Prospects for a Free Electron Laser at the FemtoMAX Beamline

Sammanfattning: A Free Electron Laser (FEL), is a device that generates extremely coherent and brilliant radiation using electrons accelerated to relativistic velocities in a particle accelerator. The process utilises similar equipment to what is used at a synchrotron source like MAX IV, but in addition there is an interaction between the relativistic electron beam and the generated radiation to achieve an exponential growth in radiation intensity. The produced radiation can potentially be several orders of magnitude more brilliant than what is possible at a synchrotron source like MAX IV, which also produces brilliant and coherent radiation in the form of X-rays. This is mainly done in the two storage rings at the facility, but also at a beamline called FemtoMAX. FemtoMAX lies in the Short Pulse Facility (SPF), and is connected directly to the linear accelerator (linac), that also injects electrons into the storage rings. When MAX IV was built, there was an idea that the linac could also be used to drive an FEL. The prospective FEL at FemtoMAX would be a so-called Self-Amplified Spontaneous Emission (SASE) FEL, similar to other major FELs like European XFEL or LCLS. In such FELs, the radiation is generated with a single pass of electrons through undulators which make the electrons emit radiation. FemtoMAX also generates radiation with an undulator driven by electrons from a linac, but what makes the difference to an FEL is that in the latter the wavelength and electron energy are tuned so that the electrons will emit radiation coherently, which causes a self-amplifying effect, and an exponential growth in radiation intensity. This exponential growth is characterised by a distance over which the intensity increases by a factor of e, the gain length, which depends on a number of parameters of the electron beam. Most importantly for this thesis, it decreases with decreasing electron energy. At the 3 GeV electron energy where FemtoMAX currently operates, the exponential growth requires a considerably longer undulator length than what is available at FemtoMAX. Lowering the energy of the linac would shorten the gain length, and could make the exponential growth long enough to generate FEL radiation. By simulating the FEL process using parameters from FemtoMAX, the prospective FEL at FemtoMAX was investigated. The results show that by lowering the electron energy to 2 GeV or lower, there is a significant lasing and exponential growth of the radiation. At 1.6 GeV, there is even saturation of the FEL process. The conclusion is that there is a real possibility to implement such an FEL at FemtoMAX, provided the energy of the linac can be lowered.