17 07 2012

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During the first trial, our langmuir probe told us that our electron beam intensity was fluctuating at 60 Hz.

This is a problem, because one of the main things we are trying to study is the way changes in beam intensity affect potential well depth, so we want a steady intensity. The frequency of the fluctuations suggest that the AC-powered hot cathode is to blame.  I don’t totally understand the details of how a hot cathode running on AC 120v 60Hz translates to this waveform:

data from the langmuir probe displayed on the oscilloscop

The important thing is to prevent it. To do that, I put a full wave bridge rectifier in the power supply. It converts the AC coming from the wall to DC

It has three essential components.

1) The bridge rectifier

This change the AC sine wave into a waveform expressed by the function abs(sin(x)):

Better, but still not steady DC.

2) The filter capacitor

This gets rid of the ripple. you could compare the capacitor to a bucket with a hole in the bottom. Even if I vary the rate at which putting water into the bucket, the rate at which it come out is always going to be more or less the same, provided that it is sufficiently large compared to the volume of water going in.

However, its impossible to get an absolutely perfect DC output with this setup, because the ammount of charge on the capacitor does affect the voltage at which the current comes out.

This 680 uF capacitor takes away enough of the ripple for our purposes:

the output of the power supply when hooked up to a light bulb

3) An isolation transformer

Usually, the diode bridge and the capacitor would be enough, but our AC isn’t coming from the wall, its coming from a grounded auto transformer. this is a problem because the rectifier only works if the AC input is floating. A transformer with an equal number of primary and secondary wingdings accomplishes this without stepping the voltage up or down.

Nest step is to test it in the chamber.

Domenick Bauer

## 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

## Electron Gun Power Supply

5 06 2012

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

18 11 2011

NYT is covering Leeds Radio. My favorite spot for electronics in NYC!

Richard is an awesome dude… very knowledgeable and great stories of EE insanity.

I’ve learned much about the history of electronics just from his inventory. The vacuum tube era was a whole nother way of doing things.

28 10 2011

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

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.

## Supercapacitors are Super

19 10 2011

Electric double-layer capacitors are a newish energy storage technology. I find them fascinating and I want to play with them.

Like a capacitor they can charge and discharge very quickly due to their low equivalent series resistance.

But supercapacitors store roughly two orders of magnitude more energy than normal capacitors!

I bought five 3000 farad, 2.7V Boostcaps:

I machined aluminum strips to form connecting bars. A 13.5 V series stack:

Check out this video of a full-charge short circuit:

## Discrepancy Between Circuit Simulation and Reality

29 09 2011

Previously we modeled the polywell coils and power supply in SPICE.

Today I returned to that model.

All resistance values in the simulation are based on real world measurements with the exception of coil inductance (code).

Starting with an estimate for coil inductance of 0.1 mH the discharge current looks like this:

The simulation’s peak of 1.5 kA is nowhere near the 2.3 kA we are getting in the real world:

OK. Maybe the value for coil inductance is off?

I played around with the value for coil inductance but the simulation would not match reality.

As a control I replaced the simulated inductor with a 1 mΩ resistor (code). Looks like this:

The simulation predicts ~1.8 kA but in reality we see 2.3kA!

Where does this discrepancy come from?

UPDATE: Reader Andrew solved the mystery:

You could try changing the ON resistance of your switch/SCR to something a bit lower than 100mOhms

.model MySwitch SW(Ron=.1 Roff=1Meg Vt=3 Vh=0)

I can’t see the part number of your SCR but 2mOhms would seem reasonable.

I didn’t notice that rather high resistance lurking in the SCR model.

Now the simulation matches reality very closely with 0.06mH coil inductance (code):

Good work Andrew and the rest of the internet brain!