Electron Beams Generated in Foilless Diodes

The operation of a foilless diode to generate a hollow relativistic electron beam has been performed on the FX-25, a Van de Graaff charged 3 MV device. The diode consists of a cylinder of carbon (graphite) supporting either a cathode tip of larger diameter or a right circular cylinder of equal diameter and a coaxial anode guide wall having either an abrupt discontinuity in wall diameter or a smooth wall. Six configurations were employed to explore the emitted electron behavior in the crossed electric and magnetic fields. Finite gyroradius effects limit the transmitted current at low applied axial magnetic field strengths. The transmitted current increases with increasing field strength until a critical field is reached, when further increase in field strength results in a decrease to a plateau in transmitted current. This critical field produces an electron gyroradius equal to the gap between cathode guide wall (anode) and is found to satisfy ¿c > 2 ¿b where ¿c is the electron gyro-frequency and ¿b is the beam plasma frequency. At high applied field strengths a very thin beam is transmitted and bounded by the space charge limited current. These features agree qualitatively with recent computer simulations of Jones and Thode.