Voltage/Pressure Tuning Advice

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Doug Tallmadge
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Voltage/Pressure Tuning Advice

Post by Doug Tallmadge » Wed Apr 18, 2018 3:02 am

Hi Everyone,

I've been a user for a while, but a lurker because its taken me a while to put my fusor together. I've got a setup right now with a stable and consistent plasma, but I've had a lot of trouble tuning my system to achieve fusion voltages. I'm using a Glassman EJ high voltage supply with up to 50 kV at 12 mA and have had no problems with the supply. What I can't seem to figure out is how to get the pressure just right for a 10 kV+ discharge to be stable. When I'm at 4 mTorr I max out on current at ~8 kV and when I pump a little past that, I get 50 kV on the grid but with no discharge and current.

What I'm wondering is if this is likely a geometry problem with the grid being too large or a pressure control problem. Is that level of fine-tuning in terms of pressure normal, and if so what strategies would you recommend for finding the sweet spot? I've tried slowly pumping down between 4 and 3 mTorr while running the power supply, but haven't been able to land more than a momentary 20 kV at ~5 mA before having the system move below the discharge range.

Thanks! And I apologize in advance if this question would be better suited to a different thread.

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Re: Voltage/Pressure Tuning Advice

Post by ian_krase » Wed Apr 18, 2018 8:18 am

Have you got a ballast?

Doug Tallmadge
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Re: Voltage/Pressure Tuning Advice

Post by Doug Tallmadge » Wed Apr 18, 2018 5:22 pm

I don't currently have a ballast. My initial assumption was that the power supply had internal capability for maintaining the current limit. Would an external ballast potentially decrease the pressure sensitivity by increasing resistance?

Would you have any recommendations for ballast options to look at?

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Re: Voltage/Pressure Tuning Advice

Post by John Futter » Wed Apr 18, 2018 6:56 pm

no you must use a ballast and it should be as close as possible to the fusor.
It is there to provide a real impedance as plasma establishment can result in negative resistance which means you now have an oscillator.
20 to 100k @ 100 watts usually works

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Richard Hull
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Re: Voltage/Pressure Tuning Advice

Post by Richard Hull » Wed Apr 18, 2018 7:12 pm

The ballast will help, however, all wonderful $$$$+ power supplies have an Achilles heel. They are so perfect and so well controlled in so many ways they typically make lousy fusor supplies. They are typically current controlled. Oddly, the manufacturer doesn't want you to burn up their supply..now your supply. Also, oddly, they expect someone who lays out $$$$+ for such a supply to have a specific need for a voltage between 0-50kv with a current between 0 ma and never more than 12 ma. In short you have no "wiggle room".

Gas discharge systems in varying pressures often demand an amp or more when striking a plasma, but might quickly settle down to 50, 20, 10 or even 5 ma current drain based on voltage and pressure. A 50kv 12ma, $$S$+ supply doesn't know or care about this; it just knows 12ma and I am outta' here...(shut down to reset).

A supply running a plasma at 5 microns and 20 kv at a fixed current limit might just snuff out if the controls are set to hold at that level. A real fusor supply will do it too, but you can up the voltage and BAM!..... It will restrike at 25kv on a relight attempt drawing an instantaneous 500ma for 0.1 seconds and settle down to 6 ma. It is the operators job to see the current falling and constantly inch the voltage forward to see that the delicate and finicky plasma never snuffs out, making sure the current is restored to the old current or a tiny bit more as the voltage is raised. Slowly, once again the current starts downward and the operator boosts the voltage by 1kv or so and the current climbs back to where it was,etc., etc., etc......

No fusor will just sit there and work unattended without the human servo mechanism or a cleverly designed replacement.

A real fusor supply should not balk at a 100 millisecond, 1 amp draw. A ballast can act as a bungie cord for weaker supplies. Its value is a matter of cutting and trying to see what works with your $$$$+ supply.

In the end, a fusor, when fusing, will work fine at a voltage of 30kv and a current of 5-10ma. This doesn't mean that a $$$$+ power supply rated at 0-50kv and 20ma will work with a fusor. What it does mean, is that this is required for a working functioning fusor as "a continuous delivery capability". You will need more current in some few instances for tiny fractions of time to get it there.

All of this is due to the nature of varying pressure electrical discharges in a gas.


Richard Hull
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Re: Voltage/Pressure Tuning Advice

Post by Doug Tallmadge » Thu Apr 19, 2018 4:33 pm

Thank you all for this additional perspective. It sounds as if the maximum load requirement from the power supply may end up being a big issue with my design. Do you think that there are worthwhile modifications to explore with this sort of electrical setup, or am I best off cutting my losses and trying to build a custom supply. I'm working on a time frame right now of about 1.5 months but am working on fusor related things full-time.

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Re: Voltage/Pressure Tuning Advice

Post by Doug Tallmadge » Thu Apr 19, 2018 9:01 pm

I've been reassessing my power supply options today and I've found an opportunity to swap my current supply for a Spellman SLM series with 50 kV at 24 mA. Would this be a good option? Are there any off the shelf commercial power supplies that users on this site have verified are sufficient for fusion?

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Richard Hull
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Re: Voltage/Pressure Tuning Advice

Post by Richard Hull » Fri Apr 20, 2018 3:34 am

More power is always better. You will still need a ballast resistor.

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.

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Re: Voltage/Pressure Tuning Advice

Post by ian_krase » Fri Apr 20, 2018 6:13 am

To expand on the whole ballast and current control thing a bit:

When gas discharges (arc or glow discharge) are struck, they go from being high impedance to very low impedance really, really quickly. This sort of thing isn't too much of a problem for your common everyday 0-60V 0-5A constant voltage/constant current bench supply. But with the high capacitance and inability to closely drive a very high voltage multiplier, it... kinda doesn't work.

Gets worse when you consider that for some of their operating domain, fusors have "negative" resistance i.e. more voltage makes less current, not more.

Anyway, adding a ballast resistor evens out much of this craziness.

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Rich Feldman
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Re: Voltage/Pressure Tuning Advice

Post by Rich Feldman » Sat Apr 21, 2018 6:14 am

Richard Hull wrote:
Fri Apr 20, 2018 3:34 am
More power is always better. You will still need a ballast resistor.
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