LCLS: the First Experiments

LCLS THE FIRST EXPERIMENTS September 2000 ii Table of Contents First Scientific Experiments for the LCLS .....................................................v Atomic Physics Experiments ..........................................................................1 Plasma and Warm Dense Matter Studies......................................................13 Structural Studies on Single Particles and Biomolecules .............................35 Femtochemistry.............................................................................................63 Studies of Nanoscale Dynamics in Condensed Matter Physics....................85 X-ray Laser Physics ....................................................................................101 Appendix 1: Committee Members..............................................................113 iii iv First Scientific Experiments for the LCLS The Scientific Advisory Committee (SAC) for the Linac Coherent Light Source (LCLS) has selected six scientific experiments for the early phase of the project. The LCLS, with proposed construction in the 2003-2006 time frame, has been designed to utilize the last third of the existing Stanford Linear Accelerator Center (SLAC) linac. The linac produces a high-current 5- 15 GeV electron beam that is bunched into 230 fs slices with a 120 Hz repetition rate. When traveling through a sufficiently long (of order of 100 m) undulator, the electron bunches will lead to self amplification of the emitted x-ray intensity constituting an x-ray free electron laser (XFEL). If funded as proposed, the LCLS will be the first XFEL in the world, operating in the 800-8,000 eV energy range. The emitted coherent x-rays will have unprecedented brightness with 1012-1013 photons/pulse in a 0.2-0.4% energy bandpass and an unprecedented time structure with a design pulse length of 230 fs. Studies are under way to reduce the pulse length to tens of femtoseconds. This document presents descriptions of the early scientific experiments selected by SAC in the spring of 2000. They cover a wide range of scientific fields. The experimental teams consist of many internationally recognized scientists who are excited about the unprecedented x-ray capabilities of LCLS that surely will lead to new scientific frontiers. More generally, this document serves to forward the scientific case for an accelerator-based XFEL source, as requested by the BESAC subpanel on Novel Coherent Light Sources, chaired by Stephen R. Leone. Two general classes of experiments are proposed for the LCLS. The first class consists of experiments where the x-ray beam is used to probe the sample without modifying it, as is done in most experiments at current synchrotron sources. In the second class, the LCLS beam is used to induce non-linear photo-processes or matter in extreme conditions. The same source can be used for both types of experiments by utilizing six-orders-of-magnitude changes in photon flux density by focusing the LCLS beam, and by exploiting the strong dependence of the photo- absorption cross section on photon energy and atomic number. The first five experiments, • Atomic Physics Experiments • Plasma and Warm Dense Matter Studies • Structural Studies on Single Particles and Biomolecules • Femtochemistry • Studies of Nanoscale Dynamics in Condensed Matter Physics are based on the design parameters of the LCLS (given in Table 1, next page) specified in the Linac Coherent Light Source (LCLS) Design Study Report: The LCSL Design Study Group, prepared for the U.S. Department of Energy by SLAC in 1998. Some of the experiments assume that the x-ray beam can be focused to 100 x 100 nm with 50% of full photon flux, which is today’s state of the art. v TABLE 1. Design parameters of the LCLS. Wavelength range 15 Å 1.5 Å Peak sat. power 11 GW 9 GW # coherent photons/pulse 2.2 x 1013 2.2 x 1012 Energy bandwidth 0.42% 0.21% Pulse width (FWHM) 230 fs 230 fs The order in which the experiments are described does not reflect their relative priority but rather indicates a thread of connectivity and dependency. All LCLS experiments involve the interaction of a high-power x-ray beam with atoms, and so the first experiment is aimed at understanding this process at a fundamental level. This basic knowledge is important for the second experiment, which uses the high power of the LCLS to create an interesting state of matter, so-called “warm dense matter,” and proposes to probe it by a second delayed LCLS pulse. The third experiment is tied to the first two in that it depends on the time scale over which a biological molecule disintegrates after it is hit by the LCLS beam. Radiation damage is one of the main obstacles today in determining the structure of proteins that cannot be crystallized. The experiment is based on the use of LCLS