ERL X-ray Science Workshop 2 Abstracts

ERL Overview and Charge to Workshop

Don Bilderback

Cornell High Energy Synchrotron Source, Cornell University

The principle of operation of an Energy Recover Linac (ERL) is explained. The status and goals of the Cornell ERL project are summarized. Relevant characteristics of a 5 GeV ERL of the type we hope to build are described and some example experiments are given.

Femtosecond Timing Distribution and Synchronization for XFEL's

Franz X. Käertner

Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology

For future advances in accelerator physics in general and seeded free electron lasers in particular, precise synchronization between low-level RF-system, photo-injector laser, seed radiation as well as potential probe lasers at the FEL output is required. We propose a modular system based on optical pulse trains from mode-locked lasers for timing distribution and timing information transfer in the optical domain to avoid detrimental effects due to amplitude to phase conversion in photo detectors. Synchronization of various RF- and optical sub-systems with femtosecond precision over distances of several hundred meters can be achieved. First experimental results demonstrating sub-50fs timing distribution in an accelerator environment will be presented.

Ultrafast X-Ray Absorption Spectroscopy at an ERL X-Ray Source

Christian Bressler

Laboratoire de Spectroscopie Ultrarapide, Ecole Polytechnique Fédérale de Lausanne, Switzerland

Electronic structure changes are at the origin of chemical reactivity, which drive the forming and breaking of bonds. The latter can be visualized by measuring the geometric structure during the course of a chemical reaction.

Time-resolved x-ray absorption spectroscopy delivers both the electronic (via XANES) and geometric (via EXAFS) transient structure changes, when interfaced with a femtosecond laser in a pump-probe scheme. In addition, XAFS methods are extremely flexible, since they are element-selective and can be applied to disordered bulk systems. We have recently recorded high-quality transient XAFS spectra at the Swiss Light Source with currently <100 ps temporal resolution, corresponding to the electron bunch width of the synchrotron. Examples of excited state structures of solvated coordination chemistry compounds (Fig. 1) and of short-lived atomic radicals in aqueous solution will be presented. The extension of these studies using a high repetition rate, ultra-short pulse ERL x-ray source will be discussed.

Time-resolved X-Ray Scattering of Proteins in Solution: the Pros and Cons of an ERL

Philip Anfinrud

Laboratory of Chemical Physics, NIH/NIDDK

The use of synchrotron radiation to solve protein structures has been enormously successful. However, the high-salt, crystalline environment in which structures are determined is quite unnatural. For example, crystal packing forces can restrain the conformational flexibility of the protein. Indeed, it is this flexibility that enables proteins to perform their designed tasks with such exquisite efficiency and selectivity. To deliver oxygen efficiently from the lungs to the tissues, tetrameric hemoglobin undergoes a cooperative R-state (high-affinity) to T-state (low-affinity) quaternary structure transition while passing through oxygen-poor capillaries. This structural transition is not accommodated by the crystal, and when executed, the crystal cracks. Consequently, structural studies of hemoglobin have thus far have focused only on the end points of the quaternary structure transition. The next frontier in structural biology is not amassing more static structures of proteins, but will be in developing an understanding of how they function in mechanistic detail, i.e., the time-ordered sequence of structural events that connect the initial and final states. Clearly, such studies require time-resolved methods. We recently developed the technique of 150-ps time-resolved Laue crystallography and used this method to study structural changes in proteins on the picosecond to millisecond time scale. To study processes that cannot occur in a crystal, we are currently developing the method of time-resolved X-ray scattering. We take advantage of the fact that the radial intensity distribution of the scattered X-ray photons is related to the size, shape, and structure of the protein. Because this technique provides one-dimensional data, assigning time-resolved scattering patterns to specific structural transitions requires much help from theory. One might suspect that this method will be sensitive only to large amplitude changes; however, our preliminary time-resolved studies have shown that this method is sensitive to the subtle tertiary conformational changes that occur when photolyzed carbon monoxy myoglobin transitions from its carboxy to its deoxy state. Most of these changes are too fast to be resolved with the 150-ps time resolution available on current 3rd generation synchrotrons. The parameters proposed for an ERL would allow this technique to probe this structural transition on the time scale in which it occurs, and would even access the chemical time scale of femtoseconds. The parameters required to pursue time-resolved X-ray scattering measurements on an ERL will be discussed.

Mechanisms for Ultrafast Generation of Coherent Phonons, Polaritons and Spin Excitations

Roberto Merlin

FOCUS Center and Department of Physics, University of Michigan

Recent work on the generation of coherent low-lying excitations by ultrafast laser pulses will be reviewed, emphasizing the microscopic mechanisms of light-matter interaction. The topics covered include phonon polaritons [J. K. Wahlstrand and R. Merlin, Phys. Rev. B 68, 054301 (2003)], long-lived phonons in ZnO [C. Aku-Leh et al., Phys. Rev.B 71, 205211 (2005)], squeezed magnons [J. Zhao, A. V. Bragas, D. J. Lockwood and R. Merlin, Phys. Rev. Lett. 93, 107203 (2004)], and spin- and charge-density fluctuations in GaAs quantum wells [J. M. Bao et al., Phys. Rev. Lett. 92, 236601 (2004)]. In addition, unpublished results on surface-avoiding phonons in GaAs -AlAs superlattices [M. Trigo et al., unpublished] and magnons in ferromagnetic Ga1-xMnxAs [D. M. Wang et al., unpublished] will be discussed.

