Linear accelerator high voltage Ion source - floating ground issue

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Dennis P Brown
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Linear accelerator high voltage Ion source - floating ground issue

Post by Dennis P Brown »

Well, I learned a few things (finally) about a Villard voltage multiplier - first time I ever had a real one to play with and modify. The diagrams I studied on Villard VM's all show a supply source that oscillates (like the one this unit has) and that one side of the MV unit - the anode - is grounded.

Aside – I am finally starting to understand how these units are used for fusars and other high voltage systems – especially playing with the charged circuits (and very safe circuits so the ouch factor is the only concern …strange but the hands on did orders more for my understanding these units than numerous attempts to study these things on line or from a book .)

The first thing I discovered was that grounding my anode of the Villard VM shorts my small battery operated driver circuit causing a shut down. Stranger still, the unit was often arching along a Plexiglas plate it was sitting upon - ten centimeters arches along clean plastic! (max volt is 15 keV.) This didn't make any sense - worse, while powered, it would back feed into my small power supply and arch across various parts to the metal case Now I think that this last part makes sense due to the lack of an anode ground for the VM. I believe that diode leakage for RMS voltages works both ways ... .)

What also shocked me - literally! - was discovering that the high voltage from the VM leaked back through the 13 volt battery lines. The high voltage lines were far away and would have had to travel through the case which I guess it did - experimental experience in action. I also discovered that after grounding all VM points and the supply two and even three times, I still got shocked a number of times ... .

To solve these problems I just potted the entire Villard unit, then stripped off the metal case for the VM power supply and potted key areas that I saw shorting. I also enclosed the entire system in a plastic box with more potting material.

The system is now stable - it holds 15 keV with no audible shorting and the battery lines haven't shocked me anymore - but I still worry that the lack of an anode ground on the VM may be an issue once I install it. That is, when installed in the secondary Van De Graaff globe, the only available ground is the battery electrode (cathode.)

I will also ground the battery cathode side of the driver unit to both the inner wall of the metal globe (but this floats, too) and to the anode side of the gun. Hope these ground loops don't cause issues (currently, I do use a true ground for the battery anode while testing the unit.)

Still unsure why I can't ground the anode side of the Villard voltage multiplier. without tripping my supply's fail safes - I guess they just built it that way. Considering they sell it with the Villard unit, I would think they would provide a note about this issue.

Considering this issue of a floating ground, I wonder if anyone else has used an ion gun that is battery powered in this manner?
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Chris Bradley
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Re: Linear accelerator high voltage Ion source - floating ground issue

Post by Chris Bradley »

I'm unclear what's connected to what, and where, in your description. An annotated diagram and/or photo would help explain the material you are trying to get across.
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Dennis P Brown
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Re: Linear accelerator high voltage Ion source - floating ground issue

Post by Dennis P Brown »

Here is a copy of the basic unit in microsoft.

Edit: One point I just notice in the diagram - my unit has no anode ground on the first diode since this shorts off my power supply; maybe a ground fault interrupter built in?
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Chris Bradley
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Re: Linear accelerator high voltage Ion source - floating ground issue

Post by Chris Bradley »

right .. .thanks...! I might've seen one of those before. :}

I mean, what is your "small battery operated driver circuit". Clearly, if you ground the top of the stack, you push your battery down to very low potentials and it will arc to whatever ground potentials are around it. But this is pretty obvious, so I was presuming you weren't referring to this, but some other feature of your circuit, as yet un-described.

If you've tried to ground the top *and* the bottom, then, again, there is an inevitable outcome. So I'm not sure what you are trying to describe here.
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Dennis P Brown
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Dennis P Brown »

Sorry - for not being clear - not a strong point for me ... .

Here is a more accurate diagram that better represents my system.

My concern is floating the Villard voltage multiplier at D1. For my system, I can't ground this and was worried this would cause issues.
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Steven Sesselmann
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Steven Sesselmann »

Dennis,

If this power supply is floating inside the HV head of your VanDeGraaf, then that is your local ground, so simply ground it to the head.

