Polywell Assembly Overhaul

26 08 2012

It’s clear from the inconclusive results of the symmetry test that our experimental apparatus isn’t durable enough. It needs to for a long period of time over the course of many trials, without breaking. Every time it breaks, we need to open up the chamber and fix it. In doing so, we inevitably make some slight change to the alignment of the components, the material makeup of the assembly, etc. Each of these changes introduces unknowns. This makes it difficult to compare the results of one set of trials to another, and thus difficult to accumulate the date we need to actually demonstrate something conclusively. To make matters worse, we aren’t getting deep vacuums because the much of the plastic and rubber components of the assembly are out-gassing.

It’s as though instead of running one experiment a thousand times, we run a hundred slightly different experiments ten times each.

In a word, the three main problems with the old polywell assembly are alignment, structural integrity, and vacuum compatibility. I designed a new assembly which should remedy these problems. instead of multiple separate components, it will be one solid piece of 3D printed ceramic which include a core, acclerator anode, hot cathode, and langmuir probe, all bolted to the 8” conflat flange:

The two feet (bottom left) will be bolted to the conflat flange.

Close-up of the hot cathode holder:

The cathode holder is actually two pieces which sandwich two 2o mm lengths of 10-gauge solid copper wire between them into the inside grooves (A). The grooves on the outside of the cathode holder accept zip ties which will hold the copper wires firmly in place (B). Once the wires are secured, the whole thing is put into place against the left column, and another zip tie is slipped through the hole (C) on the extreme left of the cathode holder and looped around the column. This connection will be strong  enough to prevent the cathode holder from moving during setup/normal operation.  A light bulb filament, serving as the cathode itself, is soldered to the ends of the copper wires.

Close-up of the anode holder:

The anode, a copper cylinder,  is put into the crescent moon shaped space (A), and a zip tie goes around it and through the hole (B), and secures it to the assembly.

Langmuir probe holder:

The langmuir probe fits int the groove in the cylinder (A), and attached with a zip tie or perhaps teflon tape. It extends into the center of the core, indicated by the blue sphere (B). The cylinder is oriented and positioned such that the langmuir probe will extend into the center of the core.

Here’s the new langmuir probe:

The new langmuir probe is a strand of wire inside a very thin ceramic tube. The 9-volt battery on the right is for scale.

Core

The coils fit into the cavities in the core, and then covers go over them. The covers will be secured with zip ties or perhaps hose clamps.

Other than the zip ties and wires, all of this will be made of ceramic. The zip ties will be made of tefzel, a strong, heat tolerant, and highly vacuum compatible material similar to teflon. All wire insulation will be teflon.  This assembly will be heat resistant, electrically insulating, and much more rugged than previous designs. Ideally, we will be able to put it together, put it into the chamber, pump down to much deeper vacuum, and do hundreds of trails without anything breaking. moving, or changing shape.

In order to work at all, this design has to be compatible with the vaccum chamber and conflats that we already had. If one dimension is even slightly off, then the whole thing fails. To prevent that, I first took measnurements of the chamber and flange, and maodeled exact copies of them in OpenSCAD, and built this assembly inside the chamber :

Here you can see the conflat flange (left) and the chamber. Notice that the blue sphere, which indicate the center of the polywell is not centered in the chamber. By offsetting the core slightly, I was able to get more clearance between the walls of the chamber and the core, which in turn allowed for a larger coil radius.

Another pic of the whole thing:

While I have uploaded these models to shapeways, they will probebly not be the ones we actually have printed. This is more of a first draft.

The source code is here

Domenick Bauer





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





Symmetry Test

27 07 2012

All photos

This project is moving into a new phase. Now that we have a working electron gun/polywell setup, it’s time to begin studying the way they interact. To do this, we need a more comprehensive measurement apparatus.

In light of last week’s murky trial results, It’s apparent that a single langmuir probe in the center of our polywell core does not give us a clear picture of the whole well. I think a good first step would be to try to understand how the differs at different distances from the center.

