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

Hot Cathode Electron V2

29 04 2012

All photos.

Today I’m taking some small steps toward building hot cathode electron gun v2.

Here is a physical diagram and schematic:

This is the hot cathode intended for  an electron beam welder. It has 34 mΩ resistance:

The parts of the electron gun will be arranged like this:

This is more simple than most electron guns. I don’t need a carefully focused beam, I just want to shoot a crude beam of electrons into the Polywell.

The next step is to design and build a simple armature to hold the pieces together.


Comments : 4 Comments »

Categories : Electron Gun, Physics, Plasma, Sydney Experiment

Electron Gun Redux

16 04 2012

The hot cathode electron gun has been a real pain in the ass.

Based on comments we may have one or more of these problems:

1) The hot cathode has been fouled by operation in normal atmosphere. I tried to avoid this, but who knows.

2) When I removed the CRTs glass casing, I may have removed a high voltage anode.

3) The phosphor screen may not be grounded well enough, causing deflection of the electron beam by charge accumulation.

4) The vacuum pressure is too high, scattering the electrons.

5) Something else.

My plan is to set aside the CRT based  electron gun, and try to build a simplified electron gun from scratch.

Previously I purchased a fresh hot cathode designed for electron beam welding.

The gun will consist of a hot cathode floating at -100V and a copper tube segment at +2000V for acceleration. A grounded phosphor screen will reveal any beam that forms.


Comments : 1 Comment »

Categories : Electron Gun

Vacuum Check

11 02 2012

All photos.

I have Swiss TV journalist Yves Gerber in the lab today. I will try the electron gun again while he is here.

Previously in the comments, Olivier suggested the vacuum pressure is too high. Indeed, at 1.66 millitorr the pressure was higher than I wanted.

My first goal today is to check the empty vacuum chamber with blank flanges. A best case scenario.

With the initial pump down I only got down to ~3 millitorr…  about the same as last time. I used the stethoscope to listen for a leak but did not hear one.

I tried tightening the flanges one last time, and suddenly the pressure started dropping again.  I forget how much torque these conflat flanges need to fully seal.

Now I am seeing pressure in the range of 0.098 millitorr and dropping. Much better!

So now let’s install the electron gun, and see what we get.

UPDATE:

With the electron gun components in the chamber I am able to get down to 0.27 millitorr… not bad!

But when I turn on the electron gun… still no beam.

Exhausting!


Comments : 11 Comments »

Categories : Electron Gun, polywell, Sydney Experiment

Disappointment

13 12 2011

All photos.

Today I did another test of the CRT based electron gun in the vacuum chamber.

Spoiler alert: I didn’t see an electron beam.

This time I put a grounded piece of phosphor right in front of the beam path:

 

Here is a closeup of the grounding wire:

I printed the phosphor holder with 1% infill so it’s basically hollow, then I drilled it with holes to allow gas to escape:

 

The vacuum reached 1.66 millitorr which is not amazing, but good enough for this run.

I tested the wiring with the unbroken CRT, and it looked good:

 

Then I wired the power supply to the vacuum chamber feedthrough:

The I fired it up, you can see the hot cathode working:

But no beam.

:(


Comments : 5 Comments »

Categories : Electron Gun, MakerBot

Just Look at this Print

5 12 2011

All photos.

Previously I tested out the electron gun and I didn’t see a beam on the phosphor. I wanted to rule out the possibility that there WAS a beam, but it wasn’t hitting the phosphor screen. By moving the screen closer I can see the beam no matter where it is pointed.

So I designed and printed this sweet phosphor holder:

I have a small piece of broken phosphor glass from the CRT. I used the excellent OpenSCAD polygon editor to make a form fitting mount:

In real life:

That was easy!


Comments : 4 Comments »

Categories : Electron Gun, MakerBot

SCR Upgrade

28 10 2011

All photos.

I received the new SCR. It can handle 3000 amps peak current. It is much larger than the SCR I recently destroyed:

IRKT136-14 data sheet.

Here it is installed. Everything works:

I also added leads to the current sense resistor to avoid touching the back of the power supply:

The pressure necessary for plasma is still too high. I tried another tweak to the electron gun:

No joy. It requires about 24 millitorr for a stable plasma. UG.


Comments : 1 Comment »

Categories : Electrical Engineering, Electron Gun, Sydney Experiment

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