I recently visited John Futters lab, where I was shown an ion source that was powered by a floating generator, the generator was driven by a motor at ground potential, via a long plastic rod.

Absolutely stunning to watch (to touch too I imagine)

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
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Chris Bradley
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Chris Bradley »

Sorry, I have no idea what the issue is (that is;- I do not understand the issue you are raising).

The 'Us' in the diagram is just 'any old' source of alternating/pulsed input. You can connect any point of that circuit to whatever you like as a reference potential - so long as you don't connect two points of it together! - and the potentials in the rest of the circuit will follow suit.

There is no reason the circuit will 'trip' unless you have accidentally connected two parts of it together through some common potential.

As Steven says, forget 'ground'. It is a local ground that this is referring to. You can connect any reference potential to any part of the circuit, be it 'earthed' or +100kV.

Sounds to me like you have been 'earthing' a point further up the stack, in which case the battery will be held at a negative potential wrt earth and you've got a shock off it.

(PS keV is a unit of energy, not of Voltage)
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Carl Willis
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Carl Willis »

You probably intend the left-hand side of C2 to be at local DC ground, but you do not show that in the figure. On the other hand, one terminal of the battery is grounded in your diagram. My guess is that you have simply failed to provide a ground reference for the HVDC in the multiplier at C2 and are getting sparking in places where you don't intend a high potential, but have one anyway because of the omission of the ground at C2.

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Dennis P Brown
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Dennis P Brown »

Sorry, had to be away for a while.

Yes, the base of cap C2 where it connects with diode D1 is a normal ground point for any Villard voltage multiplier stack (and boy, do I realize now why that is so important!) That point I cannot ground due to the limitations of the oscillator power supply built into my HV system - I am sure now that it has a ground fault interrupter circuit built into it and that is the villian. Since Steven has pointed out (along with Chris) that I can float the system as long as I have a common point that can act as a grounding point - that is good news!

I will say I do know about standard electronics and grounding issues, so, that I have avoided, so far! I grounded the battery only because the darn thing was shocking me and the battery! so I gave it a better path. In the ion gun (currently mounted), it will just float and I feel, now, that will be ok.

Thank you all for the inputs - it really did clear that and some other issues up. I'm far from knowing these things but feel a lot more confident now - getting shocked sometimes does get a good point across!
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Carl Willis
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Carl Willis »

Your power supply might have a balanced or even CT output and thus might be able to drive a full-wave CW stack rather than the half-wave assembly you are using now. Just another hypothesis, since I have no idea what your HF supply actually is.

-Carl
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Dennis P Brown
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Dennis P Brown »

Carl, excellent point and for reasons that elude me, I didn't, for some not very smart reason, give this rather important information to you all ... .

The power supply is a minimax 3 made by

http://www.amazing1.com/hv-hf-power-supplies.htm

and I added their five stage half wave voltage muplifier.

The company say's the circuit is fault proof so I guess I now know what that really means - no grounding the base (C2) of the Villard MV.

Again, thanks for everyone helping me to clarify this issue and I finally think I understand its behavior. This should work as my ionizer voltage source - 14 - 15 keV at up to 20 ma; of course, at 10-5, I figure the current will be very low compared to that number. The grounding should be a non-issue and hopefully, this thing will work - of course, the ion gun is the critical part of the accelerator and determines much of its possible performance; hence, my concern about its operatrion.
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Chris Bradley
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Re: Linear accelerator high voltage Ion source - floating ground issue; new diagram

Post by Chris Bradley »

Dennis P Brown wrote:
> This should work as my ionizer voltage source - 14 - 15 keV at up to 20 ma

At 1mA and 15kV, you'd achieve a 15keV output in 1.6x10^-16 s.

And you won't get 20mA at 15kV out of it either.