It is now widely accepted that stimulated Raman scattering (SRS) is the main mechanism responsible for the coherent coupling. Results will be presented showing that SRS is described by two separate tensors, one of which accounts for the excitation-induced modulation of the susceptibility, and the other one for the dependence of the amplitude of the oscillation on the light intensity [T. E. Stevens, J. Kuhl and R. Merlin, Phys. Rev. B 65, 144304 (2002)]. These tensors have the same real component, associated with impulsive coherent generation, but different imaginary parts. If the imaginary term dominates, that is, for strongly absorbing substances, the mechanism for two-band processes becomes displacive in nature, as in the DECP (displacive excitation of coherent phonons) model. It will be argued that DECP is not a separate mechanism, but a particular case of SRS.

In the final part of the talk, an attempt will be made to identify emerging areas of research on coherent excitations and coherent control, relevant to condensed matter systems, that could benefit from ultrafast x-ray diffraction studies.

Potential for Ultrafast X-Ray Intensity Fluctuation Measurements

Mark Sutton

McGill University

Many of the properties of a material depend more on its microstructure than its atomic structure. Intensity fluctuation spectroscopy (IFS) is an ideal way to study dynamics and fluctuations in this microstructure. In this talk I will review the current state of IFS measurements and discuss the potential to extend these into the picosecond time regime. I will also discuss the proposed speckle experiments for the x-ray free electron laser project at Stanford.

Plasma Wakefield Experiments at Cornell's Energy Recovery Linac

Mike Downer

Department of Physics, University of Texas at Austin

Tabletop plasma accelerators driven by intense ultrashort laser pulses have captured much attention recently by producing nearly mono-energetic electron beams approaching 1 GeV energy.1 Plasma accelerators driven by charged-particle bunches, however, are more likely to impact the future of high-energy physics because of their potential to double the energy of a parent accelerator by inserting a small, low cost "plasma afterburner" before the interaction point.2 A recent series of experiments at SLAC demonstrated the plasma afterburner principle using nC bunches ranging in duration from 20 ps to 100 GeV /m). However, the requirements of stable propagation and low emittance growth demand that these drive bunches simultaneously focus to a transverse dimension σr < 10 μm less than a plasma wavelength and maintain bunch density greater than the plasma density (nb >ne). The high transverse emittance of the SLAC beam limited access to this so-called "blowout" regime at high plasma densities. The comparatively low transverse emittance of Cornell's ERL for bunch durations down to 20-50 fs, on the other hand, is ideally suited to explore the "blowout" regime in dense plasmas. Because of the highly nonlinear beam-plasma interaction in this regime, the physics of plasma wave generation is non-trivial. To date particle-in-cell simulations have been the only available tool for its direct study. Measurements of the plasma accelerating structure with micron space- and fs-time resolution are needed. The high repetition rate (MHz vs. 10 Hz for the SLAC experiments) and long operating lifetime of Cornell's ERL provide unprecedented opportunities for exploring basic physics, as well as technological development, of dense plasma afterburner accelerators. A dedicated facility could spur the development of an important new capability for experimental high-energy physics.

As an initial basic physics study, I will propose to take holographic "snapshots" of wakefield accelerating structures generated in dense plasmas by the Cornell ERL beam. Because of their microscopic size and luminal velocity, critical features of these structures that determine energy, energy spread, collimation and charge of the plasma-accelerated electrons have eluded direct single-shot observation, inhibiting progress in producing high quality beams and in correlating beam properties with wake structure. Recently my group demonstrated single-shot visualization of laser-generated wakefield accelerator structures for the first time, using Frequency Domain Holography (FDH), a technique designed to image structures propagating near the speed of light.3 Our holographic "snapshots" captured evolution of multiple wake periods, detected structure variations as laser-plasma parameters changed, and resolved wavefront curvature, features never previously observed. FDH is equally applicable to beam-driven plasma wakefields, requiring only probe laser pulses synchronized with the ultrafast photocathode laser of Cornell's ERL. We can reconstruct wake morphology in real time, providing experimental feedback and optimization. I anticipate that FDH measurements will provide unprecedented insight into the physics of dense afterburner accelerators, and a first step toward developing a fully-optimized plasma afterburner accelerator.

