Field Simulation

I have only modeled the electrostatics.  I have yet to add the magnets to the simulation.  I must say that you can see the symmetry bias quite clearly.  First, SimIon represents 3 dimensional space via small cubes.  So the icosi-grid structure has some significant aliasing.  Nothing really horrible, but certainly noticeable.  It becomes even  more noticeable when you plot the potential contours for the beast (shown on the right).  I know it's a really complicated contour plot, but it is using the same isometric view of the cage shown above.

The immediate thing that stands out from the field is the one great circle that happens to lie on the Y-Z plane.  This is the only great circle that lies on the icosi-grid which aligns with the 3 ordinal planes of the cubic system of SimIon I might add.  Why am I even mentioning this?  Well, technically, all 6 great circles that make up the icosidodecahedron are the same great circle - just in 6 different orientations.  This should result in a five fold symmetric figure - i.e. all 6 great circle's should have identical fields.  But such is not the case because all I have to divide up the space with is cubes.  Cubes results in an aliasing of the space partition, and thus the imposition of symmetries not in the original model.

<sigh>  That's okay.  The simulation is far better than required to do some serious engineering with, however!

BTW, the simulation took up 270 megabytes of grid memory alone!  It took SimIon to calculate the potentials for the seven electrodes (six great circles on the icosi-grid plus the spherical chamber ground) about an hour or so on my 1.7 GHz P4 laptop.  Non trivial calculation times.

Right now, the simulation is a solid ground potential spherical shell surrounding the the icosi-cage electrode in the center of that sphere - held at -30,000 volts potential.  To make this more accurate, I'll need to poke the holes in the sphere that approximate the actual geometry of the vacuum chamber.  It will significantly change the field - that much is clear.  So it's necessary.  I can also try to rotate the entire icosi-cage before calculating the fields, removing the aberration of the Y-Z great circle.

I ran some electron trajectories using this set up, and they were very promising.  As I said, more than good enough to do some real engineering.  On the right is a view of one of the runs.  Click on the picture for the full size picture.

The grid and vacuum chamber are not shown in the picture, but roughly the same orientation is used as the previous graphics on this page.  The electrons were shot into the open pentagram of the icosi-cage, sharing the X-Axis of the model view.

What I was hoping to happen has indeed shown to work - at least in simulation.  The electrons are shot into the icosi-cage with a potential of 15,429 volts.  As you can see, they actually penetrate to the center of the cage and spin out - tracing the field contours shown above.  There was some question as to whether one could really inject electrons into the negative potential inner grid.  Since the grid is mostly empty space, there field isn't uniform.  So only 15.5 kV is needed to inject the electrons using an electron beam.

I found that if you inject the electrons on the same radius as the icosi-cage - i.e. an electron emitter sitting right in the open pentagon at the same radius of the cage, you can inject electrons with only 1.5 kV of potential.  Pretty nifty - if true.  Far easier to build an electron gun that only requires 10 keV or less.  And what I think I can show with more simulations is that I can simply use the electron gun equivalent of a flood lamp - i.e. an unfocused electron source.

This vastly simplifies the complexity of this hair brained idea I have.

What's also interesting from the electron trajectories I have modeled so far is the very nice spiraling and symmetric splitting around another axis.  Obvious from looking at the field contours, and obvious in hindsight, but pleasing to see none-the-less.  I think once I add the magnetic circuit of the icosi-cage, I'll have a very nice electron trap.  A very nice one indeed.

The magnets are going to alter the momentum of the electrons.  They don't provide a conservative force, rather an orthogonal force via the cross product.  But the whole point of the icosi-cage is to provide an advantageous geometry for making use of these cross-product forces.  The way that I see the electron trajectories bouncing around, the addition of magnets should be more than enough to push the electrons into a nice bouncing orbit around the insides of the icosi-cage.

I did a few ion simulation runs, and they were pretty fun, too.  The trajectories are much more chaotic.  The focusing of the beam is much more important.  And I can't effectively simulate the space charge of the virtual grid of electrons...  Once I get the magnets in, and assuming that the structure actually is an effective electron trap, I can simulate some of that in SimIon.  I was thinking of using some point electrodes (pixels) to simulate the space charge as well.  Hope that works.  Should provide a good approximation...

In any event, it'll be interesting to find out.  Next step is to add the magnetic circuit and see if there is the effect I believe it will.

In more real news (as opposed to simulated news), I got the electro polished cages back.  They're absolutely gorgeous.  The stresses in the cages have also evened out nicely, so they look very symmetric - not a lot of irregularities.  Very nice.  I also built the ceramic insulator standoffs.  I used the vacuum grade epoxy and constructed 9 of them.  I need 10, but I only ordered enough parts for 10 and I broke one of the cylindrical standoffs when I was trying to drill a venting hole.  Can't really drill fired ceramic without special equipment.  As the standoffs have screw holes, and I epoxy'd a ceramic ball on top of one of these holes (I'll be posting pictures later), I may have a virtual leak situation.  I'll have to see.  Hope not.  The epoxy forms a tight seal...  Fingers crossed.

I tried to come to grips with the wiring of the grid from the electrical feed through.  Got a little bit ugly, but I think I may have a way to do it simply.  We'll see.  I have to drill out a hole on those electro polished pieces of beauty...  Then I have to mount the insulators, then mount the magnets.  Joy of joys.  The magnets are always fun to have flying about :)  So, still lots to do.  Hopefully I'll have more time soon.