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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.

In the frame of conceptual studies for a compact (less than 6 GeV) X-ray FEL at PSI, we report here on the design and beam dynamics simulations of a two-cell two-frequency RF gun-like cavity. The goal of this study is to demonstrate the feasibility and the RF flexibility of such a cavity for manipulating the longitudinal phase space while preserving the initial low transverse emittance. Shaping the longitudinal phase space is mandatory for efficient velocity bunching at an early stage of the accelerator. The cavity, located after the gap of a 500-kV to 1-MV diode, is dimensioned to operate at 1.5 and 4.5 GHz. The RF design of the two-cell cavity and the coupling schemes for both frequencies are presented. Mode separation, which ensures adequate operation, is achieved by profiling the cavity walls. The flexibility of such a design to shape the longitudinal phase space is illustrated with beam dynamics simulations. The transverse emittance is shown to be nearly unchanged by combining emittance compensation and a beam matching scheme. The deleterious effects of the transverse RF fields are consequently reduced to a minimum.

The Photo Injector Test Facility at DESY Zeuthen (PITZ) was built to test and optimize electron sources for Free Electron Lasers and future linear colliders. The focus is on the production of intense electron beams with minimum transverse emittance and short bunch length as required for FEL operation. The experimental setup includes a 1.5 cell L-band gun cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains with variable temporal and spatial pulse shape, an UHV photo cathode exchange system, and an extensive diagnostics section. This contribution will give an overview on the facility and will mainly discuss the measurements of the electron beam transverse phase space. This will include measurements of the transverse and longitudinal laser profile, beam charge as a function of RF phase, and transverse emittance as a function of different parameters. The corresponding measurements of momentum and momentum spread as well as the RF commissioning results will be summarized. As a first application of the PITZ electron source it will be installed at the TESLA Test Facility Free Electron Laser at DESY Hamburg in autumn 2003.

The Photo Injector Test facility at DESY Zeuthen (PITZ) was built to develop electron sources for the TESLA Test Facility Free Electron Laser and future linear colliders. The main goal is to study the production of minimum transverse emittance beams with short bunch length at medium charge (∼1 nC). The facility includes a 1.5 cell L-band cavity with coaxial RF coupler, a solenoid for space charge compensation, a laser capable to generate long pulse trains, an UHV photo cathode exchange system, and different diagnostics tools. Besides an overview of the facility, its main components and their commissioning, this contribution will concentrate on the first measurements at PITZ with photoelectrons. This will include measurements of the transverse and longitudinal laser profile, charge and quantum efficiency, momentum and momentum spread, transverse electron beam profiles at different locations and first results on transverse emittance.