Large charge extraction from metallic multifilamentary Nb3Sn photocathode.

The current density limit for photoemission from metals was measured in an rf photogun to be below 10(9)  A/m2. We have achieved 1.6×10(11)  A/m2 by photofield emission from a new type of photocathode made from a metallic-composite, multifilamentary Nb3Sn wire driven by a 266 nm picosecond laser pulse and a 2 ns, 50 kV accelerating voltage. This cathode has a micrometer arrayed structure with tens of thousands of Nb/Nb3Sn filaments embedded in a bronze matrix. Our measurements revealed the existence of a new electron emission regime at high laser fluence (100  mJ/cm2). We have extracted stably, and without any surface ablation, up to 4800 pC of charge. This corresponds to 0.9% quantum efficiency, 100 times larger than what is measured from conventional metallic photocathodes. The unexpected large and stable charge extraction cannot be explained by the 3-step model. Thanks to the small emitting area, the measured emittance (0.6  mm·mrad) is low in spite of the high current density and space charge effects. This cathode will be of benefit for many applications based on short and bright electron bunches.