Versatile biasable filament supply

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Post Reply
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Versatile biasable filament supply

Post by ChristofferBraestrup »

For a plethora of applications, it would be nice to be able to feed an ion/electron source with low voltage filament current, biasable to roughly +-1000V.

This is the beginnings of my design/construction of that.

So far my plan is to end up with a simple, double width NIM-module (no NIM power) that can be fed from a variac, has shielded connectors for the filament, and also a BNC/MHV/SHV connector to input a bias voltage. Many designs has the filament sitting at either + or - some voltage to slightly repel the formed charged species.

The attached schematic should show it all; a variac supplies voltage to the primary of a 220V -> 2x 6,3V filament transformer (seller, a HAM, was nighe enough to insulation test the secondary to 3kV)

I may ommit the current limit pot, as this can easily be set with variac voltage.

A small panel current meter (0-3A) is mounted on perspex inside the grounded box - and viewed through a perspex window in the front panel

Any comments? feature ideas and design changes are welcome!
filament2.jpg
User avatar
Richard Hull
Moderator
Posts: 13893
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Versatile biasable filament supply

Post by Richard Hull »

Good design! Biased filaments on an ion gun are a must and allow for varying ion currents along with extractor potential. If an electromagnet is used as on many ion guns (Pigatron), also figures in ion current.

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.
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Versatile biasable filament supply

Post by ChristofferBraestrup »

Thanks!

Yes, it's also needed for some of my accelerator and mass spec experiments.

The current monitor is a bit tricky with AC with 1000V on it.. I'm thinking of scrapping the inline/floating meter for a non-contact current transformer kinda thing. Isolated by design; can be added once I find a good part.

I was considering integrating a variac into the front panel and doing a proper 3U 19" cabinet, but it'd be a lot of space and weight for 3-4 components..
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Versatile biasable filament supply

Post by ChristofferBraestrup »

The NIM module is finished! I just need to add an earthed plug to my variac and the filament system is ready!

Thankfully I have a huge supply of empty unused NIM boxes
20211224_175830.jpg
20211224_175841.jpg
User avatar
Richard Hull
Moderator
Posts: 13893
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Versatile biasable filament supply

Post by Richard Hull »

Nice volume of air in that module. I too have a few NIM blank modules. Found them at the Raleigh NC hamfest a few years back.
The very first Tektronix digital scopes tried to maintain that famous old scope "deep" profile, large deep case, CRT screen, etc. Opening one up showed only the front panel with all the electronics, a small board for the CRT electronics sitting on a vast, flat plane of smooth steel, blank chassis front to rear. What a joke! There was more volume of air space and flat steel than guts. Their next iteration was the "lunch box" LCD screen small scopes that so many folks now use to in most DSO's. In college back in the 60's our lab scopes were as big as a dog house, acted as space heaters and demanded big roll around carts! They were fabulous and fabulously expensive. I see them at hamfests now with "FREE" notes taped on them. Bring your own truss to lift them.

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.
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Versatile biasable filament supply

Post by ChristofferBraestrup »

I agree, it's slightly ridiculous to have one component in that huge of a box. I wanted the spare front panel space for a filament current meter addon, and I like to have a large safety margin on HV stuff. Also the 4 unit NIM box was the least pretty of the ones I have, so it had to find a use or go.

I've just tested this with a small variac and it works exactly as expected. Will test HV soon but I don't expect a problem.

One additional feature I'm considering is a BNC connector with its center pin shorted to the shield/case, so one side of the filament can be tied to GND, but I'm not sure it's needed.

I've also seen these instruments with a CRT sized box and then a flat LCD - I have a suspicion some fron the display twilight era (~1988-2000) might have originally been spec'd for CRT's and then hastily re-designed for LCD's and nobody wanting to re-design the case.
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Versatile biasable filament supply

Post by ChristofferBraestrup »

At some point, I would like to mount the variac into the desk of the control console for vacuum/particle/fusor systems, this is seen on the surviving control room desk of the first cyclotron in Copenhagen, set up by Bohr and de Hevesy for isotope production in the 1930's and operated to the 1990's.

A lot of health physics research was done with the isotopes produced by this, including the unfortunate hunt through the Fælled park in Copenhagen for a radioisotope-doped cat that had escaped an open window.

My variac is even the same brand! In Europe, Lübcke is probably the most well known variac manufacturer. Still is, I believe.

Here are some shots of what I'm aiming for:

Image

Image
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Versatile biasable filament supply

Post by ChristofferBraestrup »

And a final test, here is 500V bias on a small light bulb with roughly 6Vac across it!

If it will stay stable for an hour ish I claim project completion.

Also, please disregard the jankiness of putting a instrument bulb with springy legs into 2 BNC sockets with 500V on them, I only fo this because I'm alone and I know the ortec 428 is quite anaemic.

In the final design I'll make a diecast box to go over the feedthrough that will contain the ion source so nothing is exposed.
20211226_230947.jpg
User avatar
Richard Hull
Moderator
Posts: 13893
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Versatile biasable filament supply

Post by Richard Hull »

I'm glad it worked out for you. Now, it's on to gun construction and testing.

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.
Jerry Biehler
Posts: 938
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Versatile biasable filament supply

Post by Jerry Biehler »

You can find used varian filament transformers for e-beam coating system. Big red plastic encased windings on a laminated iron core. Good for at least 10kv and probably quite a but more. They can typically do around 40 amps at 6v. There are two output windings and can be strapped for higher current lower voltage or higher voltage lower current. I think they can put out at least 40A. Id have to look up exact specs.
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Versatile biasable filament supply

Post by ChristofferBraestrup »

Meter has been added and project is complete!

I added a panel meter and a current sense transformer to measure output current roughly, ended up working a treat!

Here is the final bench setup, and some pictures of a simple electron gun/fawaday cup combo

And..oops the filament heat melted my electron gun!

That'll teach me to use soft solder in vacuum!
Attachments
20211231_211539.jpg
20211231_211521.jpg
20211231_211417.jpg
20211230_223729.jpg
20211230_223751.jpg
User avatar
Dennis P Brown
Posts: 2591
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Versatile biasable filament supply

Post by Dennis P Brown »

Silver solder is a necessary joint for most higher temp applications; however, I'd think using a solid tube wouldn't be too difficult to make and use a very small copper bolt/nut holder to create a support.
Post Reply

Return to “Ion Gun Design and Construction (& FAQs)”