Emitted electron beams from velvet cathodes

Velvet cathodes are widely used in field-emission electron diodes, in particular in induction accelerators used for flash X-ray radiography. The emission from velvet could be different from pulse-to-pulse in multi-pulse operation, such as in the double-pulse option studied for the third radiographic axis of the Epure facility in France. As a consequence, we are revisiting the beam homogeneity and the emittance of the electron beam produced by velvet cathodes. First, we used a single-pulse diode connected to a 100 ns, 400 kV, 1-4 kA, 100 ns Blumlein generator. Various cathode-anode configurations were tested and we present here the analysis of the emitted beam along with numerical simulations performed both with MAGIC2D and CST-PS3D. The Cerenkov emission produced by the beam in a fused silica target is observed with fast cameras. A Pepper pot was installed in order to measure the emittance whose value is compared to the predictions of PIC simulations. Planned studies with the MI2 double-pulse injector prototype will be discussed.