Some theory musing...
- In a basic magnetic mirror machine there is a loss cone with a width inversely proportionate to the mirror ratio between magnetic field strength at the choke points vs. in the middle. This loss cone is independent of particle mass or energy.
- The magrid produces a polyhedral well configuration, giving a zero magnetic field dead center, and a fairly high mirror ratio on the inside, at the expense of more cusp points.
- Plasma at beta=1 inside this well excludes the magnetic from an extended zone inside the magrid, raising the mirror ratio high enough that the magnetic mirror loss model no longer applies. Instead a cusp loss model is used where loss area is proportional to cyclotron radius. Note that cyclotron radius is proportionate to particle mass and energy, allowing fuel ions and fusion products to escape much more easily that electrons.
With fuel ions escaping the wiffleball magnet field so easily, a different containment model is used, based on an electric potential well from an excess of electrons in the plasma.
- Outside the cusps, the magnetic mirror model still applies, and an electron with proper momentum has little difficulty reaching the wiffleball zone.
However, unless the electron is following the right magnetic field line, I see it being diverted by the same magnetic field layer that keeps electrons from escaping. It would therefor be important for the electron injectors to be fairly compact and accurately placed.
The Wiffleball, going in and out
I seem to recall the modified n-body sim someone made (horrible with names, so apologies for not remembering 
) showed that having the e-emitters to far from the cusp, or with too low energy and the electrons fail to enter the wiffleball area (bounce off), but too high an energy and they shoot right through and out the other side. This seems to me to fit what you describe/propose.
) showed that having the e-emitters to far from the cusp, or with too low energy and the electrons fail to enter the wiffleball area (bounce off), but too high an energy and they shoot right through and out the other side. This seems to me to fit what you describe/propose.