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





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





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





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





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:








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