Art Carlson wrote:I think Bussard's idea that the density in the cusp outside the magrid is very much lower than inside is based on (wrongly) neglecting quasi-neutrality in the cusps. My working picture of the cusps is just holes (or more precisely a straw or a slit between two parallel plates) that let plasma out. In fluid dynamics you usually don't get more than a factor of 2 drop in the density in those circumstances. A somewhat more sophisticated way to look at, that is also found in the literature, is that there is a "sheath" around the plasma ball that gets bled off into the cusps, whereby the flux tubes have a constant magnetic field, at least as long as they run along the plasma ball. This would also lead to a cusp density close to the density in the plasma ball, again dropping by a factor of 2 or less.
In the theory of plasma-wall interactions, including Langmuir probes, the density drop would be called the "pre-sheath". The value can depend on various conditions, particularly on how the plasma is replenished, but a drop by a factor of 2 is very typical.
A few more questions come to mind then. What is a ballpark density difference between core and cusp needed for things to work as Bussard envisions? A factor of 10, 100, 1000? The obvious follow on being if a factor of 2 is typical, what is the atypical extreme and is there a theoretical limit that can be placed there as well?