Brief Summary
The Polish Free Electron Laser (FEL) is a fourth generation light source with 100 000 times more powerful pulses than the third generation sources – synchrotrons. Light pulses shorter than 100 femtoseconds (10^-13s) are produced by electrons speeded up by a linear accelerator.

Scientific potential of Free Electron Lasers
Free Electron Lasers (FEL) are generators of coherent beam of radiation of wavelength tunable from millimeters to UV range. These devices open a brand new area of research of possibilities that can hardly be evaluated today. Intensity, brightness and quality of radiation delivered by FELs are several orders of magnitude higher than currently used sources of radiation. In some cases FELs allow the delivery of radiation in currently unavailable ranges as THz range. FELs may be used in physics, chemistry, biology, material science, environmental science or medicine helping to understand processes in living cells, chemical reactions or material structures.

General assumptions
A Free Electron Laser (FEL) facility in Poland will create for broad scientific community a unique opportunity for active participation in the European R&D programs on physics and technology of coherent light sources and superconducting accelerators. An interdisciplinary character of the research, development of new technology and educational and training possibility at the facility bear out its importance already at the preliminary stage of design studies and for many years of its exploitation. Such a role can and should play in Poland a facility providing a source of coherent light, which is fed by the electron beam from a superconducting linear electron accelerator.

Technical description
In general, FEL processes require low emittance (εn < 2 μrad) bunches with charge of the order of 1 nC. The emittance specification is even tighter (εn ~1 μrad) for the electron injector, because subsequent acceleration process causes further dilution of the emittance. To fulfil the emittance and charge requirement, the most demanding part of an injector, an RF-electron gun, must operate at high accelerating gradient to mitigate the emittance growth due to the space charge force. The entire injector should operate in the cw and near-cw mode to keep its compatibility with the driving superconducting linac, allowing for these advantageous operation modes enabling a very high average brilliance of the facility. The electron injectors operating at present do not fulfil these requirements, since they operate either in a short pulse mode with high gradients (and low duty factor) or at low gradients in the cw mode (and low charge <<1 nC). The only feasible technical approach to the cw operating RF-gun, generating the low emittance and highly populated bunches is a design based on the superconducting technology (SRF-gun). Several institutions worldwide work on this approach: BNL, Beijing University, Forschungszentrum Rossendorf (FZR), TJNAF and DESY, with contribution from IPJ in Swierk.

Realization of the project
Cooperation with DESY could create large synergy between FLASH, XFEL and POLFEL projects. IPJ already contributes to TESLA/FLASH/XFEL projects participating in the R&D programs. Using technology developed for XFEL could be beneficial for both DESY and IPJ.