Fusor Forums

Setup:
Bell jar demo fusor running on air. Outer grid is constructed of 0.025” stainless steel safety wire and has a diameter of 6". Inner grid is constructed of 0.020” tantalum wire and has a diameter of 1.24”.

Power supply consists of a full-wave rectified 12 kV, 30 mA neon sign transformer controlled by a variable transformer. (Variac)

Vacuum system consists of a small Leybold D1.6B two-stage direct drive pump. (1.6 cu-meters / hour) Lowest attainable pressure in the bell jar is ~15 mTorr. An MKS 10 Torr capacitance manometer was used to measure the chamber pressure. The bell jar had been vented to and was at atmospheric pressure just prior to pump-down and image capture.

Images were taken with a tripod mounted Canon EOS 400D. All images were captured using the same camera settings. (1/8 second, f5.6, ISO 400) Images were cropped, sharpened, and reduced in size using Paint Shop Pro.

For the sake of load times I’m dividing up the ten images I have to share into two separate posts. This post will comprise of 1 Torr through 100 mTorr, and the next post will comprise of 50 mTorr through 15 mTorr. I took two photos at 15 mTorr; the first was taken when the chamber pressure first reached 15 mTorr. (20 ~ 25 minutes into the pump-down process). The second image was taken after a good five hours of pumping.

Jon Rosenstiel

The remaining images...

Very nice series, Jon!

This should make a good set of reference shots for everyone.

I particularly like that you were able to maintain 5 mA current level so that the rise in potential with the fall in pressure can clearly be seen.

You apparently had no problems from the plasma "rays" doing localized heating at the bell jar walls, at lower pressures.

Nicely done, indeed.

Dave Cooper

Nice tutorial and photographs.
Hope this gets reference status here and can be found again later. As a FAQ maybe?

I second previous comments - nice work Jon - well worth while.
Tks.

I have FAQ'd this one on my own. This is a real winner for newbies to the vacuum world. and a great set of "gauge blocks" for folks wondering whether their surplus TC gauge is telling the right story or not.

Often the old hands have seen an image like the 100mtorr image and the user has said his gauge was reading 15 microns. We knew better. With this set of images, the amateur who hasn't got a clue can set his TC gauge pot to a relatively close value. (though a simple current match to the gauge tube spec is what is really needed) Some amateurs have no way of hitting the required submicron level needed to zero a TC gauge.

A long overdue exercise and very well presented Jon.

Ricahrd Hull

Dave-

I did a couple of things to minimize localized heating of the bell jar walls. One, I kept run times as short as possible (~10 seconds). And two, I used an NIB magnet to "steer" the main electron beam away from the bell jar wall.

Jon Rosenstiel

Richard-
Adding "FAQ" to the subject line was a good idea... something I hadn't even considered.

Jon Rosenstiel

Very helpful Jon....as a newbie this gives a little illustration to put with the instructions.

Welcome to the Plasma Club, Jon. (Hah!)

This is a beautiful and definitive photo set. "FAQ" would be an appropriate designation even if no writing whatsoever accompanied them.

-Carl

I know it has already been mentioned here in these forums many, many times… but nothing beats a bell-jar demo fusor for observing the plasma as it responds to changes in pressure, voltage, current, magnetic field, etc. Totally fascinating!

Here’s something interesting that runs counter to our everyday experiences…To our eyes the plasma “appears” to be a cloud of moisture, a fog, but try shining a flashlight or a laser pointer at it and see what happens.

Jon Rosenstiel

Jon has already mentioned the constant harping, mostly by me, for the beginner to 'experience' the plasma. The demo device is not something you hit and then move immediately to fusion.

Stop and smell the roses. Play with the INSTRUMENTED plasma. Observe the current-voltage raltionship to visual imaging. Check for the subtle shades and core relationships presented. Alter the pressure. Put in an odd gas.

This is a playground and everything is new. Investigate and discover. If you have read a bit about plasmas in books, stuff that you have read will take on a whole new meaning. I spent 8 months in 1997 and 1998 with two different demo machines before I homed in on fusion.

On my tapes, I show how I investigated magnetic phenomenon as well as rapidly changing electrostatic field gradients from outside a bell jar.

Nothing but pure fun turned into real hands-on knowledge.

Richard Hull

Very nice pics. Were the photos taken from above?
Is your jar clamped down, or held down by atmos.pressure?
Do you use an O-ring seal, grease, or both?

Tony Webb

Anthony-

Photos taken from the side.
The bell jar is held down by atmospheric pressure.
Just the normal bell jar "L" gasket, no other sealer. I got the gasket from Duniway Stockroom.

Jon Rosenstiel

Turned into a formal FAQ in 2019 with annotations in the images by me with Jon's permission. Fabulous and valuable forever to the vacuum newbie.

Richard Hull