The “inverse polywell” concept started from an off-hand comment by Tomas Ligon while we were talking at the Dallas International Space Development Conference (ISDC) 2007. He was talking about an early polywell problem. A massive current surge out of the reactor destroyed some essential equipment when the experiment was shutdown after a run. Where was this charge hiding? Comments like this catch my attention. While it is nice to have things work perfectly, it is the unexpected events that can be gifts from heaven.
A fusor is a direct electrostatic device. A voltage is applied to the hollow core electrode which provides the electrostatic motive force for accelerating the ions.
A polywell is not a direct electrostatic device. The magnetic fields are there to capture electrons which then provide the electrostatic motive force. As an indirect device it is important to know where the electrons are being held. Dr. Bussard’s view was that when electrons are forced into the interior of a polywell that the magnetic fields create a sheath around the interior that holds the electrons in the center, the whiffle ball effect.
An electrons flight path curves inside of a magnetic field gradient. Much like shooting marbles up a hill. A slow moving marble is going to quickly arc back down the hill. A faster moving marble is going to make a larger arc before returning to the base of the hill. A very fast marble is going to go over the hill. The electrons would treat the sheath like a hill.
There is also another effect that magnetic fields have on electrons. The slowest moving electrons follow the magnetic field lines as the path of least resistance.
Assuming that there is such a thing as a magnetic sheath on the inside of a polywell, there are two types of electrons that are going to exit it: the ones going too slow and the ones going too fast. Only the just right ones are going to stay inside. Where do the not just right electrons go when they exit the interior? If their are not inside then they must be outside. Since the slow ones are already trapped in the magnetic lines, they will be in the same magnetic lines outside the toroid structure.
Magnetic field lines make loops. There is no such thing as magnetic monopoles so a magnetic field must flow from pole to opposite pole. In this picture it is difficult to see where the loops are because they are out of plane.
This picture makes it easier to see loops. This picture shows a plane through a polywell at a 45 degree angle so that only the front and back coils are intersected. The black line in the inset shows the plane.
There are still flow lines out of plane because this is a three dimensional device. However it is easier to see that the loops enter and exit the polywell through the center of each toroid and gap between the toroids at the corners of the square. There are eight exit holes and six entry holes.
The next thing to notice about these field lines is that each hole is a pinch point. This is important because of another effect magnetic fields have on electrons called the magnetic mirror effect.
This is a picture of two coils where the plane is through the center of the coils. A magnetic mirror is created by moving two coils far enough apart that the interior magnetic field instead of being smooth expands between the two coils. In this picture the expansion is in the vertical direction up and down. The difference in magnetic field strength at any point is shown by how far apart the magnetic field lines are. An electron with the right forward velocity will reverse direction as it approaches a stronger magnetic field. Another way of looking at this effect is that electrons following magnetic field lines will reverse direction when approaching a pinch point. In the picture above there are two pinch points. So electrons on the inside field lines with the right forward velocity with oscillate inside between the two coils.
As can be seen in this picture there are also magnetic field lines outside of the coils. Assuming that there are no conducting surfaces obstructing electron flow, the electrons will also oscillate between pinch points on the outside field lines of the coils. In fact there is a lot more area outside of the coils then inside the coils so for the same electron density many more electrons could be outside then inside.
Relooking at the polywell magnetic field line picture we can see that as with the two coils there is a lot more room for electrons to be stored on the outside then on the inside. During polywell operation electrons are being pumped into the inside of the polywell greating a high electrostatic pressure. The electrostatic pressure on the interior electrons combined with the magnetic mirror effect means that no electrons are going to reenter the interior once they are forced out.
Because of all these electrons accumulating outside of the polywell, a polywell could be considered a solid sphere of electrons with most of them being outside the structure. Therefore a polywell is the inverse of a fusor. A fusor has all the electrons located at the electrode in the center. A polywell has a majority of it’s electrons outside of the center.
Ion movement in a polywell has the same principle as a fusor. At any point that is not at the exact center of the device there is an electrostatic force acting on the ion accelerating it toward the center. With a little bit of redesign a polywell could have very few electrons inside of the structure. This would greatly simplify electron injection since injection would be into the outside magnetic fields, reduce power consumption, and reduce thermalization between electrons and ions.