In other words, how is the well different at different points on the red line?

Before I try to answer this question,  I want to make sure that each of the six coils behaves in more or less the same way. In other words, I want to make sure  If the red line above was pointing in the direction of the left coil instead of the right one, it would made no difference. I’m going to call this a symmetry test, because it will reveal whether or not the well is electromagnetically “symmetrical” about the x, y and z axes.

The symmetry test will work like this: Two more langmuir probes into the core, one on the extreme right of the chamber, and the other at the extreme left.

Right Langmuir

Left Langmuir

If our oscilloscope readings from the two probes are the same, then we may safely assume that the faces are interchangeable.

We’re rapidly running out of feedthrough pins

Core, from the right.

I can’t imagine that the thing’s not symmetrical, because the construction of each coil is identical to that of all others. However, thoroughness never hurt anybody. Also, I’m going to leave the center probe in, so we will be able to compare the center of the well to the edge, which will be interesting in its own right.

Domenick Bauer





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





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





E-Gun + Polywell Setup

2 07 2012

All Photos

Now that we have a working electron gun, the next challenge is to pulse the Polywell core while we shoot a beam of electrons into it. Trivial as it may sound, the hardest part of this is just getting all the components in the chamber aligned correctly and with electrical feed-throughs.

My previous e-gun armature was way too bulky, so I scraped it. After several different designs, I came to this:

The round base goes around the feed-through pins and fits snugly into the hole in the conflat:

The two screws visible in these pictures allow for adjustments to the distance between the anode and the cathode.

As for the cathode holder, I cut the filament off of a light bulb, and soldered it’s ends to two pieces of 12 gauge copper wire, each about an inch-long. I attached them to the feed-through pins with two crimp connectors. Not as a cool as a 3D printed holder, but much more compact, and no plastic to out-gas or melt from the heat of the filament.

Here’s the whole thing

Then we cleaned everything with acetone and ethyl alcohol.

Began the assembly. After much trial and error, I got everything in and aligned correctly, and got the vacuum down to about 8 x 10^-4 torr.

The only thing left to do is to hook up the Polywell power supply, so we’re pretty much ready for the run!

Domenick Bauer





More E-Gun Progress

15 06 2012

all photos

We’re slowly inching towards an electron gun test. We want to make sure everything works before trying it, because repeatedly sealing and unsealing the chamber is not only a pain in the ass, but includes the risk of contamination/damage to the inside of the chamber.

But I think we’ve really got it this time.

I simplified the armature. It’s now one piece that attaches to a ceramic column which screws into the eight-inch conflat flange on the chamber. I also switched from the big ceramic light bulb socket to a smaller, lighter one.

Much easier than the three-piece setup I had before, and its still somewhat variable; the distance between accelerator and cathode, and Langmuir probe and accelerator are both variable.

Closeups:

The accelerator anode is very close to the hot cathode

The Langmuir probe

I also made a couple of changes to the power supply. We found that the high voltage box which we were going to use to give the accelerator it’s high positive potential has a built in potentiometer:

So we don’t need the extra one.  I also made a piece which holds the HV output wire in place:

it’s a cylinder with an inside diameter the same size as the piece on the HV box, so the threads dig into the plastic and hold the HV out in place

The updated schematic:

So the electron gun seems to be totally ready for a test.

Here it is in the chamber, viewed from the glass on the other side

Hopefully we’ll be able to do it in the next couple of days. One thing I’m worried about is the interaction between the beam and the accelerator. Will the potential on the accelerator sag because it is being bombarded with electrons? Another worry is how vacuum compatible this whole assembly is. Probably not very, but thankfully, vacuums don’t need to be that deep for electron beams.

This is a very exciting moment, because if this works, we can start thinking about how to make it more powerful. If we succeed there, we will be able to use an electron gun to deepen our potential well, which is uncharted territory.

Domenick Bauer








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