NST issues

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
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Dennis P Brown
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Re: NST issues

Post by Dennis P Brown » Thu Feb 14, 2019 10:49 am

Richard is correct - plywood conducts easily at those voltages. Again, since neither power nor high voltage is the issue for your experiment (I assume you just want a real, negative DC source), I again suggest you use just a single pole on the NST as a source and ground the other pole. I do this and get 7.5 kV on mine but I use it to power a voltage multiplier - a rather low load so current isn't an issue. A plasma is a near short. A ballast resistor could help.

Using two diodes (or three as Richard suggests) in series will prevent arcing if you either mount them in the air (using ceramic stand offs for the wires) or again, place them under oil. A current meter will have issues if you do not carefully follow the FAQ's on installing. Don't run the high kV directly through the meter.

Pablo Llaguno
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Re: NST issues

Post by Pablo Llaguno » Thu Feb 14, 2019 2:39 pm

I am in the process of installing a current meter so I can confirm the obvious: the fusor is pulling too much current and that’s why I am getting these voltages.

Dennis, I did try the single pole approach and indeed get 7.5 kV when there is only the HV divider as load, when demo fusor is operated I got around 3kV, however maybe if I drop the pressure to <25milliTorr, I’ll get closer voltages to the 7.5kV.

Out of topic: do you use a NST and multiplier as your fusor’s power supply?

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Dennis P Brown
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Re: NST issues

Post by Dennis P Brown » Thu Feb 14, 2019 5:31 pm

A NST can't power any real fusor even with a voltage multiplier. The frequency is far too small. I used that system to generate 100 kV for an ion gun using very large caps.

I use a conventional high voltage transformer for my fusor.

A few good things about a NST: a NST isn't likely to kill you if you make a mistake, but a real fusor power supply is absolutely lethal. Also, at voltages above 15 kV, x-rays start to be a real concern.

A ballast resister (50 k-ohm or in a similar range) can lower the current draw of the circuit and maybe allow higher voltages to develop across the plasma.

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Richard Hull
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Re: NST issues

Post by Richard Hull » Thu Feb 14, 2019 6:12 pm

There is never a need for a ballast resistor on any neon transformer as they are self limiting and auto ballast through their magnetic shunts.

A hard core, regular transformer will need a ballast in a real fusor.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Pablo Llaguno
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Re: NST issues

Post by Pablo Llaguno » Fri Feb 15, 2019 3:55 am

Got a 100ohm 10-watt resistor (read 99.8 ohms in the DMM) and measured its voltage across like this:
Image

Also, I soldered three diodes in each leg and floated the string in the air as Richard said –> no more arcing.

Couldn't test voltage and current under no load but I do have some data from my demo fusor running with the new power supply system. Here's a nice plasma (star mode? there are ion/electron beams) at 47milliTorr:

Image
-3,700V at 23mA

Getting this chamber below 30 milliTorr has been challenging, I'll try to pump tomorrow for a long time and see what kind of pressures and voltages can I achieve.

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Dennis P Brown
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Re: NST issues

Post by Dennis P Brown » Fri Feb 15, 2019 10:50 am

Those pipe fittings will not provide as good a seal as proper vacuum components, so, not gong to be easy to seal the system to get good performance. Also, your valves are not likely to permit good vacuum performance at all.

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