So are either output, and/or input, connected to the chassis ground of the inverter?
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Re: Linear accelerator high voltage Ion source - floating ground issue

Post by John Futter »

Dennis
It is time that you start using the appropriate terms.
You can have islands of potential that are referenced to a potential other than ground.
Normally to distinguish this from ground (read "Earth") this is called terminal.ie a floating point in potential that other supplies reference to.
Standard single ended VDG's use this term and usually this is the potential that the ball at the top is at.
This now becomes a local ground hoisted up in potential by whatever voltage. The terminal in commercial VDG's contains much electronics all floating at Terminal --all of this powered by an alternator that is part of the top roller of the charging belt. Communications with real earth is done via optical fibres or an optical link.

The beauty of doing it this way is that the terminal electronics can be safely tested by not powering the terminal and actually earthing the terminal to probe what is going on with the terminal electronics with ordinary service tools--multimeter HV probes ETC.
1/

I'm still mystified why you have made a water cooled target as most in air VDG's can only move a few watts of power to the terminal ie a few uA @ 0.5MeV and usually the max volts and max current are exclusive of each other.
Max tube current also tends to mean lack of focus ability due to the beam spread --ie the ions repelling each other sideways while travelling down the acceleration tube.

2./

Nealy all VDG's waste a reasonable portion of the available power via a resistor divider to setup the correct potentials on the acceleration segments of the tube. Typically this waste power is in the order of 10 -50% to keep the tube segments @ the correct voltage. It does not take many ions hitting the tube segments to upset this division and loose focus with no resultant beam. so a reasonable current has to flow down the divider chain to force the correct potentials.

But
You are building --not just armchair jawing so keep up the good work--- remember that without the VDG going you can earth the terminal (ball) to troubleshoot your ion source and focus supplies.
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Dennis P Brown
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Linear accelerator high voltage Ion source - grounds

Post by Dennis P Brown »

A lot of good information and why this is such an outstanding place to ask questions.

Chris I will ground both the input and output of the ionizer relative to the VDG globe.

Thanks for that question.
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Dennis P Brown
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Re: Linear accelerator high voltage Ion source - resistors for Einzil lens, terminology issue

Post by Dennis P Brown »

John - I fully aggree that correct terms are critical if useful communication is desired so thanks for the definitions - I need to follow protocol! and more to the point, aids me in better understanding the topic when I need to search these terms.

The reason for the water cooling (which I never considered till a few months ago) was due to both the original accel. having it and an other one I read about. This later one had an image of their target and it was glowing red hot! - while their size and power were greater than mine, that made me worry about the deuterated plastic I was going to mount on the copper support. I was concerned it would heat up too much and either kill my high vacuum quality and/or erode rapidly; hence my sudden decision to make things far more complex.

Since then and after installing the water-cooling, I've decided to go with a titanium targeted 'doped' with deuterium gas. Have a feeling you are correct and due to my rather low power, the target will not overheat.

Plate resistors - ok, once again I realize how useful this forum is - two years ago, when I started collecting items for the accelerator project, I bought enough 50 M-ohm ceramic resistors to use for the purpose of equilibrating the potential field of the collector plates (I even tested one yesterday to check the power supply for the gun.) Completely forgot why I even had them!

Your important point on using resistors to equilibrate the potential on the Einzel lens is valid and I never, even when I had first got these resistors, understood their importance in that regard (just that I read that most Electro-static accelerator's had these so I got some.) Loss of particle beam focus due to ion impact along the collector lens would completely ruin my accelerator.

This further demonstrates the importance of sharing ideas and discussing one’s approach with knowledgeable people here. Forgetting to add these resistors could have doomed my project. Again, further proof of the value of this form.

I will figure out today how I will mount these resistors between plates.

I have also decided to try something with the ion gun but will post that in the correct forum with photo’s.

Thank you all for taking the time and interest in aiding me with this project. While not a fusor, I hope some newbies get some ideas so when they do build fusors, my project gives them both a better understanding of equipment application and some ideas on component’s impact on design. Most important of all, researching their approach, testing, and getting good information before throwing out their ideas. Besides, this really does add to all the fun of building real projects - more to the point - such inputs will result in a better project and one that has a far better probability of working as desired!

Aside - I have been trying harder of late to create more informative titles so searches go a bit better for others - please forgive their length.
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