Linac Coherent Light Source (LCLS) design study report

The field of x-ray optics struggles to develop optical systems with the versatility and sophistication of their visible light counterparts. The advent of fourth-generation light sources will make the struggle even more difficult. Fourth-generation light sources include laser/plasma sources, x-ray Free Electron Lasers (FEL), inverse Compton scattering sources, and the National Ignition Facility. LCLS, (Linac Coherent Light Source), and its European cousin, will be the first of the x-ray FELs. The LCLS, to be built at the Stanford Linear Accelerator Center (SLAC), takes advantage of the existing SLAC linear accelerator to send intense, low emittance electron bunches through a 100 m long undulator structure. Through a process called SASE (Self Amplification of Spontaneous Emission) the electrons interact with the radiation fields they produce while in the undulator causing them to collect into micro bunches that emit coherent light. In the case of the LCLS the coherent radiation will have a wavelength in the x-ray regime, and will be tunable from 1.5 to 15 {angstrom}. The LCLS will deliver x-rays in individual coherent packages lasting < 300 fs, making the LCLS a very important source for studying short time phenomenon and for performing high-resolution x-ray structural analysis of molecular sized systems. Moreover, each coherent packet packs 10{sup 12} photons in a very small volume of space- time; the interaction of this packet with matter, and the subsequent aftermath offer new opportunities in physics. To make full use of the LCLS, experimenters must be able to manipulate and focus the LCLS beam, and therefore must contend with the fact that the unprecedented energy density per unit area in the FEL beam is enough to melt most materials in a single pulse. Take the example of the Warm Dense Matter experiment, in which LLNL researchers hope to use a focused LCLS beam to instantly (that is fast enough that the density doesn't change) bring solid pieces of matter to a plasma state. The experiment operates with an 8 keV photon beam, and although the focusing requirements are modest, present-day x-ray optics that can meet these requirements will melt when exposed to the beam from the LCLS according to figure 2. Figure 2 suggests that lower Z materials like beryllium, lithium, and carbon would fare better at the LCLS and that optics fabricated from low Z materials might offer a solution to the problem of survivable optics for the Warm Dense Matter experiment. Now both reflective and refractive x-ray optics are in use at these wavelengths, with reflective optics dominating because standard techniques have been developed for their manufacture. Common reflective optics make use of high Z materials which give higher performance. The less common refractive optics are also made of high Z materials but would work better if fabricated from low Z materials, which have less absorption. Based on these arguments it seems that low Z refractive optics would be the optics of choice for the Warm Dense Matter experiment. The main objective of this project was to demonstrate that refractive optics for the Warm Dense Matter experiment could be fabricated from low Z materials, which would survive in the LCLS beam. We also wanted to gain familiarity with the LCLS so that LLNL experimenters would be better prepared to take advantage is of its capabilities. And finally, we wanted to understand how these technologies could be applied to programs within LLNL.

The X-ray free-electron laser (XFEL) based on self amplified spontaneous emission (SASE) is an unprecedented coherent hard X-ray source [1,2]. However, due to its huge size and construction cost, it would not be easy to have this magnificent scientific facility in your neighborhood. Therefore, there have been studies on why it must be so big (why its electron beam energy must be so high and why its undulator is so long) and how its size can be reduced (how its electron energy can be lowered) without degrading radiation quality [3, 4]. Size reduction of a XFEL machine is basically reduced to how a lower energy electron beam can generate a XFEL without degrading the radiation quality. The reason a high energy electron beam is needed is that a very small electron beam emittance is needed for lasing as the relation = n/γ indicates. Here, is the electron beam emittance, n is the gun emittance, and γ is the relativistic factor. Hence, if n is low enough, a lower electron energy can make the same . A compact XFEL without its quality being degraded obviously means a XFEL with high FEL efficiency. A FEL’s efficiency can be roughly estimated by using the FEL parameter ρ. We can make ρ high enough, even at a low electron beam energy, by using a low emittance gun as explained above. Therefore, the issue is reduced to developing an electron gun with improved gun emittance. The emittance performance of the photo-cathode gun was widely known to be around 1−1.2 μrad, and the design of Linac Coherent Light Source (LCLS) was based on this number [5]. However, in the recent LCLS com-

