Inconclusive Symmetry Test

14 08 2012

The symmetry test was an experiment we ran to determine if our potential well was symmetrical across its x-axis. It featured three langmuir probes, one in the center, on at the extreme left, and one at the extreme right. more details are in this post.

While there were a few interesting results few interesting results, on the whole the experiment was inconclusive, and totaled our electron gun assembly.

The first problem we encountered was the vacuum level. We barely got into the 10^-4 Torr range, and when we turned on the electron beam, the pressure went up into the 10^-3 Torr range. High pressures like these don’t render the experiment impossible, but they certainly don’t help. Ideally, the only particles in the chamber would be electrons, and so everything else just adds to the list of unknown factors.

The e-gun was running normally, giving us readings of about -50VDC on the oscilloscope.

The glow from the hot cathode

For the first shot we did a control. We hooked up one probe to the center langmuir, and one the the shunt resistor on the on power supply, and we got a small well.

 

So everything was working as expected, despite the unusually high pressure. This is good, but also strange in light of the last test results, in which the charge at charge at the center of the core became less negative when we fired the coils.

Then we switched the oscilloscope probe on the coil power supply to the left langmuir probe, and fired.

Top: Left langmuir probe
Bottom: center langmuir probe

Nothing on the left langmuir. We tried again with more power going into the coils.

Here’s what we got

The charge at the leftmost extreme of the well is about -3VDC, and the charge at the center is about -10VDC. Not surprisingly, electron density has some relationship to distance from the center of the core. We intend to eventually define this relationship precisely, but to do so would require much more data.

Notice how even before the coils were fired, the top line is a slightly below its zero point (indicated by the crosses at the left of the screen). This means that for some reason, the left langmuir is brought to a slightly negative potential by the electron gun, even though it’s not pointed anywhere near the probe

We then switched the oscilloscope probe on the left langmuir to the right langmuir, and fired the core again. The moment we did, we heard a metallic noise from inside the chamber, like a coin dropping onto a metal surface. Can’t be good. This was the readout on the oscilloscope

Not especially meaningful to us.

We rightly assumed that the noise meant our trial was over, so we opened up the chamber.

We found the accelerator anode laying in the bottom of the chamber. The heat from the filament melted the plastic enough for the screw mounted in it to come loose. Everything around it was coated in a thin film of blue plastic, and much of the wire insulation was burnt as well.

 

Obviously, ABS plastic and rubber insulated wires just aren’t right for this experiment. They can’t take the heat of the cathode, and they out-gas so much that they ruin the vacuum.

Back to the drawing board.

Domenick Bauer





Armature Upgrade and More Trials

24 07 2012

All photos(1/2)
All Photos(2/2)

We weren’t satisfied with the layout of the inside of the chamber last time. The alignment between the center of the polywell core, the electron gun and the lagmuir probe wasn’t very good, so I upgraded it.

The main advantage of this version is simplicity. It puts the filament, accelerator, and probe, and polywell core all on one flange, rather than two, which means the whole thing can be assembled on a desk and then put into the chamber Much easier than trying to make electrical connections and get alignment right with the thing half way in — something which, rest assured, is a royal pan in the ass.

Another advantege of this one is strength.

Dosn’t look like it, but that’s actually a very strong connection

A big problem last time was that we were unable to get the whole assembly into the chamber without accidentally bumping it a little, and ruining the alignment. This version is more securely attached to the flange, making it easier to keep all the components in the right place and pointing the right way.

Another flaw in the earlier version: The screw which attached the accelerator to the armature extended pretty far inside the copper sleeve which we were using as the accelerator anode, so it was partially blocking the beam.

In this version, that connection has a much lower profile.  I also switched out the old copper sleeve — which was too big and full of holes — for a new smaller one.

While trivial things like holes in the accelerator probably don’t matter, I’m trying to correct them because at this stage of the game, our goal is to eliminate as many variables as possible before we really start collecting data on a large scale. The cleaner the setup, the better.

We also took Remy Dyer’s advice and grounded the positive side of the DC output going to the hot cathode, in order to maximize the potential difference between the accelerator anode and the cathode.

Then, I pumped down the chamber and tested the electron gun.

If you look closely, it’s clear that all the components are aligned pretty well

The lower line is the voltage picked up by the Langmuir probe. The little cross shaped marker on the left side indicates the zero point. Every box in the y direction is equal to fifty volts, so our electron bean is delivering -50 volts to the Langmuir probe, with almost no AC disturbance! Not bad, and this is with the voltage across the hot cathode at about 60 volts out of a possible 120, so it could get even higher.

With this bigger, badder electron gun, we ran another set of trials.

We pumped down.

Hooked everything up.

And took our first shot.

