Comments : 4 Comments »
Categories : Core, Electron Gun, polywell, Sydney Experiment
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!
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Categories : Electron Gun, Physics, Plasma, Sydney Experiment
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 : 11 Comments »
Categories : Electron Gun, polywell, Sydney Experiment
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.
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.
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Categories : Electrical Engineering, Electron Gun, Sydney Experiment
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.
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Categories : Electron Gun, Sydney Experiment
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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Categories : Sydney Experiment
Today I wired up the Polywell.
I made an armature to accomodate the new high voltage probe:
I tested the coils: definitely hitting 2.3 kA, which is what we want.
But when I tested the electron gun I saw a problem: It needs 25 millitorr of air to strike a plasma, up from the 8.5 millitorr of the original configuration. This is a problem because high pressure reduces potential well formation.
I was able to take some test shots… everything is working. The potential wells are shallow due to the high pressure.
Here you see the current on channel 1 (the bump that goes up) with a peak of 800A. The potential well is on channel 2 (bump that goes down) with a peak of -8V. For current in amps, multiply by 1000. For potential well in volts multiply by 1000.
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Categories : Langmuir Probe, Sydney Experiment
I’ve been working towards re-running the Sydney experiment with 2.3 kA of current through the coils. This required completely re-arranging the pieces of the experiment.
A big change was moving the langmuir probe to the same flange as the coils for better alignment.
I’ve been waiting for two weeks for this part from shapeways (thingaverse). It’s a new holder for the langmuir probe:
It holds a ceramic tube like so:
Now the probe is always nicely centered:
I cut down the electron gun for the new arrangement:
I made new ceramic standoffs because the previous ones were poorly crafted. These ones turned out nicely:
Here you can see the electron gun on the left and coils in the center:
I buttoned everything up:
I got the vacuum down to 1.1 millitorr. Not great, but good enough for this experiment.
Tomorrow I will wire up the electrical and run the sydney experiment at full power.