## Getting Current to the Coils

24 08 2011

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:

## Coil Power Supply: Current Measurements

22 08 2011

Today I took a step back and measured the current going through the coils.

Previously I measured 1,200 amps going through the coils with the capacitors charged to 450V.

My setup is a little different now: There are 2 meters of cabling + feedthrough between the coils and their power supply.

For all of these shots, the capacitors were charged to 450V. The shunt resistor shows 100mV across for 100A through. Multiply the voltage by 1000 to get the current in amps.

Surprisingly I’m seeing significant variation of current for identical conditions.

The most current I saw was 1,095 amps:

But with the same conditions I saw this much lower 344 amp current:

The median current reading was around 734 amps:

Disconnected, the coil shows 0.8 ohm of resistance.

I’m rather surprised by this variation. What could be the cause?

I tried some other conditions.

With the capacitors charged to 200V, I got 560 amps of current:

I also tried charging 5 out of 10 capacitors to 450V. With an average current of 500 amps.

## Deepest Potential Well Yet: 43 Volts

17 08 2011

I’ve been running shots on the Polywell yesterday and today.

Just got my deepest potential well yet: 43 Volts.

10KV, 10mA on electron gun. 420V through coils. 8.5 millitorr air:

Be sure to check out the conditions I ran yesterday and today. Each shot has an oscilloscope photo with experimental parameters in caption.

## Vacuum Degreasing

15 08 2011

A vacuum specialist suggested a better technique for degreasing vacuum equipment:

Lightly swab with acetone, followed by reagent grade ethyl alcohol.

The acetone degreases, and the alcohol removes residual acetone.

I used this cleaning technique before reassembling the Polywell:

Back in the chamber soon!

## Coil Power Supply 10 Cap Test

8 05 2010

Today I did a full 10 capacitor 450V test fire of the coil power supply into the coils:

No problems, everything went well.

On a disappointing note… it looks like Goddamn vacuum pump is broken again: the vent value going into the turbo pump is stuck open, allowing atmosphere to flow into the chamber. Maybe I fried the valve with a voltage spike.

## Coil Former Complete

16 02 2010

Behold:

I plan to replace the screws with stainless steel screws and clean it all in muriatic acid before putting it in a vacuum.

## Coil Former Progress

15 02 2010

Did some more work on the coil formers. Stuart and I used the computer controlled milling machine to precisely drill the holes. First we mount the teflon coil former in a chuck:

Then we center the spindle:

Drill the holes:

Next we take some aluminum angle bracket:

Cut them down to size:

Drill the holes:

And tap:

Assemble:

## Coil Power Supply

13 02 2010

Joe Khachan just sent me details on the power supply for the coils. They are designed to produce a brief high current pulse.

Joe says:

The part that took the most time to build was the power supply. I’ve attached a diagram that looks something like our circuit without dump resistor to dump charge of the capacitors when we need to service them. We used a hockey puck type of SCR (a type of thyristor) that can take 1000 A continuous or 10 kA pulsed. This may have been an overkill because we found that a maximum of 300 A was needed. However, we may need higher current as we increase the size of the polywell. The capacitor bank was make of 5 X 1500 microFarads electrolytic capacitors that can take a maximum of 450V all connected in parallel. You need some kind of transformer that can step up the voltage from the mains and be able to charge the capacitors within a couple of seconds. That means it shoud be a reasonably hefty transformer. We control the voltage output of the transformer with a Variac on the input side. You need to protect the SCR from back EMF with a diode across it. The diode should be able to take a few amps. The wire diameter about one millimeter and there were 10 turns per former.

Joe’s coil power supply looks like this:

Here is the preliminary bill of material (click for Mouser produce page):

1500 microFarads electrolytic capacitors max 450V

Diode Diode to protect the SCR from back EMF

Variac

Power Rectifier

Trigger Isolation Transformer

18 AWG magnet wire

100 watt 2KΩ resistor for bleeding capacitor bank

Rack mountable chassis.

Please comment if you notice wrong parts.

## Coil Former Dimentions

9 02 2010

Here are the final dimensions of the coil former:

## Sydney Plan

8 02 2010

Coil formers: First I must machine a teflon rod on a metal lathe. I have a machinist lined up for this. Teflon rod is at the lab.

Coil electronics: waiting for schematics from Sydney team.

Electron Gun: fabricated. Awaiting second test.

Langmuir Probe: Looking for a suitable HV feedthrough. I need to purchase ceramic tubes. I already have the high voltage divider.

Here is what it would look like:

This setup would use the existing vacuum chamber.