FEL simulations for the LCLS

The fourth international user workshop focusing on high-power lasers at the Linac Coherent Light Source (LCLS) was held in Menlo Park, CA, USA, on October 3–4, 2016 [1–3]. The workshop was co-organized by Los Alamos National Laboratory and SLAC National Accelerator Laboratory (SLAC), and garnered the attendance of more than 110 scientists. Participants discussed the warm dense matter and high-pressure science that is being conducted using high-power lasers at the LCLS Matter in Extreme Conditions (MEC) endstation. During the past year, there have been seven journal articles published from research at the MEC instrument [4–10]. The specific topics discussed at this workshop were experimental highlights from the past year, current status and future commissioning of MEC capabilities, and future facility upgrades that will enable the expanded science reach of the facility.

Start-End Simulations for the LCLS X-Ray Free-Electron Laser

The advent of ultrafast X-ray free-electron lasers (XFELs) opens the tantalizing possibility of the atomic-resolution imaging of reproducible objects such as viruses, nanoparticles, single molecules, clusters, and perhaps biological cells, achieving a resolution for single particle imaging better than a few tens of nanometers. Improving upon this is a significant challenge which has been the focus of a global single particle imaging (SPI) initiative launched in December 2014 at the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, USA. A roadmap was outlined, and significant multi-disciplinary effort has since been devoted to work on the technical challenges of SPI such as radiation damage, beam characterization, beamline instrumentation and optics, sample preparation and delivery and algorithm development at multiple institutions involved in the SPI initiative. Currently, the SPI initiative has achieved 3D imaging of rice dwarf virus (RDV) and coliphage PR772 viruses at ~10 nm resolution by using soft X-ray FEL pulses at the Atomic Molecular and Optical (AMO) instrument of LCLS. Meanwhile, diffraction patterns with signal above noise up to the corner of the detector with a resolution of ~6 Ångström (Å) were also recorded with hard X-rays at the Coherent X-ray Imaging (CXI) instrument, also at LCLS. Achieving atomic resolution is truly a grand challenge and there is still a long way to go in light of recent developments in electron microscopy. However, the potential for studying dynamics at physiological conditions and capturing ultrafast biological, chemical and physical processes represents a tremendous potential application, attracting continued interest in pursuing further method development. In this paper, we give a brief introduction of SPI developments and look ahead to further method development.

Short wavelength FELs using the SLAC linac

Recent numerical studies of Free Electron Lasers (FELs) operating in the Self Amplified Spontaneous Emission (SASE) regime indicate a large sensitivity of the gain to the degree of transverse overlap (and associated phase coherence) between the electron and photon beams traveling down the insertion device. Simulations of actual systems imply that accurate detection and correction for this relative loss of overlap, rather than correction for the absolute departure of the electron beam from a fixed axis, is the preferred function of an FEL amplifier's Beam Position Monitor (BPM) and corrector systems. In this note we propose a novel diffractive BPM with the capability of simultaneously detecting and resolving the absolute (and relative) transverse positions and profiles of electron and X-ray beams co-propagating through an undulator. We derive the equations governing the performance of the BPM and examine its predicted performance for the SLAC Linac Coherent Light Source (LCLS), viz., for profiling multi-GeV electron bunches co-propagating with one-to-several-hundred keV X-ray beams. Selected research and development (R&D) tasks for fabricating and testing the proposed BPM are discussed.

