Using I-TOF spectrometry to measure photon energies at FELs

Abstract

Due to the stochastic nature of the Self Amplifying Spontaneous Emission (SASE) process and the resulting pulse-to-pulse fluctuations of the Free Electron Laser (FEL) photon energies, experimenters working with FELs need to get real-time feedback about the photon properties for their experiments. Investigations of narrow atomic or molecular resonances, phase transitions, or any other kind of effect heavily dependent on photon energy would need to know the precise FEL photon energy for each individual photon bunch. Furthermore, any spectrometer developed to deliver the information of these properties should not significantly interfere or degrade the FEL beam. Therefore, the group at the Free Electron Laser in Hamburg (FLASH) has developed an online photoionization spectrometer that uses ion time of flight (I-TOF) measurement methods on noble gases to measure the photon energy of each pulse. This paper presents the first test results for the viability of this online photoionization spectrometer (OPS).

A non-invasive online photoionization spectrometer for FLASH2

“…Schram et al, 1966;Stockli & Fry, 1997). For the model used in the ion spectrometer and similar multipliers, this behavior could be confirmed already in earlier measurements at a synchrotron source (Juranić et al, 2009), at FLASH (Guichard et al, 2013) and at SACLA (Kato et al, 2012). Accordingly, intensity correction factors have been introduced for the ion spectra analysis,…”

High-resolution single-shot spectral monitoring of hard x-ray free-electron laser radiation

“…Several technical approaches have been pursued to this end, for example, single-shot spectral measurements of the SASE pulses in the soft x-ray range [3][4][5]. For the hard x-ray range, a method using a focusing mirror to create a divergent x-ray beam and a flat silicon crystal for spectral dispersion [ Fig.…”

Single-shot analysis of hard x-ray laser radiation using a noninvasive grating spectrometer

“…In the soft x-ray range, time-of-flight spectroscopy on the photoemitted electrons [5] or ions [6] in a gas has been used to obtain emission spectra without interfering with an experiment downstream. However, in the hard x-ray range when using applicable gas pressures, the photoionization cross sections become too small for this method to work well.…”