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


Comments : 12 Comments »

Categories : Core, Data Acquisition, Electron Gun, Instrumentation, polywell, Power Supply, Vacuum

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


Comments : 4 Comments »

Categories : Core, Electron Gun, polywell, Sydney Experiment

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


Comments : 17 Comments »

Categories : Electrical Engineering, Electron Gun, FUCK YEA!, Instrumentation, Langmuir Probe, Power Supply, Uncategorized

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


Comments : 3 Comments »

Categories : Electron Gun, Langmuir Probe, polywell, Uncategorized

Electron Gun Power Supply

5 06 2012

all photos (1/2)
 all photos (2/2)

The light bulb electron gun test had to be put on hold because we didn’t have suitable power supply, so we built one. Here’s the schematic:

The schematic of the power supply which will drive both the hot cathode and the accelerator anode.

Here’s the real thing:

It’s really two power supplies in one. Put simply, it will convert the 120v AC current coming out of the wall into a source of lower voltage, higher current, AC power for the hot cathode, as well as a source of positive potential for the accelerator anode.

This is how the hot cathode power supply will work:

1) AC 120 volts from the wall to a switch.

  

2) From the switch to a 0.5 amp fuse. This, as suggested by Rehan, will prevent a current overload should the cathode arc to the wall of the vacuum chamber.

  

3) The the current will go to a variac auto-transformer which will allow us to regulate the voltage and current of the hot cathode.

  

4) Out to the hot cathode.

This is how the accelerator anode supply will work how it will work:

1) Current from the wall will enter the box and go to the same switch as the cathode power supply.

(see above for photos)

2) The current enters a stand-alone DC power supply. It is essentially a transformer which steps the voltage down to 24v and then a rectifier which converts it to DC.

  

3) The current goes into a high voltage power supply which steps it up to 500 volts DC.

  

4) The negative HV output is capped, because we have no need for it. The positive high voltage output (which will ultimately create the large positive potential on the accelerator anode) goes into a potentiometer, which will allow for a variable potential on the anode, anywhere from zero to the maximum voltage output of the power supply.

  

5) A 2 MΩ resistor. This will act as a sort of safety-net resistor. If there is an arc from the hot cathode to the anode, this will prevent the cathode’s larger current from flowing into the HVDC power supply.

  

6) A voltmeter. Self explanatory.

  

7) Out to the accelerator anode.

And that’s all there is to it.

Domenick Bauer

 


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Categories : Electrical Engineering, Electron Gun, Electronics

Electron Gun Trial Run Setup

31 05 2012

While the armature is approaching completion, we have yet to test the electron gun which it holds.  As described in earlier posts, it will consist of the cathode from an electron beam welder, a piece of copper tube for an accelerator anode, and a shard of phospher screen so that we can be sure it is actually shooting electrons.

This weekend, we will test a simplified version of this design. Instead of using the welder cathode, we use the tungsten filament from a broken light bulb, as suggested by Rehan:

Instead of using the phosphor screen, we will use the langmuir probe to detect the electrons.

In preparation, I have printed holders for the ceramic light bulb socket, and the accelerator anode so that the filament and the axis of the accelerator anode  are on the same line with each other and with the tip of the langmuir probe.

light bulb socket holder

ccelerator anode holder

both pieces together

Hopefully, this will work as a rudimentary way to inject electrons into the center of the reactor, deepening the potential well. If it does work, and we decide that we want an even deeper well, we will continue work on the original electron gun design.

Domenick Bauer


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Categories : Electron Gun, Fabrication, Rapid Prototyping, Uncategorized

Electron Gun Progress

29 05 2012

The work on the electron gun armature is progressing nicely. Shapeways is currently printing this version:

Similar to the last one, but this time with a phosphor screen holder in the right shape for our phosphor shard.

This project demands a high level level of precision for its components. Everything needs to be exactly the right size and shape, and so in order to  use 3D printing effectively, we need to understand its limitations and work around them.

