Damage thresholds for blaze diffraction gratings and grazing incidence optics at an X-ray free-electron laser

Abstract

The Linac Coherent Light Source is upgrading its machine to high repetition rate and to extended ranges. Novel coatings, with limited surface oxidation, which are able to work at the carbon edge, are required. In addition, highresolution soft X-ray monochromators become necessary. One of the big challenges is to design the mirror geometry and the grating profile to have high reflectivity (or efficiency) and at the same time survive the high peak energy of the free-electron laser pulses. For these reasons the experimental damage threshold, at 900 eV, of two platinum-coated gratings with different blazed angles has been investigated. The gratings were tested at 1 grazing incidence. To validate a model for which the damage threshold on the blaze grating can be estimated by calculating the damage threshold of a mirror with an angle of incidence identical to the angle of incidence on the grating plus the blaze angle, tests on Pt-coated substrates have also been performed. The results confirmed the prediction. Uncoated silicon, platinum and SiB 3 (both deposited on a silicon substrate) were also investigated. In general, the measured damage threshold at grazing incidence is higher than that calculated under the assumption that there is no energy transport from the volume where the photons are absorbed. However, it was found that, for the case of the SiB 3 coating, the grazing incidence condition did not increase the damage threshold, indicating that the energy transport away from the extinction volume is negligible.

Change of the relative line strengths due to the resonance induced population transfer between Fe XVII and FeXVI ions

“…The photon intensity I p used in the simulations can be obtained from pulse energy P E , pulse duration τ and the effective focal area with f d the focal diameter of the photon beam by . Unfortunately, these three parameters were not well defined in the experiment 5 , we can only infer the ranges of them from the LCLS specifications 11,16–18,33,34 . The raw X-ray pulse energy before the monochromator is about 1–4 mJ 11,16 .…”

“…For example, based on our simulations, the measured ratio mainly depends on the ratio of the pulse energy and the effective focal area, i.e., P E / σ d , for which some diagnostics are being developed at the LCLS 40,41 . On the other hand, some attenuators 18 may be used to reduce the XFEL intensity where the resonance induced population transfer effect is negligible, or alternatively, one can extract the pure Fe XVII ion from the EBIT known as the Electron Beam Ion Sources (EBIS) 42 for the experiment.…”

“…Alternatively, it was proposed that other physical processes beyond those included in the atomic structure calculation of an isolated atomic system, such as the one due to the ultra-high intensities of XFEL with ultra-short pulse period, may be responsible for this discrepancy 10,14,15 . By scrutinizing the experimental conditions at the LCLS 5,11,16–18 , the intensities of the XFEL pulses are not sufficiently high to support this proposed interpretation 10,14,15 and it is not likely that the discrepancy between theory and experiment could be attributed to the high order nonlinear effect. Thus, it is worth exploring further other possibilities to explain the experiment 5 .…”

Soft x-ray seeding studies for the SLAC Linac Coherent Light Source II

“…A weak or nonuniform x-ray seed along the e-beam raises the relative SASE background. Further, limitations on the allowable 20 mJ=cm 2 fluence on the SXRSS monochromator optics directly limits the ratio of seed to SASE power in the second stage [30]. At LCLS, SASE usually accounts for ∼10% of the total output pulse energy in the linear gain regime.…”

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single-shot ablation threshold

“…Only recently the first papers on resistivity of metal films to multi-shot X-ray radiation were published [2,3]. Earlier, Hau-Riege et.al.…”