Fusor Forums

Below is a picture borrowed from a Wikipedia article about magnatism.
http://en.wikipedia.org/wiki/Magnetic_field
It shows a looping electron beam shaped presumably by a magnetic field. Looks like a replacement for a wire loop. Looks like a cathode/ electron gun behind the round disk. Is this a shield or a magnet? The loop passes between two verticle bars with extensions- is this the magnet(?) or an antenna for microwaves. Anode appears to be the flat sheet metal in the back.
While the verticle bars are large as a replacement for a single wire loop, it intriges me. Through increased strength magnets, etc. could the bars be moved far enough away, shrunk in size to effectively reduce the impact area of the physical structures- ie- have a 'mostly virtual' cathode?



Dan Tibbets

Hello Dan,

the vertical struts serve as a ruler so that you can estimate the diameter of the electron loop. It has no electrical function.

The Wikipedia article should explain what this tube does and how it works. This is a classical school experiment to determine the e/m ratio of electrons. The electron beam is bent into a circle by an axial magnetic field from two Helmholtz coils, one on each side of the tube. If you know the strength of the magnetic field, the acceleration voltage and the radius of the circle, you can then calculate e/m.

When the elementary charge e is also known by other means (Millikan oil drop experiment), you can use this simple experiment to determine the mass of an electron!

I have such a tube (without the internal ruler), the image below shows how the electron gun is constructed.

Wilfried

I have thought about using electron beams to weave a 'grid' but I suspect there would be space charge issues with the + ions de-focusing the e-beam. When I get back to actually making stuff I plan to experiment with a cylindrical topology both using a spiral e-beam grid and a hollow 'doughnut' profiled e-beam.

Oliver

The idea of a hollow e-beam grid is great. The smaller you make the donut, the better. This means a very high field. In a cylindrical device, the donut is formed in a right angle field which puts the magnet structures right in line with the ion beam lines.

Secondly, there is that tremendous electrostatic field at the positive anode tending to cork screw your well constructed grid.

Flies in the oinment.

Richard Hull

Good idea. If you turn the tube at a slight angle to the magnetic field, you'll see a nice spiral e-beam instead of a circle. This virtual cathode could attract ions from outside, if you can set up a high enough potential.

Just a straight beam would do as well, or maybe a couple of crossing electron beams.

This was Farnsworth's original intention with the multipactor fusor. It was rather high powered, but produced no detectable fusion whatsoever. Why not? I think this design was never fully tested.

Having spoken with all the techs and engineers in person on the Farnworth team for many hours of interviews in person in Fort Wayne and over any number of phone interviews over the years, Farnsworth refused to accept that his electron bombardment idea was not all that valid. As such, They, to the man, told stories of the tedium between 1960 and 1964 of no results, numerous modifications to the idea and concept, alterations of the device, several un-named complete device retoolings, etc. , with no neutrons in evidence.

It was only with the arrival of Hirsch in his final year of his doctoral work, (summer of 63), as a guest worker that he, somehow, got Farnsworth to alter the design to an ion machine. With no results to that point, ITT was working on canceling the entire program and Farnsworth knew it. ITT and the Admiral decided to permanently hire Hirsch to give the entire effort a Phd somewhere in its midst. 1964 saw the first neutrons of even moderate significance. ITT decided to continue funding at a bit higher level.

You, of course, are more than welcome to strike out anew chasing this electron bombardment rainbow.

One engineer told me "the whole concept was wrong headed". I asked if they objected to Farnsworth directly during those 4 fallow years. "Yes", was the answer, but Farnsworth was sure it would work and at first listened to arguments, but ultimately became a bit miffed with the complaints and ulitamely said "build it as I tell you". They liked their nice jobs and complied as one did in those days.

Richard Hull

Cork screwed E-beam- sort of. With 6 microwave magnets spaced evenly around the pressure cooker demo fusor The e-bean was curved and appered to be cork screwing to the bottom of the Fusor. Difficult to appreciate in the picture. More obvous visually.
The magnets had alternate poles faceing inward. When all of the same poles faced inward there was a diffuse 'Star' like appearance of the glow discharge going through all of the donut holes.


Dan Tibbets