Towards a dynamic compression facility at the ESRF

Abstract

Results of the 2018 commissioning and experimental campaigns of the new High Power Laser Facility on the Energy-dispersive X-ray Absorption Spectroscopy (ED-XAS) beamline ID24 at the ESRF are presented. The front-end of the future laser, delivering 15 J in 10 ns, was interfaced to the beamline. Laser-driven dynamic compression experiments were performed on iron oxides, iron alloys and bismuth probed by online time-resolved XAS.

Keywords: dynamic compression; laser shock; time-resolved XAS.

Figure 1

(a) Single-pulse XAS measurement of Fe (3.5 µm deposition on a diamond substrate) under ambient conditions in different configurations. Black, red and green lines are spectra acquired during the 4-bunch mode, while the blue spectrum was acquired in the 16-bunch mode. In the black spectrum, the spectrometer is aligned so that the Fe K-edge is in the middle of the energy range; in the red spectrum, the edge is moved towards low energies to increase the energy range; the green spectrum is the same as the red spectrum, but with the beam focused vertically using an additional mirror (VRM); and the blue spectrum is the same as the green spectrum but in the 16-bunch mode. (b) The corresponding direct beams (colours matching).

Figure 2

(a) Energy resolution as a function of energy for a Bragg polychromator using an Si(111) crystal focusing at a distance 1.1 m. The detector is placed at 2.3 m (blue curve) and 3 m (purple curve). (b) XANES spectra of Fe, where the blue and purple lines are spectra acquired on ID24 under the same conditions, and the green line is a reference spectrum from BM23.

Figure 3

Optical layout of the laser front-end; see text for details. PC is the Pockels cell, Amp are the rod amplifiers with their diameter for indication, SA are the apodizing apertures, L are the lenses, FI are the Faraday Isolators, Pol is the thin-film polarizer and QR is the quartz rotator.

Figure 4

Schematic view of the synchronization system. The synchrotron RF is distributed to all instruments through a set of two cascaded in-house electronics (BCDU8 and OPIOM). The frequency and corresponding period are shown for each clock, while only the repetition rate is shown for trigger events.

Figure 5

The configuration of the beams (not to scale). The beamline pink beam is diffracted and focused normal to the sample surface (pink then sketched as a few rainbow rays). The laser, in red, arrives at 30°. Downstream is the X-ray detector. A flip mirror can be inserted in the X-ray beam path for VISAR analysis (green). A direct line-of-sight camera can be used to record the laser focal spot in the absence of samples (dashed red). The vacuum vessel, its windows and the two in-vacuum microscopes for sample observation are not shown for clarity.

Figure 6

A photograph of the experimental hutch in November 2018. Beams are shown as solid arrows, whereas instruments are shown as dashed lines. X-rays are shown in pink, the drive laser in red and the shock diagnostics in green. For scale, the laser is 2 m long.

Figure 7

A photograph of the beam on burnpaper at the laser output (not focused and without a phase plate, left). Horizontal and vertical profiles recorded with a beamviewer (Gentec Beamage) and the corresponding super-Gaussian fits (right) with profile .

Figure 8

Laser temporal profile recorded with a fast photodiode (EOT ET-3500) connected to a fast oscilloscope (LeCroy WaveMaster 20 GHz). Orange dotted lines represent the rise time between 20 and 80% of the maximum, i.e. 300 ps.

Figure 9

Capture of the laser spot at the sample position using a hybrid phase plate designed to generate a 250 µm focal spot. The profiles shown at the top and to the right are 40 µm thick lineouts through the focal spot at Y = 0 and X = 0, respectively. Full width at half-maximum (FWHM) is close to 200 µm along both axes. Enclosed energy measurements on this figure show 50% of the energy inside a 180 µm diameter, 70% inside a 226 µm diameter and 80% inside a 290 µm diameter.

Figure 10

Fe, Fe alloys and Fe₂O₃ target designs, and the corresponding hydrodynamics simulations for the case of Fe₂O₃.

Figure 11

Normalized single-shot XAS spectra of (a) Fe–10 wt%Ni (4-bunch mode) and (b) Fe–3.5 wt%Si (7/8+1 bunch mode) recorded at various time delays between the optical laser and X-ray pulses. The spectra under ambient conditions, representing the bcc structure, are shown in black.

Figure 12

(a) X-ray absorption spectra of Fe₂O₃ at pressures of 5, 35 and 59 GPa and ambient temperatures, and at 81 GPa and 2100 K, using the LH-DAC apparatus at ESRF ID-24 (Boulard et al., 2019 ▸). (b) X-ray absorption spectra of Fe₂O₃ under shock at an estimated pressure and temperature of 120 GPa and 1500 K, and under adiabatic release at ambient pressure and 1000 K (SESAME EOS 7440). The black dotted spectrum is the ambient reference.

Figure 13

(a) Com­parison of the VISAR measurement and the ESTHER simulation of a target made of a window of 200 µm of sapphire and 50 µm of Al. (b) The pressure map of a Bi target (sapphire/Bi/polyimide) when the same simulation parameters are applied. The used EOS in the simulations are: SESAME 3720 for Al, SESAME 7411 for sapphire and BLF for Bi.

Figure 14

(a) Bi foil reference spectrum (black), Bi target spectrum obtained summing 10 X-ray pulses (blue) and shocked Bi target at 8 ns of laser delay (average of 10 X-ray pulses, red). (b) Observation of the Bi^I to Bi^V transition at 8 ns and (c) com­parison with literature static data. (d) Recovery of the Bi^I phase at 25 ns (blue spectrum, average of 5 X-ray pulses).