Studies on charge production from Cs2Te photocathodes in the PITZ L-band normal conducting radio frequency photo injector

TL;DR

This study investigates charge production from Cs2Te photocathodes in an RF gun, revealing how laser profile and space charge effects influence emitted charge, with simulations aligning well with experimental data.

Contribution

It introduces a detailed analysis of laser profile effects on charge emission and improves simulation accuracy using measured profiles and semi-analytical models.

Findings

Charge depends linearly on laser energy at low levels.

Charge saturation occurs at high laser energies due to space charge.

Simulations match experimental results when using measured laser profiles.

Abstract

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.