A single-shot spectrum, here with a fundamental wavelength of 13.49 nm
Fluctuations in wavelength – within the FEL bandwidth of approximately 1 percent – from pulse to pulse are specific for the SASE process. The FEL wavelength spectrum can be measured by a grazing incidence grating spectrometer at the end of the linac tunnel. However, to perform the measurement, a mirror has to be moved into the beam blocking the radiation to the user experiment, and hence these measurements are only carried out for machine commissioning and tuning on user demands.
A Variable Line Spacing grating spectrometer (VLS) enables optional online spectral measurements of the FEL pulses actually used for experiments on a single shot basis. It is installed at the non-monochromatised BL-beamline branch. The blazed grating replaces the second mirror of the FLASH beam distribution system. The optical design of the spectrometer was carried out by a collaboration of DESY, Scientific Answers, and Solutions (SAS) in Madison and the Council for the Central Laboratory of the Research Councils (CCLRC) in Daresbury.
The spectrometer makes use of two interchangeable plane VLS diffraction gratings and covers the wavelength range from 6-60 nm. Shorter wavelengths can be reached using second order light. The mechanical system was designed jointly by Helmholtzzentrum Berlin (HZB) and DESY. The major fraction of the radiation, ~85–99% depending on wavelength and grating, is reflected in zeroth order to the experimental stations, while the rest is dispersed in first order and used for the online measurement of the spectral distribution. The dispersed radiation is focused onto a Ce:YAG single crystal screen which is imaged by an intensified CCD camera with a repetition rate of 10Hz. Up to now, the instrument has reached a resolving power of 1000 at 25 nm FEL wavelength.
A mirror with both, carbon or nickel coating can be moved in place of the gratings if no information of the spectral distribution is requested, or if the intensity on the user sample is very critical.
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References:
G. Brenner et al., First results from the online variable line spacing grating spectrometer at FLASH,Nucl. Instr. and Meth A 635, 99–103 (2011). Article
