This paper discusses the behavior of electron bunch charge produced in an
L-band normal conducting radio frequency cavity (RF gun) from Cs2Te
photocathodes illuminated with ps-long UV laser pulses when the laser
transverse distribution consists of a flat-top core with Gaussian-like decaying
halo. The produced charge shows a linear dependence at low laser pulse energies
as expected in the quantum efficiency limited emission regime, while its
dependence on laser pulse energy is observed to be much weaker for higher
values, due to space charge limited emission. However, direct plug-in of
experimental parameters into the space charge tracking code ASTRA yields lower
output charge in the space charge limited regime compared to measured values.
The rate of increase of the produced charge at high laser pulse energies close
to the space charge limited emission regime seems to be proportional to the
amount of halo present in the radial laser profile since the charge from the
core has saturated already. By utilizing core + halo particle distributions
based on measured radial laser profiles, ASTRA simulations and semi-analytical
emission models reproduce the behavior of the measured charge for a wide range
of RF gun and laser operational parameters within the measurement
uncertainties.Comment: 15 pages, 16 figures, 2 table
