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Thoughts on the electron beam.

Posted: Sun Jul 01, 2001 9:23 am
by teslapark
This may be of little valuable substance, but reviewing video of the fusor tonight has got me to thinking a little about the electron beam coming out of the poissor.

The beam exits the poissor and then passes through the incenter of a grid opening, repelled by the negative grid wires.

At some point after it leaves the grid, the beam seems to converge to a focal point. I think this must be caused by alteration in electron trajectory due to electrostatic repulsion from the grid wires, "sqeezing" the beam.

After the focal point, the beam continues, but the angle made after focus is larger than the angle made before focus, due to collisional scattering I suppose.

This model makes sense when one looks at how the beam is affected by adjusting voltage.

As voltage is increased, the focal point moves closer to the grid and gets brighter. Stronger electrostatic field has more direct effect on electron trajectory, and makes the focal point more dense. The angle of scattering after the focal point is also much larger.

Eventually, as voltage is raised, the beam seems to "bugle out" or not have a focal point at all. I'm not exactly for sure why this is, or if this bugle is the same mechanism that is seen in the bugles I get at higher pressures.

As mentioned earlier, this may already be common knowledge and not of any practical value, I've just never heard it mentioned before and it seemed interesting.


Re: Thoughts on the electron beam.

Posted: Mon Jul 02, 2001 8:53 am
by guest
I have done some simple 2D field plots of a symmetrical 4 electrode array at ( -1000 V) inside a large square ground electrode.

The electrodes represent a slice right through the middle of a spherical electrode cage made from two intersecting circles of wire. The equipotential lines create a small region of about -755V (or about 25% more positive than the electrodes, and also indicate 4 regions like valleys between the electrodes that have a potential that goes steadily more positive towared ground. These regions appear to coincide with the zone from which either the bugle (at fairly high pressure...a few 10's of microns) or the ray, at lower pressures, emerges. Halfway outtowards the ground planes, the equipotentials are nearly spherical (rather, circular) .

Unfortunately, my program cannot account for space charge, so the actual poissor effects I have to guess at.

Maybe this all suggests that a minimum number of cathode rings are needed to make a tight virtual electrode inside. I will try to model with 8 electrodes (4 rings) next. It may also indicate what all plasma people know instinctively, that plasmas and charge clouds are inherently unstable. The stability (apparent) of the glow discharge mode is intriguing. I still don't understand what's really happening..yet.

I will try in the near future to post a few of the plots that look interesting.

Dave Cooper

Re: Thoughts on the electron beam.

Posted: Mon Jul 02, 2001 2:49 pm
by Richard Hull
The large angle after the focus is, no doubt, more likely to be electrostatic repulsion of the electrons amoungst themselves once in a more open and uniform field region. In the grid and just after the focus effect of the grid wires, the beam is rather confined. Once out in the open, the beam is free to diverge rapidly via coulombic repulsion.

Remember that the light is not the electron beam! It is ions recombining! So the visible light is really not the electrons or the ions, it is ions going neutral. The complex interplay of forces and electrostatics is often tough to visualize and verbalize.

Electrons have a huge mean free path compared to ions at any reduced pressure and at the same given energy. A figure of 10X would be about right untill the pressure gets really low and then it is thousands of times the mean free path of the ions. (I gave the equations in an ancient post on

At higher pressures and moderate voltages, the bugle is noticed due to mean free path of the ions recombining as they exit the grid and try to turn to return to it, but start recombining. This is the light you see.

At lower pressures and higher voltages, and, thus, higher currents, the bugling can return due to gas density (pressure) in the local areas increasing due to the increased current flow (number of ions) in the inner grid region's small volume. Thus you have effectively raised the local pressures in these areas.

Remember, also, that we are right at the edge of glow discharge in some regimes here and a 10 to 30 micron local variation due to ion pressures is possible. We absolutely rely on this to keep the fusor working at or near 1 micorn where most simple gas systems are extinguished.

We are trying to keep the area of high pressure near the center while lowering the overall AVERAGE pressure of the chamber to achieve decent mean free ion paths out side of the grid for recirculation purposes.