Beam position monitors (BPM) are critical for beam-based alignment and operation of the world’s first X-ray free electron laser, Linac Coherent Light Source (LCLS), being built at Stanford Linear Accelerator Center. Fabrication of the BPMs requires welding components made of copper alloy that is extremely difficult to weld due to its low absorptivity to laser light and a high thermal conductivity. This paper presents a laser welding technique for jointing the BPM components made of copper alloy. In this technique, a surface coating and a set of pulsed laser process parameters were developed to enhance the absorption to the laser beam and to produce sound cylindrical butt welds. Results from a welding monitor used to real-time monitor the process for weld quality control are also reported.

We present a method for generating femtosecond duration X-ray pulses using a single-pass free-electron laser (FEL). This method uses an energy-chirped electron beam to produce a frequency-chirped X-ray pulse through self-amplified spontaneous emission (SASE). After the undulator we consider passing the radiation through a monochromator. The frequency is correlated to the longitudinal position within the pulse, and therefore, by selecting a narrow bandwidth, a short temporal pulse will be transmitted. The short pulse radiation is used to seed a second undulator, where the radiation is amplified to saturation. In addition to short pulse generation, this scheme has the ability to control shot-to-shot fluctuations in the central wavelength due to electron beam energy jitter. We present calculations of the radiation characteristics produced by a chirped-beam two-stage SASE-FEL, and consider the performance of the chirped-beam two-stage option for the Linac Coherent Light Source (LCLS).

We present a method for generating femtosecond duration x-ray pulses using a single-pass free-electron laser (FEL). This method uses an energy-chirped electron beam to produce a frequency-chirped x-ray pulse through self-amplified spontaneous emission (SASE). After the undulator we consider passing the radiation through a monochromator. The frequency is correlated to the longitudinal position within the pulse, and therefore, by selecting a narrow bandwidth, a short temporal pulse will be transmitted. The short pulse radiation is used to seed a second undulator, where the radiation is amplified to saturation. In addition to short pulse generation, this scheme has the ability to control shot-to-shot fluctuations in the central wavelength due to electron beam energy jitter. We present calculations of the radiation characteristics produced by a chirped-beam two-stage SASE-FEL, and consider the performance of the chirped-beam two-stage option for the Linac Coherent Light Source (LCLS).

News

For a lab that made its name in particle physics, shifting into new directions was never going to be easy. “Changes are hard and people do not like them,” admits Persis Drell, director of the SLAC National Accelerator Laboratory in California. “But we are now opening up new frontiers in another direction and that is very exciting.” That focus, which lies in photon science, reaches new fruition this month with the opening of the world's first X-ray free-electron laser (XFEL) – the Linac Coherent Light Source (LCLS).

ExaFEL is an HPC-capable X-ray Free Electron Laser (XFEL) data analysis software suite for both Serial Femtosecond Crystallography (SFX) and Single Particle Imaging (SPI) developed in collaboration with the Linac Coherent Lightsource (LCLS), Lawrence Berkeley National Laboratory (LBNL) and Los Alamos National Laboratory. ExaFEL supports real-time data analysis via a cross-facility workflow spanning LCLS and HPC centers such as NERSC and OLCF. Our work therefore constitutes initial path-finding for the US Department of Energy’s (DOE) Integrated Research Infrastructure (IRI) program. We present the ExaFEL team’s 7 years of experience in developing real-time XFEL data analysis software for the DOE’s exascale supercomputers. We present our experiences and lessons learned with the Perlmutter and Frontier supercomputers. Furthermore we outline essential data center services (and the implications for institutional policy) required for real-time data analysis. Finally we summarize our software and performance engineering approaches and our experiences with NERSC’s Perlmutter and OLCF’s Frontier systems. This work is intended to be a practical blueprint for similar efforts in integrating exascale compute resources into other cross-facility workflows.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Heinz-Dieter Nuhn; LCLS: Status and simulation results. AIP Conf. Proc. 1 June 1997; 413 (1): 103–119. https://doi.org/10.1063/1.54422 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search