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.

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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.





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!





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.

:(





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!





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.





Electron Gun Tweaks

20 10 2011

All photos.

Today I tweaked the electron gun.

My goal is to bring the electron gun closer to ground. This should reduce the pressure needed to strike a plasma.

I used Stuart’s DIY spot welder to make a hoop from welding wire:

I installed this hoop on the electron gun:

This arrangement brings ground potential closer to the electron gun.

And it worked somewhat.

With this hoop, I can strike a stable plasma at 18 millatorr. Better than yesterday’s 25 millatorr. Still needs work to get back to 8.5 millatorr.

When I went to fire the coils I discovered my SCR is shot:

Turns out the SCR’s peak current is 2,000 amps. At 2,300 amps we are well beyond that!

I have to order a new SCR tonight. Suggestions welcome.

Elsewhere… I drilled a hole in the langmuir probe holder to allow trapped gas to escape:

Also tested out the supercapacitors in my friend Arnie’s forklift:

They were able to lift Arnie twice!

Video of operation:








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