The ideas of "flux lines" and "B field" help us visulize the observed behavior. As the B field increases, flux lines get closer together. The gyro radius shrinks - or you can say the particles circulate around the same number of flux lines and since the flux density is increasing, the particle orbits shrink. Same obversvation - different way to describe it.
Electrons inside the wiffle ball are mostly trapped. Only a few in the "loss cone" velocity region will escape the center, and the electrostatic field from the MaGrid helps to bounce them back. But that assumes the B field is high enough to hold the plasma. If it really runs in a beta = 1 mode (beta is ratio of magnetic field pressure to plasma pressure) then I would guess stability is a question of electrostatics only.
One of the things that makes plasmas so much fun and challenging is that the electric field extends across all space. You can't really think of a "stray electron". You can kinda think about bulk currents - if the density is high enough MHD (magnetohydrodynamics) works reasonably well. The whole system of particles and fields is all one big huge mess that affects itself continuously.
If you really want to see how weird plasmas can be, look up lightning. High density plasma does what ever it feels like, when if feels like it. Sometimes it takes the copper wire to ground, some times it blows holes thru walls instead. Another thing to play with is an arc welder. Just playing with an arc will give you a lot of feel for what goes on inside a fusion reactor of any kind. It squirms, it jumps, it's amazing and it's fun!
Oh yeah - put a lit candle in a microwave and fire it off at even 600 Watts. Very cool plasma balls will form. Not exactly high density, but fun to watch! Don't expect to save the microwave for cooking
