Propagation of an intense electron beam in neutral gas along a strong magnetic field

Two propagation chambers were used, one with a length comparable to the beam diameter, and a second long chamber with a length much greater than the beam diameter. It is concluded that the application of a magnetic guide field has three principal effects on beam propagation: (1) the Alfven current limit is relaxed when the guide field is much stronger than the beam self magnetic field, (2) the magnetic guide field constrains secondary electron motion, and (3) filamentation of the beam in the diode is seen at high magnetic fields. It is concluded that this has little effect on the space charge neutralization process, but strongly affects the current neutralization of the beam. A one dimensional model of the virtual cathode, generated when the beam is first turned on, is developed. This model includes both beam velocity spread and effects of the ionization of a background gas. The results of this model are compared with experimental results.