CW, plasma, Paschen curve

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Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

Here is the LTSpice result of our CW under a 3 MOhm load, from 2019

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Richard Hull
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Re: CW, plasma, Paschen curve

Post by Richard Hull »

I still worry that you case of the transformer may be floating if you are using the two knobs and a simple voltage doubler. Again read the FAQ I supplied in the prior post. A 15kv transformer's metal case can be lethal if not grounded!! The transformer is set up to be, (left knob 7.5kv... zero volts @ grounded case center tap....7.5kv right knob). Voltage doubling across the knobs and grounding the plus of the doubler to use the negative as hot will leave the metal transformer case at a deadly 7.5kv to any one touching the case!

A few fool-hearted souls have done this but wisely placed the transformer in a thick plastic case where no one could touch the case during operation. One still runs the risk of the secondary to primary insulation breaking down as the core is at 7.5kv. By reading the FAQ specific to the neon transformer you will see the current at voltage is grossly reduced. Add to this fact that the doubler has low capacitance in its storage filters and the current available really plunges even further to almost nothing.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

I will go and read that ... however I can assure you that the center tap of the secondary on the NST is grounded. What gives you the impression that it might not be? Well I guess I can answer that, you doubt that it would output the current we are seeing if it were. (I missed your last post as it ended up on the previous page.)
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Joe Gayo
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Re: CW, plasma, Paschen curve

Post by Joe Gayo »

@gunnar

Heed the words of Richard about safety.

Also, my comment was about multiple milliamperes, like what will be needed ... 5-10mA ... I still suspect you will see a sizable drop ... ~30%+
Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

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Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

@Joe, 32 KV over 3.3 MOhms = about 10 mA. What am I missing?
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Richard Hull
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Re: CW, plasma, Paschen curve

Post by Richard Hull »

I see the transformer is grounded in your image.

Are you floating the two doubler output leads?! If you properly ground the hot positive of the doubler, as you should for safety sake, the doubler might only give negative15kv or above output reference to ground. If you float both leads then you will, indeed, get 30kv or more no load across them but touching either lead will be lethal with reference to ground!

What has saved you might be that no one has touched anything while the power supply system is running and none of the floating leads is within arcing distance to true ground. In this manner, you only have one single lead to insulate and keep away from grounded objects.

Floating HV leads is a bad idea. You only want one single, deadly, hot lead in most all HV applications relative to ground.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

@Richard, you can look at the CW picture at the start of this thread. Yes, only the negative output is used, the left and right outputs are floating. In oil. With nobody closer to the box than 6 ft, and even that is only when we are testing it outside of the normal setup, when nobody is closer than 40 ft, and I can shut down everything manually with an emergency switch outside of the room.
Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

Reload of a log file from last week. This was not in the vacuum system, but in the lab with the 3.3 MOhm load resistor in oil. Note: The NST rms measure only one knob, to ground.

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Last edited by Gunnar Mein on Mon May 31, 2021 3:36 pm, edited 2 times in total.
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Joe Gayo
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Re: CW, plasma, Paschen curve

Post by Joe Gayo »

You mentioned you measured a current of 1mA. I agree 32kv across 3.3Meg ~ 10mA. If you actually measured 10mA, then I'm surprised the NST can source that current at the full 15kV p-p.
Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

Gotcha. Typo in post. Fixed. Thanks!
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Richard Hull
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Re: CW, plasma, Paschen curve

Post by Richard Hull »

Yes, floating and safely out of the way in oil the full doubler will function just not much current at the doubled voltage unless lightly loaded. As most fusors are metal shelled, this would not be possible in a real fusor due to all the vacuum plumbing and all attached instrumentation being at real electrical ground.

We tend to think fusion functioning fusor here when talking HV and safety. Safety grounds abound and there is only one viper to worry about.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: CW, plasma, Paschen curve

Post by Dennis P Brown »

A 60 Hz NST will never delivery any significant voltage and current for a CW that drives a plasma; once you hook it up to any real device to generate a plasma, the voltage will drop like a rock and the cap's will delivery very little current. Yes, a scope with its high impedance will show a different story.

None of this matters to join the plasma club! The device will ignite and hold a plasma so it will function for that purpose. Just don't lose sight of the fact that a VM for a real fusor requires a very high frequency driver circuit to deliver real current at a high voltage.
Gunnar Mein
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Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

@Dennis, I don't know what else to say - we measured 32 KV across a 3.3 MOhm load, that works out to 10 mA for us. There was no high-impedance scope involved, just that load resistor and the actual divider boards we use in real scenarios.

We are going run an LTSpice simulation to convince ourselves that the lowside return on the CW is sufficiently standing in for the highside current. We don't see how it accounts for losses, but we would love not to do the whole optical link to the Arduino and stuff. When that's done, and we have a working current measurement, we'll gather all the evidence to apply.

Today and tomorrow, I am working with 7 students on a couple of improvements - camera under chamber, new 5V offset supply for divider board, maybe shielded cases for some electronics. I will post more pictures.
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Richard Hull
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Re: CW, plasma, Paschen curve

Post by Richard Hull »

I went back to your introduction and you noted you have a "beefy" neon sign transformer. It would be great to know the name plate rating of your transformer. A stock non-beefy 15kv neon is rated at 30ma on the name plate. In the past, 15kv neon xfrmrs were made in 60ma and way back, 120ma current ratings. A 60ma version would support HV at a much higher current level and the very rare 120ma transformer would easily supply 20-30ma at nearly full voltage. Now that is beefy! Those ancient 120ma units were used to drive massive neon tube lengths in giant neon signage. Such signage is rare today and those massive hunks of iron have given way to custom designed electronic switching type supplies for modern neon. One day iron, shunted neon transformers will be talked about around campfires referring to the good old days.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Gunnar Mein
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Location: Kirland, WA

Re: CW, plasma, Paschen curve

Post by Gunnar Mein »

I believe our NST is 15 KV peak-to-peak at 60 mA. Will try to find a picture of the plate.
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Richard Hull
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Re: CW, plasma, Paschen curve

Post by Richard Hull »

That helps explain why you can get the 10ma. 30ma - normal, 60ma - beefy, 120ma - super beefy

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: CW, plasma, Paschen curve

Post by Dennis P Brown »

The output current the the NST isn't the main issue here and I wasn't being clear - the voltage multiplier caps will determine the output power and at 60 Hz that isn't going to be very much. The caps are charging too slowly.
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Richard Hull
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Re: CW, plasma, Paschen curve

Post by Richard Hull »

At 60hz all caps charge at the same rate, (assuming a suitable low impedance transformer), however 200pf caps will charge fully and not be able to support any real current at the multiplied voltage output of the multiplier. With 1ufd caps you will have far more energy to support high currents out of the multiplier. The hitch is that 1uf caps at 10kv are very expensive and rare. If you triple the voltage at 60hz with 1ufd caps, the net capacity is down to .33ufd when you need the current.
If you dream of 60hz multipliers, be ready to spend a lot of money for large capacitance, high voltage caps and dealing with the monstrous stored energy found in a suitable higher current drain capacity to hold up the high voltage you demand from your voltage multiplier.

A good rule of thumb is the impedance of the high voltage transformer, (non-shunted types only), must be must be so low that its rated output current must be (multiplication factor X the desired current output of the multiplier), i.e. a 10kv quadrupler that you want to drain 40ma out of will require the transformer to be fully capable of 160ma at 10kv. .... And.... you must figure out the huge capacitance required at 60hz in the multiplier to deliver the 40ma desired.

Power multiplication at 60hz is pretty much of a fools errand.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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