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

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.


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

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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