MEIS studies of oxygen plasma cleaning of copper for fast response time photocathodes used in accelerator applications

The performance of a fourth generation light source is to a greater extent reliant on the properties
of the electron bunches, with the fundamental limit controlled by the photocathode where the
electrons are emitted. Normally conducting RF guns often use metal photocathodes, mainly due
to their fast response time that allows very short pulses to be generated. However, they typically
have very low quantum efficiency (QE) compared to semiconductor alternatives (GaAs or
Cs2Te). The drive to use higher QE metals is motivated by the need to minimize the laser power
required to generate sufficient bunch charge for the downstream accelerator.
The use of Cu as the metal photocathode of choice is long standing. However, the preparation of
an atomically clean surface is thought to be key to achieving a high enough QE to be used in an
RF gun. At STFC Daresbury laboratory a preparation procedure has been developed that has
allowed a QE of approximately 10-5 to be achieved. This coupled with a high power UV laser
system has allowed electron bunches of up to 250 pC to be generated in the Versatile Electron
Linear Accelerator (VELA) facility [1]. However, there is very little understanding of how the
cleaning procedure leads to the required surface properties for electron emission and hence a
detailed study has been initiated.