Joint Research Activity 5
Development of an Optimized Radio Frequency Photoinjector for X-ray Free Electron Lasers (PITZ2)

This joint research activity aims at the optimization of the existing rf photoinjector at the Photo Injector Test facility at DESY in Zeuthen (PITZ), with the purpose of providing an electron source to be used at approved FELs like FLASH, European XFEL, and other free electron lasers. Therefore, the second phase of the PITZ project, called PITZ2, focuses on FEL-related requirements, e.g. production and conservation of high beam quality, overall reliability, rf technology, and user aspects. Due to the vicinity to FLASH and the XFEL, a direct transfer of the project’s results to the FEL community is guaranteed.
The joint research activity focuses on many different subjects, relevant for FEL operation. They are organized in five work packages:

Work Package 1 - Beam dynamics simulations
The design concepts of the current and future injectors for FEL operation (e.g. FLASH, SPARC, LCLS, XFEL) are based on numerous simulations, which show the possibility of achieving smaller emittance using the emittance conservation principle (Fig. 1). In order to reach small beam emittance, the optimized working point has to be found by simulations and confirmed with experimental results. Further development of simulation codes and optimization of all subsystems of the photo injector are necessary. The simulations are mainly done by DESY and INFN-LNF Frascati.

Fig. 1: Simulation of the emittance development along the PITZ beamline
when applying the emittance conservation technique.

Work Package 2 - Booster and gun development
According to simulations, electron beams of very small emittance can be produced when using a booster cavity. Thus, the booster cavity is the key technical element of PITZ2. A preliminary booster was installed in the PITZ beamline in 2005, allowing for an energy increase from ~5 MeV to ~12 MeV. The final booster, designed by INR Troitsk and currently under construction at DESY in Hamburg, will allow reaching the goal energy of about 30 MeV. The gun development is also urgent since the FEL operation parameters can only be reached with a higher accelerating gradient in the gun – which requires a new gun design with improved cooling possibilities. Such a gun with powerful cooling system was developed and built by BESSY and is now under tests at a special gun test stand at DESY in Zeuthen (Fig. 2).

Fig. 2: BESSY gun with improved cooling on the test stand in the PITZ tunnel.

Work Package 3 - Diagnostics beamline
For detailed studies of the electron beam parameters and a complete characterization of the electron source at higher energies, a new diagnostics beamline has to be developed and installed. It includes devices that allow efficient and precise measurements of longitudinal and transverse phase space parameters for the full range of beam energies from 5 to 30 MeV: a dispersive section for momentum and slice emittance measurements is being developed and built in collaboration with CNRS-LAL Orsay. Three new emittance measurement stations have been built by INRNE Sofia and are already installed at PITZ (Fig. 3). Finally, the development of a beam dump for the special beam parameters at PITZ is a challenging task.

Fig. 3: View of the new PITZ beamline with the emittance measurement stations.

Work Package 4 - Laser and photocathode development
In order to produce electron beams with optimal beam quality for FEL operation, the driving photocathode laser must provide UV light pulses of adequate longitudinal and transverse shape. In order to permanently monitor the longitudinal pulse shape, an optical sampling system was developed by MBI and is now being installed at PITZ. Studies on further improvement of photocathodes represent another important component for the whole project. Two main research directions are foreseen: cathode performances (cathode lifetime, quantum efficiency, thermal emittance) and cathode handling system (preparation, transportation, and insertion chambers). These studies – as well as the production of the cathodes themselves – are done by INFN-LASA in Milano.

Work Package 5 - Operation and physics studies
The measurement program is mainly oriented in the direction of an experimental verification of the emittance conservation principle. The production and transport of very low emittance beams has to be demonstrated. Extended comparisons between simulations and measurements will help to benchmark the theoretical understanding of photo injectors. The PITZ facility is operated by DESY with the help of BESSY and INFN-LNF Frascati.

More details on JRA5...
Contact: Sergiy Khodyachykh, DESY