problem. The top spike came at a moment when the langmuir probe was reading zero. This means that there was no beam when we pulsed the core, so it’s no surprise that we just got an induced current.

The most tantalizing and baffling run we did. The well appears to be extremely deep, greater than 100V, but it’s unclear whether thats credible, because there’s so much other confusing stuff going on.

All in all, our results are little confusing, but good. We were able to create the well, which is a big win, but we weren’t able to do so consistently. In order to really study how the well is affected by tweaking variables, we need consistent baseline well to compare against.

Domenick Bauer





Viton Gaskets

14 09 2011

All photos.

I am testting out viton gaskets for the vacuum chamber.

Viton gaskets are reusable replacements for single-use copper conflat gaskets.

The gaskets are rated down to 10e-8 torr, which is fine for now.

The smaller gaskets installed easily, but the large 8″ gaskets were impossible to install… they fell off the flange.

I called MDC and asked “what’s the trick?”

The trick is a little vacuum grease to retain the gasket!

I ordered APIEZON TYPE M vacuum grease from ebay, also rated down to 10e-8 torr.

I made 8 tiny dabs of vacuum grease:

And it worked!

After a few hours the chamber went as low as 5.5e-5 torr. Totally fine for now, but could be better.

I will continue to use them.





Vacuum Degreasing

15 08 2011

All photos.

A vacuum specialist suggested a better technique for degreasing vacuum equipment:

Lightly swab with acetone, followed by reagent grade ethyl alcohol.

The acetone degreases, and the alcohol removes residual acetone.

 

I used this cleaning technique before reassembling the Polywell:

Back in the chamber soon!





Vacuum Fixed!

4 03 2011

All photos.

I had a real leak, and possibly a virtual leak.

The leak:

See the break in the groove? My knife edge has a small ding.

But it’s no problem as long as you tighten the conflat enough.

There are 20 bolts on these conflats, it’s easy to accidentally skip one or more.

To improve the process I made this paper insert:

Pretty proud of this one actually.

In addition to an actual leak, I may have had a virtual leak, which is just air trapped behind a screw. To eliminate a virtual leak, the bolts inside the vacuum need a channel to let trapped air escape:

I’ll cut those channels on the bolts today.

As a control I put the coil former and supports in the chamber unassembled:

It’s looking good!!

And dropping.

 





Outgassing Check

2 03 2011

All photos.

Last week I mentioned I would start taking smaller steps. Turns out to be a good strategy.

My small step is investigating the coil former’s outgassing.

I put the coil former into the chamber:

At first I had to troubleshoot a gross leak. By listening I traced the problem to the connection from pump to chamber:

The pump had shifted under the chamber and unseated the gasket:

I fixed this leak with a fresh gasket. Now I don’t hear any leaks.

The turbo pump spins up almost all the way:

Either I have a small quiet leak or the coil formers are outgassing like crazy.

I’m going to let the pump run overnight. If outgassing is the culprit it will abate with time.





Acid Washed

28 02 2011

All photos.

I just disassembled and acid washed all the parts for the coil former and electron gun. This degreases and cleans the parts.

I used a 15% HCL solution.

The aluminum started bubbling, so I didn’t leave it in long. The Stainless and ceramic I left for 15 minutes.

The parts will dry overnight.








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