Electron orbits in extrap during the start-up phase

Particle orbits in a linear EXTRAP vacuum magnetic field configuration are computed. Since the breakdown in a narrow channel surrounding the zero of the octupole field, orbits near the neutral line are studied. The attainable energy and suitable defined confinement time of particles in an applied electric field along the axis are calculated. The results indicate that the particles near the neutral line would gain substantial energy in the free fall, and are the most effective ones for the brekdown. The acquired energy depends on the elctric field strength; the required value of the field is determined. The direction of the applied electric field has no effect on the nature of central orbits. However, the particles with non-central trajectories drift towards the wall when the electric field is anti-parallel to the external currents, but towards the conductors when the field direction is reversed. An increase of the rod distance from the origin, or a decrease of the rod current, increases the attain