Photon beamline for the prototype 250-MeV machine is dedicated for diagnostics of the linac and the undulator by measuring the intensity and the spectrum of the photon beam. The photon beam is expected to be in the vacuum-ultraviolet (VUV) and the visible region, i.e., the wavelength range of 60-400 nm. The following figure shows the schematic view of the photon beamline, which is placed just downstream of the beam dump. The intensity of the VUV light is monitored as the photoelectron and the photo-ion currents of rare gas, such as Ar, Kr, and Xe, filled in the gas-chamber with low pressure. The two differential pumping systems (DPS) upstream and downstream of the gas-chamber isolate the vacuum. The intensity of the visible light is monitored by a photo-diode and a calorimeter. These intensity monitors serves for study of saturation and fluctuation of FEL pulses. The spectrometer with a CCD imaging device measures the spectrum at once, and gives important information of shot-by-shot spectrum, such as the fine structure, the center, and the width of the spectrum.
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