## 11 Future Developments  
Being a brand new facility and observing much progress in the field of FEL sources, FEL instrumentation, and novel types of scientific experiments, a rich variety of further developments is expected to be implemented during the coming years. Developments going beyond the baseline scope of European XFEL have already started using external funding. Most notable is the construction and implementation of self-seeding for the hard X-ray FEL sources [104; 105]. This is on-going for the SASE2 FEL and under preparation for SASE1. The FEL radiation performances of SASE3 would benefit enormously by the provision of variable polarization that can be switched between linear and circular with full flexibility [56]. The installation of an SASE3 afterburner is therefore under preparation. This afterburner consists of several 2 m long APPLE-type undulators, which will be added to the main SASE3 undulator. Furthermore, the installation of a chicane in the SASE3 undulator will enable operation at two widely separated photon energies for time-resolved X-ray-X-ray pump-probe investigations. Another development concerns the construction of additional scientific instruments. Using funds from user consortia, the additional end-station at SPB/SFX for serial femtosecond crystallography and a third beam transport system, vacuum port and experiment hutch at SASE3 are pursued. A completely different area is that of further developing the PP laser towards providing much longer wavelengths. Pumping solids in the THz regime has many scientific applications and is vigorously requested by part of the user community. The feasibility and possible implementation of laser- and accelerator-based techniques to produce intense, ultrashort duration, and monochromatic THz pulses is currently studied.  
Naturally, the completion of the remaining two yet unoccupied FEL sources and the construction of further scientific instruments are expected to become major activities of European XFEL once the regular operation of the facility is achieved successfully. In a more distant future, a modification of the superconducting accelerator to include a cw mode of operation is very interesting for scientific applications, as is also indicated by the LCLS-II project [13]. Such an upgrade first requires developing an additional low emittance injector operating in cw mode. Since the electron energy will be significantly smaller than with the present pulsed RF system, this upgrade also requires a modified concept for the FEL sources. One possibility would be to direct the electrons to a second switchyard with novel FEL undulators specifically designed for the smaller electron energies and providing at the same time space for a second experiment hall hosting additional scientific instruments.  
All contributors, both internal and external, to European XFEL are acknowledged for their outstanding contributions to build this facility. Additional instrumentation for the scientific instruments and the laboratory infrastructure provided through external contributions, in particular by the HIBEF, IRIXS, COMO, SFX and XBI User Consortia, and through BMBF Verbundforschung projects 05K10 PM2, 05K13 RF4, 05K16 BC1, 05K16 PE1 and 05K13 PE2 is most gratefully acknowledged.  
[PERSON] conceived the article, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] contributed sections to this paper.  
The authors declare no conflict of interest.