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A single-shot spectrum, here with a fundamental wavelength of 13.49 nm
The Spectrum of a Single Shot
An FEL single-shot spectrum can be measured by a grazing incidence grating spectrometer at the end of the linac tunnel. The grating diffracts the light – depending on wavelength – to different positions on a CCD camera. However, to perform the measurement, a mirror has to be moved into the beam blocking the radiation, and hence these measurements are only carried out for machine commissioning and tuning on user demands.
A new Variable Line Spacing grating spectrometer (VLS) will enable optional online spectral measurements of the FEL pulses actually used for experiments. The blazed grating will replace the second mirror of the FEL beam distribution system with negligible side effects on the downstream beamlines. In this setup about 10 percent of the beam are diffracted and focused by the grating in the first order onto a CCD detector, while the rest of the pulse, reflected in zeroth order, proceeds to the experiment. The spectrometer will cover the wavelength range from 6.5 nm to 60 nm.
