Dummy checking my initial design before I begin construction

[Tyler_Johnson]

Hello!
I have finished designing a fusor that I plan to make, that will initially be a demo fusor as a proof of concept before having its voltage upgraded in order to do actual fusion. I will have access to a lab at that point, so neutron detection and radiation protection are not a huge concern. I want to give a basic idea of what I have designed to make sure from the experts that I have not missed anything. I have opted to go for a cylindrical chamber after reading quite a few posts discussing their use, as well as due to trying to keep prices down wherever I can. The tube will be stainless steel, with one end open. This open side will be covered with a .5 inch thick acrylic (still sourcing so I can’t give exact details of what kind) in order to both have a viewport for a camera and to avoid implosion problems with glass which some of the FAQ’s and posts warned of. The biggest part I am concerned with is the electrical element. I plan to use an HV power supply around 30 KV (link to the product - https://tinyurl.com/ypv8ed3k). In all honesty, I do not fully trust this listing to be fully accurate in terms of watts and voltage, however if it falls short I can opt for a more expensive option. My main concern is the current. I can use a current regulator, however from what I have seen the ideal current is around 1-2 mA, which is considerably lower than the 10 mA the supply advertises as putting out.
In terms of the vacuum system, I plan on using a vacuum pump I got 2nd hand a while back. Its rated for 5 microns, and is barely used. Should it not do the trick, I have a source for a more expensive option that should be a bit of overkill.
In terms of the inner grid, I plan to use a circular grid placed directly in the middle of the 5 in inner diameter of the chamber.
My biggest questions are as follows; should I not even try out the PS and just go for a more expensive but reliable option, and is there anything in terms of these systems I have forgotten or overlooked? Note that the gas system is mostly handled by the lab I will be working with if I can get this off the ground, which will likely feed in through a flange that will be opposite of the vacuum pump system.

I appreciate all the help, and can give any additional information that might help with dummy checking my design. Thanks!

[Richard Hodson]

That supply looks usable but be careful not to tough anything while on or even afterwards until discharged and unplug first! That supply looks adjustable. Literature on here says it should be a negative hot supply but I'm not sure what that means even though I've built power supply circuits before. Negative was always the electron source. I don't know about eBay being reliable but a cheap Chinese supply may do okay and if it fails it was cheap and not $500. Check your pump closely for old oil or dirt. This site has info you need. They spec a large resistor in series with the negative power supply connection. I forget the ohm range. To limit current in case of arcing etc.

[Richard Hodson]

Acrylic might not work because it could lose surface molecules to plasma etching fast. Wear safety goggles or face shield near windows I guess. I would. They use cameras or mirrors to avoid x rays!!!!!

[Rich Gorski]

Tyler,

Be careful using that 0.5” thick acrylic as a viewport. With a 5” ID on your chamber there will be near 300lb of force on the viewport. It might not hold up. If you really need to stick with acrylic go thicker like 0.75” or 1.0”. Better yet don’t use acrylic. It’s a horrible vacuum material and with plasma present you will be sputtering carbon all over the place. Things inside will start turning black. You should be able to find a nice piece of glass that’s say 0.75” thick. That should work fine. Of course with glass implosion is always a possibility however you can protect yourself by covering it with a metal screen or a piece of acrylic.

For your ballast resistor (in series between PSU and fusor) use something in the range of 25k -100k. My setup uses 50k ohms. With a current of 10mA that 50k resistor will dissipate 5 watts and drop 500 volts across it.

Also, at 30kV you will be generating a significant level of X-rays. The viewport whether its acrylic or glass is transparent to 30keV X-rays. The stainless steel chamber is also just starting to become transparent to X-rays of this energy so be aware and plan on having a GM counter around to measure the intensity.

Good luck with your build.
Rich G.

[Dennis P Brown]

Your description of the fusor being a simple cylinder with one opening covered by acrylic indicates it is as bare bones a demo as I've ever heard. Without access ports and other usable connection points it can only be used to see a plasma. You mention a 'flange' so does it have other ports?

The power supply for voltages above 25 kV will likely only produce at the lowest end of the power range - maybe 1 ma. But that can be deadly. Considering the level of that proposed device I'd suggest you use a low cost NST. Vastly safer.

Without proper instrumentation, you will not have any ability to control the demo device. You need a milli-amp meter and voltage gauge at the bare minimum and that requires building proper devices first. Otherwise, you will have no idea what voltage or current your system is using. Further, a vacuum valve of some type and a vacuum gauge are essential for any real attempt to control the system's performance with a plasma. Through, with a plastic window, the later would mostly be pointless and could damage the gauge detector via contamination.

