High-current-density cathode operation and beam transport with steady-state and pulsed cathode heating methods

High-current-density cathodes are of prime importance for millimeter-wave free-electron laser devices. The present work is concerned with the development of cathodes with current densities in the 10–50 A/cm² range for pulsed and cw electromagnetic devices with Pierce-type electron guns. The primary cathode material used thus far is lanthanum hexaboride (LaB6). Electron-beam bombardment heating has been found to be a practical means for heating these cathodes to the temperature range of 1800°C needed for the above current densities from LaB6. This heating method offers high-impedance-level circuitry which is easily controllable for continuous heating, and it also can be used in a novel pulse-heating mode which is compatible with beam-voltage pulse generators such as Marx banks. Inductive isolation of the heating power source is not required in this mode. Steady-state electron bombardment heating of a one-inch diameter cathode has been used to obtain a cathode current density as high as 32 A/cm2 at 110 kV voltage operation. Descriptions of the simulated and measured heating operation of both pulsed and continuous heating modes will be given. Measurements of the resultant beam properties such as current distribution and axial velocity spread will be described. The velocity distribution is measured with a retarding potential analyzer operating up to 82 kV. The transported beam can be allowed to interact with a bifilar wiggler if desired. These measurements are compared with trajectory calculations through the beam drift-tube /wiggler region and into the simulated analyzer. Good agreement is found between the measured and simulated velocity distributions.