title: A non-invasive online photoionization spectrometer for FLASH2.
abstract: The stochastic nature of the self-amplified spontaneous emission (SASE) process of free-electron lasers (FELs) effects pulse-to-pulse fluctuations of the radiation properties, such as the photon energy, which are determinative for processes of photon-matter interactions. Hence, SASE FEL sources pose a great challenge for scientific investigations, since experimenters need to obtain precise real-time feedback of these properties for each individual photon bunch for interpretation of the experimental data. Furthermore, any device developed to deliver the according information should not significantly interfere with or degrade the FEL beam. Regarding the spectral properties, a device for online monitoring of FEL wavelengths has been developed for FLASH2, which is based on photoionization of gaseous targets and the measurements of the corresponding electron and ion time-of-flight spectra. This paper presents experimental studies and cross-calibration measurements demonstrating the viability of this online photoionization spectrometer.
authors.affiliation: Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Braune
authors.affiliation: Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
authors.forename: Günter
authors.initials: G
authors.lastname: Brenner
authors.affiliation: Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Siarhei
authors.initials: S
authors.lastname: Dziarzhytski
authors.affiliation: Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
authors.forename: Pavle
authors.initials: P
authors.lastname: Juranić
authors.affiliation: Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Andrey
authors.initials: A
authors.lastname: Sorokin
authors.affiliation: Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Kai
authors.initials: K
authors.lastname: Tiedtke
keywords: free-electron laser; photoionization; rare gases; soft X-ray; wavelength