As before, the lower line indicates the voltage on the Langmuir probe at the center of the Polywell core, where the potential well should be. Instead of a well, we have a hill! The magnetic fields generated by the Polywell are supposed to compress all the electrons within the core into its center, so the voltage detected by the Langmuir probe should go even lower. Instead, it goes up, from about -50 to -25. Very confusing.

We rant it a few more times, increasing the current sent through the coils each time, which translates to stronger containment fields. The results were similar.

Here’s a strange one where the center of the core seemed to become very positive. We suspected an arc.

Sure enough, we couldn’t get the core to discharge after this, so we worried that we fried the coils. When it was safe to do so, we opened the chamber and saw that there was an arc, but thankfully not on the core.

Evidently, there was a bad connection between this din rail connector and the core feed throughs on the inside of the chamber, and so an arc occurred, and broke the connection entirely.

So that was that for our trial.

It’s left me confused. What on earth could be making our well positive instead of negative, whereas in the last trial, we got good, negative wells?

Domenick Bauer





New High Voltage Probe

18 10 2011

Just got this Tektronix P6015 high voltage probe. It goes up to 40 kV!

Well actually it only goes to 27 kV without the fluorocarbon 114 dielectric.

It’s huge! Shown next to a normal probe:

Just in time for the next run.

This is the same probe used in the Sydney experiment.





Oscilloscope Camera Mount

13 09 2011

All photos.

Reader jsults turned my on to g3data… it’s a little open source program that helps extract data from graphs.

It looks like this in action:

I tried it with my oscilloscope photographs and it worked OK.  But it does not compensate for trapezoidal distortions.

If my camera were perfectly lined up with the oscilloscope g3data would work great.

This got me to thinking… maybe I could build a camera holder for the oscilloscope?

So I did.

I designed this mount using sketchup and had it printed at shapeways. It came in the mail today.

 

Works like a charm. Now all my oscilloscope photos will be perfectly centered and flat:

You can download at thingaverse or purchase at shapeways.





How to Read an Oscilloscope

25 08 2011

Here is a quick tutorial on how to read an oscilloscope.

Voltage increases as you go up the screen.

Time passes from left to right.

The three numbers circled below are the keys.

In this example:

1V  means the distance between each gridline bottom to top  represents 1 volt.

500µs means the distance between each gridline left to right represents 500 microseconds.

2.160V is the voltage between two lines I manually adjust. This is called a cursor.

The small cross circled on the lower left indicates zero volts.

Those are the basics of reading a ‘scope.





Reality Check

14 02 2011

All photos.

This project has been an ongoing lesson in electrical engineering. Now with an oscilloscope I can finally see a circuit’s behavior.

I’ve spent the last week reviewing my assumptions about the most basic circuits and components. Sometime my hunches are correct, but just as often I am confounded by what I see. Here is my test setup:

The most interesting behavior happens with an AC signal. Conveniently the oscilloscope has a built in square wave generator intended for calibration. I am passing this square wave through test circuits to see how the wave changes.

The oscilloscope generates a square wave that goes from ground to +0.4 V. The frequency ranges from 50 Hz to 5 MHz depending on the time setting. I start with 50 KHz. I use two probes. The first probe is connected to the signal source, the second probe is connected to various other points in the test circuit.

The first thing to note is the signal generator doesn’t provide much power. If you overload the signal generator, you will see it’s voltage sag.

Both probes ground to the oscilloscope chassis, so choosing appropriate ground points is crucial … incorrectly grounding a probe can drastically change the circuits behavior.

I started by looking at a single capacitor. I tested this circuit:

In this capacitive coupling configuration a capacitor removes the DC component from an AC signal. Probe 2 shows the same signal as Probe 1, except shifted down. Probe 1 goes from 0 V to + 0.4V whereas probe 2 goes from -0.2 V to +0.2 V. So that’s what it looks like to block the DC component.

Next I passed the square wave through a transformer. I’m using a variable resistor to limit the current into the transformer.

This is what I see:

signal generator at top. transformer output bottom

It doubles the voltage as expected, and adds quite a bit of color to the waveform. It also draws enough current to make the signal generator’s voltage sag… even the ground line as seen in this video:

From 2011-02-12

At some frequencies, the transformer really changes with waveform:

signal generator at top. transformer output bottom





60 Hz Hum

7 02 2011

All photos.

In the US the mains power runs at 115V, 60 Hz. With an oscilloscope, I can see this mains hum everywhere. It radiates from the AC power lines throughout the lab and the whole city. Appearently our bodies make great antaneas for 60 hz: touching the oscilloscope probe shows a 9V RMS @ 60 Hz. Like a nine volt battery!  From my body!

I also noticed that the using the probe clip picks up more mains hum than just the probe tip.

I’m learning how to use the scope. I suspect it will become my main measurement tool.

Today I received a Tektronix p6109 general purpose probe compatible with my scope:








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