pulses that are fast enough to determine the structure by x-ray scattering before radiation damage sets in. The fourth experiment goes to the very heart of chemistry, aiming at obtaining molecular pictures, i.e., atomic positions, bond length and angles, during chemical reactions or transformations at femtosecond time scales, an area of research previously reserved for ultrafast lasers. Finally, experiment number five pushes the envelope in probing ordering phenomena in hard and soft condensed matter on the important nanometer length scale, which cannot be seen by optical photons, over a broad range of time scales. Nanoscale dynamics is not only scientifically interesting, but it constitutes the competitive arena of advanced technological devices. The above five experiments, even at this early proposal stage, already instill dreams of improved XFEL characteristics, for example, shorter pulse lengths. Just as the application of conventional lasers has been accompanied by R&D on lasers themselves, there needs to be an R&D program to explore new accelerator and optics concepts with LCLS. This is the goal of the sixth experiment, • X-ray Laser Physics. The history of and experience with three generations of synchrotron radiation sources has taught us that the above experiments are at best the tip of the iceberg of scientific opportunities. It is safe to predict that we have not yet thought of the most important experiments that eventually will be done with this new class of radiation sources—x-ray free electron lasers! - Gopal K. Shenoy and Joachim Stöhr, Co-Chairs, LCLS Scientific Advisory Committee vi Atomic Physics Experiments R. R. Freeman, University of California, Davis, CA P. H. Bucksbaum, University of Michigan, Ann Arbor, MI K. Kulander, Lawrence Livermore National Laboratory, Livermore, CA L. Young, Argonne National Laboratory, Argonne, IL R. Falcone, University of California, Berkeley, CA I. SUMMARY All Linac Coherent Light Source (LCLS) applications involve x-ray free electron laser (XFEL)- atom interactions, so understanding basic atomic physics is one of the most important early tasks. Warm dense matter starts with atomic absorption. Even the transmission, diffraction, and attenuation optics necessary to transport and condition the beam will involve basic interactions between atoms and the intense LCLS beam. The intensity and wavelength regime of LCLS place it apart from third-generation sources, and some of the x-ray science will be different. We are in a good position to estimate how basic effects like photoionization might change, but experiments are needed to confirm the calculations, to sort out the relative importance of competing processes, and to lay the foundation for any materials science experiments with the x-rays. We propose investigating this in controlled experiments, using both the focused and unfocused LCLS beam, at both design wavelengths (15 Å and 1.5 Å). In particular, we propose five initial experiments. The first one aims to directly observe multiple core hole formation in an atom. This is a novel phenomenon that could not be addressed in the past due to limitations in x-ray flux density. The second experiment extends nonlinear optics into the x-ray regime for the first time. It involves multiphoton ionization of a K-shell electron, which can be readily observed by characteristic x-ray emission with a photon energy that exceeds the LCLS pump energy. The third experiment exploits the unique features of LCLS to produce energetic clusters with high charge states that will undergo giant coulomb explosion. This experiment is of key importance in understanding energy transfer and radiation damage of the LCLS beam in clusters, molecules and particles (see proposal on biomolecules). Advanced research topics include studies of radiation and possibly lasing from XFEL-excited matter and the formation of highly excited laser plasmas (see proposal on plasmas and warm dense matter). II. BACKGROUND AND MOTIVATIONS A. Proposed experimental investigations 1. Direct observation of multiple core hole formation in an atom by the LCLS beam This is a novel phenomenon, since conditions necessary to produce multiple core holes by direct multiple photon excitation have not existed prior to the LCLS. The experimental signature for this is production of “hypersatellite” Auger electrons with energies significantly higher than those associated with single hole states. 1 2. Direct observation of multiphoton ionization of a K-shell electron This would extend nonlinear optics into the x-ray regime for the first time. Here the process is identified through characteristic line radiation, where the photon energy exceeds the LCLS energy. 3. Observation of giant Coulomb explosions in atomic clusters (GCEC)

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