Low Emittance Beam Injector

To obtain high peak power from SASE FEL, the electron beam emittance has to be low. Emittance is a measure of paralellisity of group of particles. As seen in the figure, low emittance beam has uniform velocities, thus the radiation becomes pallarel. SASE FEL need very low emittance beam. To generate this beam, generally the rf-gun with photo-cathode combination is commonly used. By irradiating short-pulse laser beam (order of a few psec down to sub-pico-second), the short bunched beam is directly generated.

In SCSS, however, high-voltage pulsed gun with single crystal thermionic cathode is employed. This is because, this system does not requrie the short pulse laser system which is sometimes troublesome, and needs manpower to maintain. For the cahode material, we will use CeB6 cathode. It is a single crystal cathode, which maintains a very flat cathode surface after usage of cathode at high temperature by evaporation of cathode material.