Measurement of thermal emittance for a copper photocathode

Measurements of the thermal emittance of an electron beam produced by photoemission from the copper cathode of a high power RF cavity are presented. The RMS normalized emittance has been measured as a function of laser spot size, applied surface field, and polarization of the laser beam at normal incidence. Local field enhancement due to surface effects is found to increase the emittance substantially beyond that expected for a perfect planar surface. 1 THERMAL EMITTANCE The value of the thermal emittance of an electron beam produced by a photoinjector is a measure of the temperature of the electrons emitted from the metal cathode surface. It is the minimum possible emittance for a given accelerator, and is an important determinant of FEL performance and linear collider luminosity. It is difficult to accurately measure because nonlinear space charge forces, lattice errors and variation in linear space charge and RF forces along the bunch all contribute to growth beyond the thermal value. The limits on peak current and bunch length are discussed in the sections below. Lattice misalignments and wakefields are negligible due to the very short distance (65 cm from the cathode) the beam is transported. We assume a thermalized distribution of electrons is emitted from the copper surface. There is no correlation among momentum and position so that the normalized emittance is given by xxxN xx ′=′ = σγβσγβε 22 (1) Lawson’s expression [1] for the width of the momentum distribution of a thermalized beam is 22 mc E mc E k xxN k x σεσ =⇒= ′ (2) where the kinetic energy of the electrons after emission is,sin rfrfrfCuk EhE θβαυ +Φ− = (3