Electron Gun Operational!

29 08 2011

All photos.

Electron gun operational:

I got these high voltage supplies made for CRTs:

I still need to play around to get it focused, but a great start!





Coil Inductance

27 08 2011

Let’s take a look at the coil inductance.

A fun and easy place to start is look at some AC going through the coil (suggested in comments).

I used the oscilloscope’s test wave generator. I’m using two 10:1 probes connected thusly:

Channel 1 is shown above channel 2 on these oscilloscope screens:

Not much distortion at  500Hz:

At 50kHz we start to see some distortion:

At 500kHz we have obvious distortion:

Not sure what this tells us, but I’m sure it tell us something!

Now lets try a more abstract approach to the problem. We have a 6cm diameter air core coil with 60 turns. I found this handy inductance calculator. I looked up the thickness of 18 AWG wire and the relative permeability of air.

Theoretically the coil has 0.56mH of inductance. Sound right?
Given inductance and frequency, you can calculate impedance…which is what we’re after. But what’s the frequency of a single pulse?

UPDATE: The rectangular function may be useful for calculating frequency.





Ground Resistance

27 08 2011

All photos.

Now that I have a milliohm meter, I want to measure the resistance of everything.

Ground quality is important for safety and signal quality.  So I measured the resistance of my grounding network. I made two 5 meter long Kelvin probes to do so:

I have two main ground connections. One is the ground pin on a standard wall outlet. The second is a braided wire to a water pipe:

Normally these ground exits are star connected. As a test I disconnected them from one another and measured the resistance between them: 700mΩ.

For various test points on the machine and rack we see from 10 to 100mΩ

I realized that this potentiometer is not grounded at all!

I will ground this forthwith!





Wire Resistance

26 08 2011

All photos.

To reach 2.5kA with a 450V power supply, the coil cannot exceed 180mΩ.

I removed the Polywell from the chamber to measure resistance of the coil alone.

The coil contributes 227mΩ.

Our 6cm diameter Polywell uses 11.31 meters of 18 AWG wire.

Based on wire resistance charts, 16 AWG wire would contribute 149mΩ.

One pin of small feedthrough: 33mΩ:

The bellows feedthrough has a tiny 1mΩ. I should use this for the coils (currently used for langmuire probe):

One leg of the power supply cable with connectors: 39mΩ. This can be improved with a shorter thicker cable.

I ordered 16 AWG magnet wire.





DIY Milliohm Meter

25 08 2011

I asked Joe Khachan this question:

Did you go to great lengths to reduce the resistance of the coil path?

He responded:

Yes we try and use a thick enough wire so that the resistance is small but it cannot be too thick in order to be able to have many turns. More importantly is the contact resistance when connecting to the wires. We try to keep this low. From this perspective, a milliOhm meter is helpful.

Milliohm meters start around $200 on ebay. Too much.

Then I found this excellent tutorial on Kelvin (4-wire) resistance measurement.

Luckily I had a current sensing resistor on hand for the current reading:

I used the resistor on the right.

So current sense resistor plus two voltmeters and BINGO: milliohm meter. Results:

The coil path is 0.35Ω.

The dummy coil is 0.185Ω

As a calibration I tested the 0.001Ω resistor. Result: 0.00088888

Not bad.

Now I have excellent tool to troubleshoot resistance in the coil path. AWESOME.





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.





Getting Current to the Coils

24 08 2011

All photos.

I am currently repeating an experiment performed by Joe Khachan and Matthew Carr in Sydney, Australia.

Their experiment is written up here:  “The dependence of the virtual cathode in a PolywellTM on the coil current and background gas pressure” ($1.99 pay wall)

Joe and Matt were able to delivery 2.5kA to the coil:

 The coils were driven by a pulsed current power supply that consisted of a 7.5 mF capacitor bank, which could be charged to a maximum voltage of 450 V….A maximum peak current of 2.5 kA was achieved.

We are seeing an effective resistance of ~0.45Ω compared to their 0.18Ω.

We need to lower the resistance and increase the voltage.

I started with raising the voltage. I rewired the coil power supply to use 2 capacitors in series: 0.3mF, 900V

The power supply’s transformer and rectifier only go to 600V (but I pushed them to 800V without issue)

With 800V we get 1300A. More current, but effective resistance increases to 0.61Ω.

OK fine. Lets try lowering the resistance with a dummy coil directly connected to the power supply. 45 turns 6cm diameter. The Polywell coil is same size but 15 more turns.

Here it is connected:

Now we are clearing 2.5kA with 600V!  But look at the strange pulse shape. Hmm.

I thought having the coil so close to the power supply might be a confounding factor. I added 1M of 12 gauge stranded wire to distance the coil:

Revealingly, just adding that 1 meter of wire reduced the current by almost half for same 600V:

So clearly delivering current will be a design challenge.

A note on technique. Based on comments I now ensure probes are perpendicular  to current:








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