Low Velocity Spread Axis Encircling Hollow Beam Gun for the NRL Gyropeniotron

The design of a 70 kV, 5 A electron gun and beam forming system for the NRL gyropeniotron is presented. The gyropeniotron requires an axis encircling hollow electron beam with very low spreads in both axial velocity and guiding center. Simulation results using the 2-D deformable-mesh electron trajectory code DEMEOS, have produced axial velocity spreads less than 2% and guiding center spreads less than 5%. The mean radius of the electron beam is 0.045" with a perpendicular to parallel velocity ratio, α, of 2. The gun design uses a space charge limited dispenser cathode for reduced beam noise. The electron beam passes through a magnetic field reversal which causes the beam to rotate at the cyclotron frequency. There is a magnetic shield surrounding the gun and a hollow iron center post carries the magnetic flux enclosed by the beam. Since there is no magnetic field at the cathode, all the electrons enclose the same amount of magnetic flux and have the same angular velocity after the field reversal. This is important since the axial velocity spread is proportional to α
²
times the transverse velocity spread.