Electron-beam uniformity of a large-area high-current accelerator diode

Abstract

Time-resolved photography of Cherenkov emission has been used to study large-area electron-beam uniformity and electron-beam propagation in air. For the 50-ns pulse widths used, we show that the time-dependent electron emission from large-area cathodes is typically very nonuniform. Pinching of the electron beam by its self-magnetic field can be prevented by the use of an applied axial magnetic field approximately equal to the self-magnetic field generated by the electron beam. When an applied field is used, beam nonuniformities introduced by the anode support structure persist through tens of centimeters of air outside the diode. These nonuniformities can reduce efficiency and beam quality of the laser. Beam rotation in the diode caused by the applied magnetic fields can result in significant loss of electron-beam energy to the anode chamber and foil-support structure. However, we have shown that emission can be reduced on areas of the cathode which map onto obstructions on the anode so the electron energy loss can be reduced.