Soft x-ray mirrors for the Linac Coherent Light Source

Abstract

The Linac Coherent Light Source (LCLS) is a 0.15−1.5 nm wavelength free-electron laser (FEL) being constructed at the Stanford Linear Accelerator Center (SLAC) by a multi-institution consortium, including Lawrence Livermore National Laboratory (LLNL). One of LLNL's responsibilities involves the design and construction of two grazing-incidence mirror systems whose primary intent is to reduce radiation levels in the experimental halls by separating the FEL beam from unwanted high-energy photons.This paper discusses one of these systems, the Soft X-ray Offset Mirror System (SOMS) that will operate in the wavelength range 0.62−1.5 nm (0.827−2.00 keV). The unusual properties of the FEL beam translate to stringent specifications in terms of stability, material choice and mirror properties. It also precludes using approaches previously developed for synchrotron light sources. This situation has led us to a unique mirror design, consisting of a reflective boron carbide layer deposited on a silicon substrate. In the first part of this paper, we discuss the basic system requirements for the SOMS and motivate the need for these novel reflective elements. In the second part of this paper, we discuss the development work we have performed, including simulation and experimental verification of the boron carbide coating properties, and the expected performance of the final system.

Lifetime and damage threshold properties of reflective x-ray coatings for the LCLS free-electron laser

“…This effect can be understood if one considers that the 1µm-thick B 4 C and SiC magnetron sputtered films that were used in these studies have lower densities (90-95% of bulk) and amorphous morphology 7 compared to their bulk counterparts, which are crystalline. Another general trend that can be seen in Figure 4 is that the damage thresholds for both bulk and thin film B 4 C and SiC materials are close to the theoretically predicted damage thresholds, which justifies the principles used for LCLS optics design and materials selection 2,3,6,7 . An important element to consider is the mechanisms that lead to damage in the photon energies of the FLASH vs. the LCLS FEL facilities.…”

“…B 4 C and SiC were chosen as the reflective coating materials for the LCLS soft x-ray and hard x-ray optics respectively, due to their predicted high damage thresholds against the LCLS FEL beam compared to other coating materials, combined with the good reflective performance and absence of electronic absorption edges in the 0.827-2 keV (for B 4 C in the soft x-ray) and 2 -24 keV (for SiC in the hard x-ray) energy ranges of operation. Our group at Lawrence Livermore National Laboratory (LLNL) has led a team composed of national laboratories and industry in the design, specification, fabrication, coating and precision surface metrology of all the x-ray mirrors installed at LCLS, and these topics are discussed in detail in earlier references 2,3,4,5,6,7 . The B 4 C and SiC coatings of the LCLS x-ray mirrors were developed at * e-mail: regina.soufli@llnl.gov, phone: 925-422-6013 LLNL and were deposited by DC-magnetron sputtering 8 on precisely figured and polished Si substrates supplied by commercial vendors.…”

Current Status of Single Particle Imaging with X-ray Lasers

“…The Soft X-ray Offset Mirror System (SOMS) and Hard X-ray Offset Mirror System (HOMS) located in the Front End Enclosure of LCLS have harmonic cutoffs at~2.5 and~25 keV respectively. The SOMS operate in the FEL energy range from 0.20 keV to 2.0 keV and the reflectivity is above 90% [151]. The system reflectivity above 2.48 keV is below 20%, which contributes to suppressing the third harmonic FEL peak [152].…”

Development, experimental performance and damage properties of x-ray optics for the LCLS free-electron laser

“…The CXI beamline also uses two pairs of focusing mirrors and the MEC beamline uses one flat steering mirror. In collaboration with other national laboratories and commercial vendors, Lawrence Livermore National Laboratory (LLNL) led the design, specification, mounting, precision surface metrology and modeling of several of the x-ray mirrors installed at LCLS, including all the x-ray mirrors in the FEE 2,3,4,5 .…”