Instantaneous spherical electron focusing in a Penning trap

In the electron beam Penning trap, electron focusing at the trap center by means of injecting low-canonical-angular-momentum electrons has been identi fied. It is found that the focusing occurs instantaneously under the condit ion that the injection current is larger than a threshold and the effective electric potential well is spherical. A self-consistent model is proposed to describe the setup process of the dense core. According to the model the instantaneous core density is evaluated to be about one order greater than the corresponding Brillouin density. (C) 2000 American Institute of Physic s. [S1070-664X(00)01110-1].