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This work discusses the behavior of electron bunch charge produced in an L-band normal conducting radio frequency gun from Cs 2 Te photocathodes illuminated with ps-long UV laser pulses and presumed homogeneous flattop laser transverse distribution. The measured charge shows the expected linear dependence in the quantum efficiency limited emission regime at low laser pulse energies. At higher laser pulse energy, the measured charge in the space charge limited emission regime should saturate, assuming an ideal homogeneous flattop laser transverse distribution. However, this behavior is not observed experimentally. Instead of saturating, the measured charge continues to increase with laser pulse energy, albeit with much weaker dependence than in the quantum efficiency limited emission regime. Simulations with the space charge particle tracking code ASTRA show that the charge saturates as expected using a homogeneous flattop laser transverse distribution. The discrepancy between simulations and measured excess charge may be attributed to the presence of unintentional Gaussian-like decaying radial halo beyond the core of the otherwise presumed homogeneous flattop core. The rate of increase of the measured charge at high laser pulse energies seems to be proportional to the amount of halo despite charge saturation in the core of the transverse laser radial profile. 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.

The matching of a particle beam to specific transverse parameters, imposed by various operational, functional and diagnostic reasons, is an essential procedure in most types of accelerators. While well-established methods can easily match beams of negligible self fields, their accuracy is significantly reduced in the presence of strong space-charge forces. In a pursuit of time-efficient matching solutions for space-charge influenced beams, two approaches are demonstrated in this paper: a faster and more approximative one for symmetric beam transport along periodic and dense lattices, and a more analytic one for broader beam and lattice conditions. Beam dynamics simulations and experimental measurements are used to validate the performance of the suggested methods for the matching requirements of the transverse phase–space tomography at the Photo Injector Test Facility at DESY, Zeuthen site (PITZ).