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





Safer Coil Power Supply

3 08 2012

 

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

The capacitor bank which powers the coils is very lethal; when fully charged, it dumps more than a megawatt into the coils. Needless to say, If I were to accidentally complete the circuit through my body, it would rearrrange my insides. To decrease the likelihood of such an occorence, I’ve done my best to idiot-proof it. Here’s how it looked before:

Pretty easy to thoughtlessly reach in and touch the dangerous parts.

So first, I took a broken shipping pallet that someone across the street was throwing away, and made a box out of it

The box fits nicely on the crate that we’ve been using as a controls stand

 

I also left the front partially open so they we could get at the controls

With this setup, the only thing you can touch is wood, plastic controls, and the grounded metal face of the box, so it would be pretty hard to blow your hand off.

The next step was making the HVDC out wires safer. (the fat green and yellow ones in the first pic) The way we had it, the only way to dissconnect them was to reach into the supply and unscrew the ring terminlas to which they were connected. Not great. To fix this, I installed a high current outlet onto the back of the box and connected some shorter wires.

The other hole is where the cord which plugs into the wall goes

Took the wires we had and connected them to a matching high current plug

Assembled (and labeled):

 

So now it’s pretty safe. In fact, it’s so safe that getting into the power supply at all istoo much of a pain. We don’t want to have to move this box every time we need to reset the breaker. To make access easier, I put hinges on the bottom of it (see above), and chains on the side, so it opens like a tool box.

MakerBot mailed me an iron-on patch with some supplies I ordered, so I taped it on for good luck. (top right)

Look’s pretty badass, kinda like a ten-cylinder car engine.

 

 

Here’s one more safety feature I added:

If you open the “hood”, the power cord is automatically pulled out. This way, it’s impossible to open the power supply while it’s plugged in.

The coil supply is idiot-proof, but isn’t so locked down that minor repairs and upgrades are too much of a hassle. This is one more step towards a streamlined, repeatable, experiment procedure.

Domenick Bauer

 








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