Cut through the electron source of FLASH. The Cs2Te photocathode is mounted at the backplane of a 1.3-GHz 1.5-cell copper cavity. The electric field assumes its maximum value at the cathode. The radio-frequency power of about 3 MW is guided to the cavity through a wave guide and a coaxial coupler. The UV laser beam is deflected onto the cathode by a small mirror outside the electron beam axis. A solenoid coil produces longitudinal field lines around which the electrons travel on helical trajectories. A second solenoid, called “bucking coil”, compensates the magnetic field in the cathode region where the photo-emitted electrons have very low energy.
The high charge density in the bunches that is needed in a SASE FEL can be accomplished with photocathodes illuminated with short ultraviolet laser pulses. The injector at FLASH consists of a laser-driven photocathode mounted inside a Radio-Frequency (RF) cavity. The cathode is made of molybdenum and coated with a thin Cs2Te layer to achieve a quantum efficiency for photoelectron emission of typically 5 percent.
The UV laser pulses are generated in a mode-locked solid-state laser system (Nd:YLF) built by the Max Born Institute, Berlin. A difference to conventional cathodes is the rapid acceleration to relativistic energies which can only be achieved with radio-frequency fields and not with a dc electric field. The accelerating field at the cathode is in the order of 40 MV/m. A magnetic solenoid field is superimposed to force the particles on helical trajectories around the magnetic field lines in order to preserve a small beam cross section. The pulsed UV laser is synchronized to the 1.3-GHz RF of the linac with a precision of better than 100 femtoseconds.
It is impossible to generate the high peak current immediately in the gun because then huge space charge forces would arise and immediately disrupt the bunch due to the fact that the negatively charged electrons repel each other. Therefore modestly long laser pulses of 10 picoseconds duration are used, leading to a peak current of typically 50 A, but even in this case the particles must be accelerated as quickly as ever possible to relativistic energies. In the relativistic regime the repulsive electric forces between the equal charges are largely cancelled by the attractive magnetic forces between the parallel currents.