I too agree that a plastic end port cover is a bad idea but just to see a plasma (maybe) a few tests are possible. Your not knowing or caring that 20+ kV will create a dangerous level of x-rays through that port tells me you are not familiar with the FAQ's. I'd suggest that you read those valuable resources before trying to build this demo device.

Do note that the PS has a lethal output.

[Tyler_Johnson]

Sorry for the confusion, I do have access ports planned in the design but forgot to mention them. The main access port is planned to be on the back metal of the cylinder, and will be as vacuum tight as I can make it, though it will likely just be sealed with JB weld for any tests. I will switch over to glass following the advice of others, I was mainly trying to avoid implosions and have successfully used high strength acrylic in a few projects before, so I figured it could work for this purpose. In terms of the additional ports, yes it does have them, and the system for the vacuum has a gauge and valve while the electrical system also has an mA meter and a voltage meter. I did not include these in the description as I guess I figured they were “goes without saying”, which is my mistake. In terms of the PS, I am aware it is deadly. It will not be turned on when I or anyone else are anywhere near touching it. For the x-ray concern, the device will be rigged to activate from a short distance, outside of an X-Ray rated room at a nearby lab. As such, I am not super concerned with it kicking off x-rays, as I and other will be fully protected. I have gone through quite a bit of the FAQ as well as the broader forums, examining both successful and non successful examples, over the past 4 months. I believe the biggest obstacle in the designing phase is forgetting something as I did when forgetting to mention the instruments and access ports. In terms of the PS, I may run it into a voltage multiplier and have it running at a lower voltage, while also incorporating my ballast resistor at the end of it in a more controlled way, though I am aware of the issues of current reduction, as such this is just an in progress idea rather than planned. Thank you for your help!

[Dennis P Brown]

Glad your being safe - that is #1 priority, to say the least.

We can't know what you do know or equipment/parts you have unless posted. So I followed SOP in offering advice. That does not seem necessary except in one case. I am unsure why you want a voltage multiplier. These increase operating voltage and at a massive cost in power. Those two things, I do believe, you want to avoid doing.

I understand that those types of power supply's allow reduction in voltage through an adjustment setting. I suggest that you start at or under 5kV to test the system and get a feel for operating it. Learning what pressure and voltage provides a fairly stable plasma is important to begin to get a get an idea on fusor operation.

Since you are building this in a lab environment I am guessing both deuterium gas as well a Geiger counter(s) are available. In the case of the later, do use when voltages exceed 20 kV.

[Tyler_Johnson]

Absolutely, it was my mistake for forgetting to mention that stuff. Regardless, the voltage multiplier is specifically to reduce the power and up the voltage if the PS puts out too much current, which I think is likely. I will have to test it though. Another question I realized I hadn’t asked; the central grid is typically spherical, and I’ve seen mentions of steel rings as well as custom fabrication being used to make these. I assume I would need to weld it together from the rings, the only difficulty is sourcing them. Any place I should look to find them? Thanks!

[Dennis P Brown]

I've used steel welding rod. Not too difficult to bend into a spherical set of loops using a metal pipe of the correct diameter. Others here buy tungsten and make the grids. They can weigh in on how to make those.

Going to yet higher voltage via a multiplier will certainly reduce the total supply power dramatically; yet the x-ray hazard is then increased for the max photon energies even as the total flux will be far lower.

That power supply you show will certainly not produce too much power for a fusor in its higher voltage range. But a voltage multiplier might be useful in protecting the supply for runaway situations. But the ballast resistor is suppose to help in that regards.

[Tyler_Johnson]

I thought about doing steel, but I was concerned about deformation or outright failure of the grid at high temperatures. Is there any real likely benefit to tungsten other than better reliability, or is it more of a rather safe than sorry thing? Additionally, the X-Ray problem should hopefully be solved by the lab environment, but I do plan on making a basic lead silhouettes shield for any home demo tests. I really appreciate all of your help!

[Dennis P Brown]

I run very high power (600+ watts) through my steel grid for well over half an hour - glowed white hot - and no problem (outside of sputtering.) Tungsten would offer lower sputtering rates.

[Rex Allers]

From what I am hearing, it sounds like you don't understand basic electronics:
Ohms law -- E = I * R
and power -- P = I * E

You seem to think that a 30 kV supply rated at 10 mA will always supply 10 mA to the load it is connected to. The current rating of a supply is the maximum current it can supply at the rated voltage. The current it actually supplies depends on the resistance of the load.
I = E / R

Say the supply is set to provide 30 kV. If connected to the chamber but no plasma or arcing is happening, the R of the chamber will be very high, multi-gigohms. The current the supply is giving will be very low, essentially zero.
I = E / R = 30000 / inf = 0

If a plasma strikes in the chamber, we can think of it as a very dynamic resistor that depends on pressure, gas type, and possibly the voltage itself. Say the plasma has an effective resistance of 6 Meg ohm
I = 30kV / 6E6 = 5 mA, or half of what the supply is rated for.

