Fusor Forums

Chris,

I favor electrostatic focusing over magnetic where possible but when you have an insulator you don't have many other choices. WIth my POF setup in the large recessed electrodes configuration, the magnets keep the plasma off of the insulator walls and contrict it down so a several cm plamsa is only a several mm (or less) plasma when it clears the top and bottom insulators. This gives a smaller reaction area in the center of the fusor. With a small enough zone and high enough current, there may start to be some thermal effects in addition to the usually non-Maxwellian fusor. By having this type of setup, I can also induce ions sweeps within the chamber by varying the voltage diffential slightly between top and bottom electodes or the magnetic pinch from top to bottom. More here but this post is about Steven's problem.

Frank Sanns

Magnets...

I would love to give this a try, and have a couple of questions.

Do toroidal magnets still have the same field if they have been cut or broken?

What would be the best orientation for my system?

a) North poles facing cathode
b) South poles facing cathode
c) Opposite poles facing cathode
d) Location close to cathode or half way along tube.

Steven

"Do toroidal magnets still have the same field if they have been cut or broken?"

Been a while since I took engineering electromagnetics, but based on my vague recollection, I'd say you probably would want to short the polls with some steel before cutting, and then make very sure they don't heat up during the cut. Then reassemble them in place, and only then remove the steel. If you do it that way, you shouldn't lose a significant amount of field strength.

It's possible to partially demagnetize them along the cut edges otherwise, when the field from the body of the magnet expands from the edge at an angle to the local orientation.

Steven,

Cutting magents always give smaller magnets with opposing poles. Typical ring magnets have the poles on the flat faces so not much will change. This configuration is not the best for what you want but it is informative to play with some magnetic configurations around a tube of plasma.

For a nice primer on short focal lenghth, small spot size magnetic focusing check out:

http://prst-ab.aps.org/pdf/PRSTAB/v8/i7/e072401


Frank Sanns

Steven
They will cut with a Diamond saw - preferable wet. Then just stick the two bits together arround the glass tube.

Try loudspeaker magnets of you have any old speakers lying about. The big ones tend to have slightly larger holes than magnetron magnets.

Oliver

Though slightly dated, Building Scientific Apparatus is a good and very hands-on book.

For the more theoretically inclined, the company company Field Precision LLC make two good books on Accelerator physics and Charged Particle Beams availible for download on their web site: http://www.fieldp.com/educa.html
Probably this has been mentioned on this board before, but hey; repititio matre studiorum est!

Just to bring some closure to this thread, I report herewith..

The ceramic centering rings performed well, and theoretically it makes sense, that extending the dielectric inside the cathode ought to improve focus. It is clear that any future S.T.A.R. designs will include this feature.

The subject of ring magnets around the accellerator tubes also came up in this thread, and I did a rough and ready experiment, by taping a broken magnetron magnet around the upper tube. The effect was a dramatic pinching of the beam as it went through the hole in the magnet. I did not get up close to take a picture of this, as it emits a bit of x-rays, but on the video screen it was interesting to watch.

I keep having problems with the ion gun feedthrough, and it looks as if I have to get a larger feedthrough which is rated for higher voltages. No problem, just a bit of waiting..

These latest experiments were all conducted below 30 kv as I can do that without filling the capacitor with oil. Much less mess and much quicker to make changes.

After watching the now famous Richard Hull TV interview, I have hooked up a rate meter with a loudspeaker to my B10 detector, which has made it much easier to tune the reactor for the best fusion conditions. It requires incredibly careful tweaking of the gas flow and ion gun parameters, but all of a sudden I can hear the random popping of the neutron counter and then a steady humm.

Running at 30kv/0.5 ma, yes that is correct, 0.5 ma, I calculate that the T.I.E. is around 2500 n/s. This is barely enough to get a bubble once every 10 minutes (if I am lucky). on the other hand it is only running at 15 watt + ion gun 10 watt.

I am not too concerned about the neutron rate at this stage, more about eliminating annoying little problems and optimising the beam focus, so I can demonstrate the same results consistently.

Steven

Bee Research