It is weird to think about, but a true, theoretically neutral plasma ( absolutely equal numbers of ions and free electrons) should be invisible!! (temperature of the plasma is such that recombination is impossible.)

In real life things tend towards the maxwellian. This is what makes the attempts to bulk heat plasmas limited by the "maxwellian tail". (other fusion methods)

We, however, in the fusor, can control electrostatically where the hottest ions occur in "velocity space"

Richard Hull

Re: Thoughts on the electron beam.

Posted: Fri Jul 13, 2001 8:01 pm
by guest

Although several effects could cause the beam to scatter, I suspect that just as the inner grid would repel (focus) the electrons, the outer grid would attract (defocus) them.

Re: Thoughts on the electron beam.

Posted: Sat Jul 14, 2001 7:31 am
by guest
Some comments for discussion.

1. The rather long glowing rays emanating from the spherical cathode cage could either mark the path of a low energy electron beam that is colliding with and thus exciting, gas molecules. Or it could be the path of the ionized gas molecules themselves, as the ions collide with neutral gas molecules and stray electrons and thus become neutral again, glowing in the process.

2. The length of the beam is particularly intriguing. In my 12" diameter bell jar setup, I have seen beams 4 to 6" long (depending on the voltage and pressure) that always come from the same octant of the cathode structure. Now, what is intriguing, is that I have used the bell jar base plate as the ground plane! There are no other grounded electrodes around. The ray comes out of one of the octants pointing upward and away from the anode, not from an octant pointing towards the ground plane.

3. My electrode system is quite assymetrical as to the anode electrode configuration, yet the plasma sphere is reasonably central within the cage. This suggests that only the cathode cage is somewhat critical. This, at pressures around 10 -20 microns.

4. At higher current ( 1- 3 mA), but low voltage( ~ 1kV ) the central spherical glow shrinks in diameter and develops several thin rays. These seem to be one for each octant of the cathode cage. So far I have stayed below about 2 kV because of feedthrough limitations.

5. I have now modelled an 8 wire cathode cage. As expected, it has a much smaller and lower positive potential region in the center. (A complete Farady cage would of course have zero volts) This positive region is the area into which the electrons will be drawn. The potential is about 80 volts positive,( for a 4 wire cage it was 245 volts positive) to the cage (cathode) potential of -1 kV.

From field plots, it seems that as the cathode cage becomes an electron source, most of the electrons will simply move outward to the positve anode electrode (spherical fusor - to the shell), creating (mostly) positive gas ions by impact ionization on the way.

The positive gas ions are then drawn toward the negative cage. When the mean free path for the gas molecules is long enough ( pressure is low enough) for them to reach the cathode cage area, some over shoot and scatter in the central region, within the cage. Potential contours within the cage, focus ions into "rays", which exit approximately normal to the cathode sphere.

Those electrons emitted toward the interior region of the cathode, would be drawn to the virtual anode created both by the electrostatic potentials and the ions' direction change. The above may be a bit too glib. More later if anyone is interested.

Re: Thoughts on the electron beam.

Posted: Sat Jul 14, 2001 2:57 pm
by Richard Hull

We are always interested in observations and thoughts on what is going on within the fusor.

In my bell jar fusor (fusor II), as the pressure dropped, my single beam would sometimes just switch quadratrants over many quadrants of the inner grid and it looked like a frantic searchlight beacon digitally hopping around until, finally, at some point, it would stabilize to one quadrant.

This is seen dramatically on the video tapes on the fusor. I believe it is the first tape. There is more than enough interesting and bizarre activities which sometimes are not replicable given similar conditions. The delicate conditions between 20 and 1 micron with applied voltages between 2 and 20KV can supply a lot of interesting observations and thoughts on what is going on. What's more, each fusor will be slightly different . Often, each run can be different in minor respects. Overall operation, however, remains rather predictable.

Richard Hull

Re: Thoughts on the electron beam.

Posted: Mon Jul 16, 2001 7:20 pm
by guest
ignore my post. I was having a stupid day....