Vacuum Check

11 02 2012

All photos.

I have Swiss TV journalist Yves Gerber in the lab today. I will try the electron gun again while he is here.

Previously in the comments, Olivier suggested the vacuum pressure is too high. Indeed, at 1.66 millitorr the pressure was higher than I wanted.

My first goal today is to check the empty vacuum chamber with blank flanges. A best case scenario.

With the initial pump down I only got down to ~3 millitorr…  about the same as last time. I used the stethoscope to listen for a leak but did not hear one.

I tried tightening the flanges one last time, and suddenly the pressure started dropping again.  I forget how much torque these conflat flanges need to fully seal.

Now I am seeing pressure in the range of 0.098 millitorr and dropping. Much better!

So now let’s install the electron gun, and see what we get.

UPDATE:

With the electron gun components in the chamber I am able to get down to 0.27 millitorr… not bad!

But when I turn on the electron gun… still no beam.

Exhausting!

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2.36 KiloAmps

6 09 2011

All photos.

Today I tested the newly rewound polywell coils and thicker leads. Best shot was 2.36 kA.

Now we are in the right neighborhood.





Terrifying Power

4 09 2011

All photos.

Tonight I really experienced the power of the coil power supply. Whoa.

I’ve been working to increase the coil current from ~1.2kA to ~2.5kA.

Previously I discovered the coil discharge path had more DC resistance than expected.

I rewound the Polywell coils with 16 gauge wire (previously 18 gauge).

The 16 gauge DC resistance is 144 mΩ compared to 227 mΩ for 18 gauge wire.

I beefed up other wires on the coil discharge path (4 gauge):

Lets test the wiring with the dummy coil:

I took the power supply up to 100V… a small test charge…

When I fired, the noise from the coil made me flinch. It was never that loud before.

Lets turn up the power!

300V for second test.

When I fired the coil there was lightning! HOLY CRAP.  Look what happened:

The coil fucking wrapped itself around the transformer (electromagnetic forming). Then it discharged to ground:

So I haven’t measured it yet, but I think we are getting more current to the coils.





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:





Coil Power Supply: Current Measurements

22 08 2011

All photos.

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

All photos.

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

All photos.

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!








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