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:



2 responses

24 08 2011
Raymond Rogers

A couple of things; what was the resistance of your additional wire?
More to the point you should probably try reversing the the winding direction in the middle of the new coil. There are better ways to cancel the inductance but I doubt if it makes much difference.
There is a question: Is it possible that the spike is coming from ionization (or some such)?
If you would like I could do a circuit simulation; but capturing non-linearities due ionization would be difficult.
This last set of waveforms seems to be significantly different than yesterday’s. Maybe it’s due to moving the scope probe?
I like my waveforms to be consistent.

25 08 2011

The coil proximity to the aluminium sheet will produce interesting effects. The sheet acts like a short circuit secondary, producing a field in the opposite direction pushing the coil away.
Putting some of your coils in parallel is another way to increase current (at the expense of pulse length).
As the pulse has a frequency component, skin effect will add to the resistance that determines max current. (something else to think about).

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