If the plasma presented 3 Megohm, then I = 10 mA or the full rated supply current.
If the plasma presented 2 Megohm, the plasma would want to draw 15 mA but that is more than the supply is rated for so the supply might shut down or the voltage would have to drop.

Another way to look at things is as power. For this supply rated 30 kV at 10 mA
P = I * E = .010 * 30000 = 300 W
So this is a 300 W supply.

In the last example where the load wanted to draw 15 mA at 30 kV the power needed would be 450 W but the supply can only give 300 W. So if it was providing 15 mA the voltage would have to drop to 20 kV to equal the 300 W available.

--------------------
Richard Hodson tried to help you about the supply you are looking at but even after Richard Hull took the time to explain (one more time) HV supplies with Positive vs. Negative output and why you must use a Negative supply, Hodson still didn't seem to understand why.

Most HV supplies with enough power for a fusor are designed with one side (polarity) of the HV output fixed at ground level. The other side is the HV output and for a Fusor this must be negative. Most HV supplies can't be switched. If it is designed for positive HV, in most cases there is no easy way to convert it to Negative.

Many of the HV 'Precipitator' Chinese supplies are different with HV (+) and (-) wires unconnected so that one can be (*must be*, for safety in our application) connected to ground and the other used as the HV output.
[Note: the 'Precipitator', if mentioned in supply name, is because their original intended use was for cleaning dust from air in an electrostatic precipitator.]

The link you gave is for one of this type of supply. A few people have successfuly done fusion with one of these, but, as far as I know, they have used the open frame circuit board type.
Like this:

The one you linked is in a metal case. One reason to not use the cased one is that the successful users have put the supply in an oil bath to minimize any possible arcing.

---------
Please read the dedicated FAQ sections in each of the forum Areas. For this High Voltage stuff, here is the direct link:
FAQs: High Voltage
viewforum.php?f=29

Three suggestions to start with:
FAQ - Polarity of a fusor supply
#5 FAQ - Grounding in a fusor system
FAQ - Mark Rowley on the 30 & 60 kV precipitator supplies

--------
Also the currently active thread:
Joe Ballantyne fusor V2. (Low cost fusor.)
viewtopic.php?p=102218#p102218
Wed May 22, 2024 9:53 pm

Is using one of these supplies and also covering how to make a fusor that might be a very good guide.

[Tyler_Johnson]

Hello!
I’m fairly familiar with electricity and ohms law, the issue with the PS is that I am not confident it can output enough power given the resistance of the system. An open frame circuit board was another option, but I figured this PS would likely be more durable in shipping. I’ll look into finding a different PS though, as I had forgotten about the negative polarity issue. Thanks for the help and clarification!

[Paul_Schatzkin]

.
Just for the record, I would like to express my objection to anybody who finds their way to this site and expresses interest in the material calling themselves a "Dummy."

"Newbie" maybe, but just the fact that you've been drawn to this arena tells us you are no "dummy."

We all started somewhere.

--PS

[Daniel Harrer]

The resistance of the system is not fixed. The biggest variable is the pressure in the chamber, which you can (and in many builds must) fine-tune at the very end to the appropriate conductivity under the chosen voltage. The ballast resistor is mostly irrelevant: at 10 mA the 50k resistor causes a voltage drop of merely 500 V, which is far from the 30 kV.

In reality I expect that the 10 mA are very optimistic for the shown supply. For a demo fusor it should be more than enough. I've made nice plasma with only ~1W of power at 40 kV, so merely 25 µA. But it might even work for a proper fusor, just not at high neutron rates.

I would very strongly second the suggestion to put the supply into oil. My experience with such cheap supplies is that their insulation is barely sufficient at best, and the added cooling is definitely a strong help, too.

[Rich Gorski]

The ballast resistor serves no real purpose while the fusor is running in a stable manner and at a reasonable current. It's main purpose is for protection of the PSU in case of arc or other instability within the fusor (a short circuit ?). In general a ballast resistor (or inductor) will act as a current limiter. For example if there were an arc while running at 30kV there could be a momentary fast pulse measuring into the amps if no ballast resistor is present. However with a 50k ohm the current would be limited to 600mA. One might even choose to use a larger value while the system is under development for greater PSU protection and then drop the resistance when there is more trust in its operation. As indicated above at 10mA a 50k ohm resistor it will only drop 500V so it will have a very minor affect in the operation of the fusor running at 10s of kVs.

Rich G.