Wavefront sensing at X-ray free-electron lasers

Abstract

Here a direct comparison is made between various X-ray wavefront sensing methods with application to optics alignment and focus characterization at X-ray free-electron lasers (XFELs). Focus optimization at XFEL beamlines presents unique challenges due to high peak powers as well as beam pointing instability, meaning that techniques capable of single-shot measurement and that probe the wavefront at an out-of-focus location are desirable. The techniques chosen for the comparison include single-phase-grating Talbot interferometry (shearing interferometry), dual-grating Talbot interferometry (moiré deflectometry) and speckle tracking. All three methods were implemented during a single beam time at the Linac Coherent Light Source, at the X-ray Pump Probe beamline, in order to make a direct comparison. Each method was used to characterize the wavefront resulting from a stack of beryllium compound refractive lenses followed by a corrective phase plate. In addition, difference wavefront measurements with and without the phase plate agreed with its design to within λ/20, which enabled a direct quantitative comparison between methods. Finally, a path toward automated alignment at XFEL beamlines using a wavefront sensor to close the loop is presented.

Correction of the X-ray wavefront from compound refractive lenses using 3D printed refractive structures

“…This technique reproduces measurements for the particular optics, and wavefront errors recorded for Be lenses are found on a similar order as measured by the other techniques, e.g. ptychography or speckle tracking (Seaberg et al, 2019). The 1D measured wavefront errors along vertical and horizontal lines are shown in Fig.…”

“…Recently (Seaberg et al, 2019), a 3D printed phase plate in IP-S resist was used to correct wavefront errors of 20 Be lenses, and wavefront analysis was carried out using three wavefront reconstruction techniques for X-ray free-electron laser (XFEL) sources. This paper describes the use of the knife-edge imaging-based wavefront-sensing technique to determine wavefront errors from two different stacks of Be lenses.…”

Binary Amplitude Reflection Gratings for X-ray Shearing and Hartmann Wavefront Sensors

“…The past three decades have seen numerous successful demonstrations, in a variety of circumstances. Among these, shearing interferometry [ 3 , 4 , 5 , 6 , 7 , 8 ] and Hartmann wavefront sensing [ 9 , 10 , 11 , 12 ] (referred to as shearing and Hartmann) have emerged as rapid, high-sensitivity methods for measuring the wavefront slope and shape and for feedback to the shape of bendable mirrors. These techniques are relatively simple to implement and easy to align and use, and the data can be recorded in a single exposure and interpreted quickly.…”

“…are patterned mirror elements that typically modify the phase (i.e., path length) of the incoming wave, Hartmann requires high-contrast amplitude modulation. Phase gratings are used for shearing at X-ray wavelengths [ 8 , 16 ], but optimization is challenging to achieve in reflection, over a large photon energy range, at fixed incidence angles. We developed binary, amplitude-modulating reflection gratings to serve both shearing and Hartmann testing with a compact optical element.…”

Focus characterization of an X-ray free-electron laser by intensity correlation measurement of X-ray fluorescence

“…Furthermore, the shot-by-shot positional and pointing fluctuations of the focused XFEL beam, which result from the stochastic nature of the XFEL amplification processes, introduce difficulties in precise focus characterization. As an alternative, wavefront sensing has been widely used for focus characterization (Seaberg et al, 2019). However, the information obtained from the wavefront measurement is not always appropriate for focus characterization because the wavefront measurement tends to include systematic errors (Inoue, Matsuyama et al, 2018), which often lead to an inaccurate assessment of both optic misalignments and beam size estimates.…”