Journal Article FEL physics beamline endstation diffractometer hard X-rays instrumentation macromolecular crystallography photon diagnostics APPLE undulator FEL Published Erratum Research Support, Non-U.S. Gov't SLS 2.0 X-ray beam position monitors X-ray free-electron lasers X-rays additive manufacturing beam mask beam position monitors beamline instrumentation chicane collection strategy compact footprint diaphragm diffraction data front end gas attenuator high-power load high-power slits microcrystallography