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

Posted: Mon May 28, 2012 12:49 am
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

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.

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

Posted: Mon May 28, 2012 6:17 am
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?

Re: Linear accelerator high voltage Ion source - floating ground issue

Posted: Mon May 28, 2012 7:42 am
by John Futter
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 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.

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.


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.

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.

Linear accelerator high voltage Ion source - grounds

Posted: Mon May 28, 2012 12:03 pm
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.

Re: Linear accelerator high voltage Ion source - resistors for Einzil lens, terminology issue

Posted: Mon May 28, 2012 1:04 pm
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.