Jimlux - thank you so much for your very detailed response - much appreciated!
Ok - yes 500kV is likely very difficult to physically contain as you say, so I was thinking about what I will actually need.
This is my thinking from your reply...
The setup has two 'sides' so there will be two separate psu's. I'm thinking that I could aim for 250kV per side, one referenced to earth the other to positive to give opposite polarity and effectively give the system 500kV where it matters. Still a challenge, but I've seen some CW towers online working well to 100Kv per side even with only air and plexiglass, diodes mounted on the other side from the caps. I'll be printing an enclosure for the CW and will either use epoxy or oil to insulate. Oil might be better as I can't fit the build into my small vacuum chamber to remove the air = corona...
I saw some people using multiple series power resistors at the CW output - is that to put a resistive 'cap' on the CW to help minimize the losses through the tower?
___________________________________________________________
In my existing setup the signal generator channels are fed into isolated gate drivers and then to the mosfets at around 24v DC (to maximize the switching effectiveness). This is Master Ivo's half-bridge pcb with a slight modification which works well.
I also have a DC feed into the drain and source of the series mosfets so I'm planning to control the overall system voltage from here.
By taking the Source (neg) and Drain (pos) from the pcb, it seems to make sense now to feed this directly into an AC transformer using the gate driver output of 24v (variable up to 64v DC with the external DC psu). The AC transformer would then take it up to 25kV AC, into the first stage CW...
This is where there are holes in my understanding - how much of the original signal will be intact or lost on the other side of a standard AC transformer which is then rectified thru the CW multiplier?
The frequencies I am able to use with the gate driver IC is anything up to almost 2 Mc/s, but I would initially anticipate 30Kc-300Kc, just from my impulse experience...but this setup might well respond better to higher frequencies in the Mc/s... in which (future) case I may need to adopt a different input strategy to amplify my sig gen only (rated to 60Mhz), or more likely look to a much higher frequency setup. I just need the flexibility to change the nature of what is adjusting the plasma field characteristics within David LaPoint's coherent magnetic field.
So generally speaking I think this is a good initial approach to the psu, but I'm still unsure about how well the HV AC transformer will transmit the information from the sig gen to the plasma.
I looked up the cascaded AC transformers - also an interesting option, but whilst the desired output is HVDC I think the CW multiplier is likely more useful.
I have an abundance of enthusiasm, and a lot of inexperience! ...so I'm really grateful for any input.
Thank you!
Ian.
On Thursday, 9 February 2023 at 10:24:43 GMT-5, jimlux <jimlux@xxxxxxxxxxxxx> wrote:
On 2/7/23 9:08 PM, Ian Gault wrote:
> Hello,
>
>
> I’m interested in replicating David LaPoint’s ‘Primer Fusion’ project.
>
>
> This is my first plasma project. I have been working with high voltage
> impulses up to 7kV regularly, but am asking advice on the best way to
> build a psu providing ~500,000v.
That's quite a challenge, mostly from the mechanical side - insulating
500kV is not easy, especially things like avoiding corona.
in air, uniform field breakdown is around 30kV/cm (70 kV/inch) (round
numbers), but the field isn't uniform on most HV gear.
All your conductors need to have big radii of curvature. Immersing in
oil or SF6 helps a fair amount, but brings with it a bunch of other
difficulties (leaks, mess, etc.)
>
> I plan to use a Cockroft Walton multiplier in 20-30kV steps, but need a
> psu to provide 20-30kv to the first stage.
Yes, that works - Take a look at the C-W generator pictures in museums
(there's a nice one at the Scottish National Museum in Edinburgh)
https://www.nms.ac.uk/explore-our-collections/stories/science-and-technology/cockcroft-walton-generator/
that's 1 MV and fairly old technology for the HV rectifiers, but it is
air insulated, and gives you an idea of the size needed to get insulation.
Tesla coils get up to around 500kV, but you'll note they generate
copious ozone (indicating corona) and, of course, have sparks into free
space.
>
>
> The main difficulty I’m having is that I want to be able to manipulate
> the:
>
> 1 – Voltage 0-500Kv
Varying 0-500kV with a single knob is possible, but more likely you'd do
it with various taps (add/remove parts of your CW stack, for instance)
>
> 2 – Phase
The CW is a DC output device, maybe what you want is a cascade transformer?
>
> 3 – Polarity
You can reverse the rectifiers in a CW and flip the polarity. Not with a
switch, but more like by restacking it.
>
> 4 – Frequency
What range?
>
> 5 – Duty (possibly)
>
>
> This would require amplifying the input signal without altering its
> nature.
that's really, really hard.
You didn't mention what current you need. Microamps? Milliamps?
>
>
> It looks like ZVS into Flyback tx’rs will only work on specific
> frequencies, so could a NST work?
A NST will do a nice job of getting you ~15kVrms from line voltage, with
some limitations. you could drive it with variable voltage and
frequency (within limits).
>
>
> I plan to use an existing system I have with dual channel signal
> generator to control freq, phase, and duty into dual gate drivers/HV
> Mosfets for impulses, but also require a continuous power feed as an
> alternative.
If you're doing AC, then a cascade of transformers might be a way to get
what you need. The classic way is to have transformers with multiple
windings and really good insulation. Say you want 500kV. If you have 5
transformers that have 100kV output (and insulation to match), you run
them in series. The trick is how do you get primary power to the
transformers at the top of the stack? Say your transformers are 20:1,
5kV in and 100 kV out. The bottom transformer has 3 windings, 2 for 5
kV, and one for 100kV. One of the 5kV windings is insulated for 100kV.
You feed the low voltage 5kV, take the higher insulation voltage 5kV and
feed it to the next transformer up the stack, which sits at 100kV above
ground potential.
https://top10electrical.blogspot.com/2014/11/cascaded-transformers-method-for.html
has a nice picture.
>
>
> Any advice would be greatly appreciated.
>
>
> Many thanks,
>
>
> Ian.
>
>
> Sent from [1]Mail for Windows
>
> References
>
> 1. https://go.microsoft.com/fwlink/?LinkId=550986
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