Metallic photocathodes

The metallic photocathodes provide SRF gun another alternative to semiconductor photocathodes, especially Mg cathode for medium bunch charge application. Mg cathode is safe for the niobium cavity and can produce up to 300 pC bunch charge, which is suitable for the applications in the ELBE radiation centre.

Our Mg cathode is an Ø 10 mm bulk plug of pure poly-crystalline magnesium. The plug was mirror-like polished with different sizes diamond compound. HIgh power UV laser has been used to burn off the MgO layer. The gun laser was focused and guided into the transport chamber and scanned an Ø 4 mm area in the center of the Mg surface. The cleaned surface had a shining silver color. Also the microscope view demonstrated the surface structure change. After a proper cleaning process, the fresh QE reached the level of 10-3. The laser cleaning reduces the work function and then improve the QE, but the change of work function and surface roughness can also increase the thermal emittance ethermal, which is the limit of the electron beam emittance of the photoinjector.

Mg cathode image