In all likelihood, there will be a problem with the armature above. Possible problems include:

1) The holes in the first column which are supposed to accept the hot cathode are smaller than they should be.

2) the columns are warped by the kiln-firing process.

3) Some unforeseen problem.

To address problem 1, We have also printed a hole-gauge:

The holes have a range of diameters, all clustered around 1.28 mm, the right size for the holes in the hot cathode holder.

The range of diameters in the gauge will give us a good idea of the relationship between the hole diameters in the OpenSCAD files, and the hole diameters in the printed part. If the holes in the printed armature are smaller(or bigger) than they should be, then this will tell us how to compensate in the next printing. Source code

Here’s the hole gauge in plastic

The hole into which the cathode is inserted was designed to have a diameter of 3mm, but as you can see, it tightly holds the cathode foot, which has a diameter of about 1.3 mm, which means that the diameter of the plastic version of the hole is 1.7mm bigger than that of the OpenSCAD version.

with further measurements, it might be possible to find a formula which converts OpenSCAD dimensions to real dimensions for this material and MakerBot setting, but that’s not important right now.

Another concern (problem 2), is keeping the columns straight. I’ve been in correspondence with the Shapeways, and according to them,  long thin pieces like the armature columns sometimes warp unpredictably when fired. This is problematic because a straight line of sight from the cathode, to the center of the accelerator, to the phosphor screen is integral to the electron gun’s operation.

If they don’t warp, great.
If they do, then we must alter the design so that this doesn’t happen, and reprint. Here’s a candidate:

In this version, the columns are buttressed in the x and y dimensions, so they shouldn’t warp. If they do anyway, or if something else goes wrong, then it’s back to the drawing board for another armature design, but that’s OK, because OpenSCAD and 3D printing make the prototyping process fast and inexpensive. Source code

Domenick Bauer


Comments : 3 Comments »

Categories : Electron Gun, Rapid Prototyping, Uncategorized

Electron Gun Armature

21 05 2012

One of the components of the electron gun is an armature which will hold the hot cathode, accelerator anode, and phosphor screen all in the same line with the center of the reactor.

This is a challenge because the armature must be an excellent electrical insulator and have a  high heat tolerance. The ideal material is ceramic.

The problem with ceramic is that it we cannot machine it into the unique shapes required, but we can 3D print it! I’ve modeled the armature shape in OpenSCAD:

Here’s a link to the source code

The three curved “feet” have the same curvature as the inside of the reactor chamber, so it will fit nicely and sit still in the bottom of it.

the first column on the left holds the cathode, the middle column, the accelerator anode, and the last, the phosphor screen. the black line will be the path of the electrons to the center of the reactor. Everything here is pretty much how it’s going to be on the final armature, except the phosphor screen will have a different shape, and the distances between the columns will be different as well.

The MakerBot wouldn’t be able to print this all in one shot, so I printed it in sections, and glued them together to get a feel for the final one.

The accelerator cradle

Column for hot cathode

Base of the armature

Earlier version of the base

Assembled armature with cathode and accelerator

Hopefully when I send this file out to be printed in ceramic, they will be able to do it all in one piece. If not, I’l have to find some way of gluing pieces together

Domenick Bauer


Comments : 5 Comments »

Categories : Electron Gun, Fabrication, Rapid Prototyping, Uncategorized

OpenSCAD Model of Hot Cathode

8 05 2012

Here’s a rendering of the hot cathode and the cylinder mentioned in the last post. The important dimensions here are diameter of the holes in the cylinder, and their distance between them, because they were made according to the measurements of the actual cathode. The rest of the dimensions are approximate. We will take more measurements and tweak the model to fit them.

Here’s a link to the OpenSCAD code.

Image

Image

By DOMENICK BAUER


Comments : 1 Comment »

Categories : CAD, Electron Gun

Measurements for Hot Cathode

3 05 2012

All photos.

See photos for measurements of hot cathode.

The next steps are modeling the hot cathode (plus clearance) and subtracting that from a cylinder in OpenSCAD:


Comments : 1 Comment »

Categories : Electron Gun, Fabrication

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