Design of a nondestructive two-in-one instrument for measuring the polarization and energy spectrum at an X-ray FEL facility

Abstract

A: The free electron laser (FEL), as a next-generation light source, is an attractive tool in scientific frontier research because of its advantages of full coherence, ultra-short pulse duration, and controllable polarization. Owing to the demand of real-time bunch diagnosis during FEL experiments, precise nondestructive measurements of the polarization and X-ray energy spectrum using one instrument are preferred. In this paper, such an instrument based on the electron time-of-flight technique is proposed. By considering the complexity and nonlinearity, a numerical model in the framework of Geant4 has been developed for optimization. Taking the Shanghai Soft X-ray FEL user facility as an example, its measurement performances' dependence on the critical parameters was studied systematically, and, finally, an optimal design was obtained, achieving resolutions of 0.5% for the polarization degree and 0.3 eV for the X-ray energy spectrum. K: X-ray FEL, e-TOF, Polarization, Energy spectrum A X P : 1702.066521Corresponding author. 2Corresponding author.

Proposal of X-ray absorption spectroscopy and magnetic circular dichroism using broadband free-electron lasers

“…is the detection efficiency for the XFEL with a given energy E; N v is the number density of an atom in the sample material, and l is the thickness of the target in the incident direction of the XFEL. The XMCD experiment requires simultaneous circular polarization and energy spectrum measurement, which can be achieved by using an e-TOF experiment [25] based on photoelectrons' angular distribution [34,35] and drift time, respectively [34]. To measure the E ph spectrum, the relationship between E ph and the photoelectron energies E e is…”

“…Recently, several such methods have been implemented for absorption spectrum measurements [10,24], including the use of a grating raster for the simultaneous measurement of incident and transmitted E ph spectra [24]. However, in order to monitor the polarization properties of incident or transmitted FEL pulses, in this study, an e-TOF-based instrument was chosen because of its ability to simultaneously measure both the XFEL spectrum and polarization [25]. In our case, using an SXFEL operated at a 50-Hz repetition rate [26] and an e-TOF-based highresolution spectrometer [25], the absorption spectrum of an MnO sample was measured in 1 min after careful calibration.…”

“…However, in order to monitor the polarization properties of incident or transmitted FEL pulses, in this study, an e-TOF-based instrument was chosen because of its ability to simultaneously measure both the XFEL spectrum and polarization [25]. In our case, using an SXFEL operated at a 50-Hz repetition rate [26] and an e-TOF-based highresolution spectrometer [25], the absorption spectrum of an MnO sample was measured in 1 min after careful calibration. Therefore, the broadband XFEL offers a more efficient method for both steady-state and time-resolved XAS and XMCD experiments.…”

“…Generally, noise from phenomena such as XFEL beam fluctuation and photoelectron drift lead to lower quality deconvolutions. Compared to our previous research [25], an improved method using Wiener deconvolution [41] has been used to retrieve E ph from a signal pulse robustly, reducing the influence of the noise significantly. In the following, this method will be described.…”

“…where * represents the convolution operation; S(t) is the pulse shape of an MCP detector for a single electron without delay; N (t) is the theoretical flight time of a photoelectron from a photon with a given energy in the vacuum; N s (t) is the distribution of photon electrons in the time domain (without an offset of the earliest flight time); F (t) is the signal pulse shape for an impulse in time domain, which can be described in the shape of a Log-Gaussian distribution [25]; and e(t) is an error term due to noise and statistical fluctuations. In the frequency domain, Eq.…”