Injector Topics for Discussion High Repetition-Rate XFEL Physics and Technology Workshop Shanghai, Aug. 10, 2017 Photocathodes: QE, QE lifetime, laser wavelength (for mA-class currents QE lifetime is the critical/important parameter) FEL compression performance vs. quality of longitudinal phase space at injector FEL shortest wavelength vs. emittance and peak current. Possible tradeoffs. Select the tradeoff (using start to end simulations) to define the emittance and peak current requirements at the undulator. Define the electric field at the cathode, photocathode and beam energy at the gun to obtain the desired emittance. Consider dark current performance. Define the compression required at the injector to obtain the required longitudinal phase space quality. The injector can be a powerful source of temporal jitter. LLRF, RF source, beam parameters stability requirements must be carefully defined