Linear accelerator high voltage Ion source - floating ground issue

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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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 » Sun May 27, 2012 7:43 pm

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 » Sun May 27, 2012 7:48 pm

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 » Sun May 27, 2012 8:00 pm

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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Here is a generic VM[1].pdf
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Chris Bradley
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Re: Linear accelerator high voltage Ion source - floating ground issue

Post by Chris Bradley » Sun May 27, 2012 8:31 pm

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 » Sun May 27, 2012 8:59 pm

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 » Sun May 27, 2012 9:21 pm

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 » Sun May 27, 2012 9:34 pm

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 » Sun May 27, 2012 10:04 pm

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 » Sun May 27, 2012 10:23 pm

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 » Sun May 27, 2012 10:36 pm

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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