References:

1. Faure et al., Nature 431, 541 and companion articles (2004)

2. Joshi, Scientific American 294, 40 and references therein (2006)

3. Matlis et al., submitted to Nature Physics (2006); Le Blanc et al., Optics Letters 25, 764 (2000)

Ultrafast Dynamics in Complex Materials

Antoinette J. Taylor

Center for Integrated Nanotechnologies, Los Alamos National Laboratory

In this talk, I will discuss the development and application of novel optical spectroscopic techniques to the study of ultrafast dynamics in complex materials. I will describe ultrafast optics experiments on (a) magnetic materials, (b) superconductors, and (c) heavy fermion materials. The experimental techniques are discussed followed by a brief review of ultrafast electron dynamics in conventional wide band metals that serves as a starting point in understanding dynamics in more complex systems [Journal of Physics: Condensed Matter 14, 1455 (2002)]. Multiple probe energies are employed to characterize the ultrafast dynamics from the far-infrared to visible to x-ray regimes, and these probes reveal complementary information of the material dynamics. In magnetoresistive oxides, the quasiparticle dynamics in the ferromagnetic metallic state can be understood in terms of a dynamic transfer of the spectral weight which is influenced by the lattice and spin degrees of freedom [Phys. Rev. Lett. 87, 017401 (2001), Phys. Rev Lett. 95 2674044 (2005)]. In other complex oxides, the insulator-to-metal phase transition is investigated [J. Phys. Soc. Jpn. 75 1008 (2006). The measurement of demagnetization in ferromagnetic materials using optically induced terahertz emission will also be described [Opt. Lett. 29, 1805 (2004)]. For high temperature superconductors, ultrafast quasiparticle dynamics are sensitive to the order parameter and superconducting pair recovery occurs on a picosecond timescale [Phys. Rev. Lett. 91, 267002 (2003)]. Heavy fermion compounds reveal an anomalous slowing down of quasiparticle dynamics below the Kondo temperature [Phys. Rev. Lett. 91, 27401 (2003), Phys. Rev. Lett. 96, 037401 (2006)]. These results show that, in general, ultrafast optical spectroscopy provides a sensitive method to probe the dynamics of quasiparticles at the Fermi level.

Explosions of Clusters Irradiated at Short Wavelength

T. Ditmire

The Texas Center for High Intensity Laser Science, Department of Physics, University of Texas at Austin

In recent years, there has been quite substantial progress in the understanding of explosions of atomic clusters subject to intense laser irradiation. It is now well understood that single species clusters of low Z materials (such as hydrogen or deuterium) expand by a Coulomb explosion if they are irradiated with enough intensity. In this case, if irradiated with a pulse of sufficiently fast rise time and sufficiently high intensity to eject all free electrons from the cluster, the ejected ion energies will be simply the potential energy of the ions after ionization at their equilibrium position in the cluster. Large clusters irradiated at modest intensity exhibit different behavior. Experiments indicate that in this case collective electron oscillation phenomena are very important in determining the dynamics of such clusters. It has been found that in an intensely irradiated cluster, optically and collisionally-ionized electrons undergo rapid collisional heating for the short time (<1 ps) before the cluster disassembles in the laser field. Pump-probe experiments indicate that the cluster microplasma exhibits a resonance in the heating by the laser pulse similar to the giant resonance seen in metallic clusters. Charge separation of the hot electrons leads to a very fast expansion of the cluster ions.

Quite recently, the nature of cluster interactions with intense, short wavelength light, ie vacuum ultraviolet and extreme ultraviolet, have come under study. Such studies have now become possible because of the development of a number of intense, ultrafast sources in the VUV range.

These studies are motivated in part by the promise of XFEL imaging of large protein molecules, an experiment which is similar in many respects to the interaction of an intense XFEL pulse with a large cluster. The expansion time and mechanism of these large structures under intense short wavelength illumination is of great importance to ascertaining the likely success of such imaging experiments. As such, a complete understanding of the explosion mechanisms of large clusters (ie. proteins) under intense short wavelength illumination is critical. Furthermore, such interactions are of fundamental interest as collective effects, not manifested in long wavelength laser interactions, may play a part in the dynamics. Interactions with wavelengths shorter than 10 nm will be quite different from near IR interactions (at 800 to 1000 nm) since, even at high intensity, the ponderomotive forces (which scale as Il2) of the XUV pulses are much smaller than in the IR pulses. As a result, much of the electron heating and ponderomotive ejection of electrons, which we know to occur at high intensity in IR pulses, will likely not occur in the short wavelength pulses. Furthermore, short wavelength pulses will come into resonance with the giant dipole resonance of a cluster plasma at much higher density than do IR pulses. These are much more collisional plasmas and the absorption of energy from the pulse will differ dramatically.

In my talk I will discuss various aspects of the physics of intense XUV and x-ray interactions with small clusters. I will discuss our plans for using femtosecond XUV light generated by high order harmonic generation to study these short wavelength-cluster interactions. I will also consider how the Cornell ERL source might be used for time resolved imaging of the explosions of the clusters during these intense short wavelength interactions.

X-Raying Laser-aligned Atoms and Molecules

Linda Young

Argonne National Laboratory

At the forefront of atomic, molecular and optical science have been recent developments of laser-based techniques to align and orient individual molecules in space. By allowing the study of "fixed-in-space" molecules and the control of the relative orientations of colliding molecules, the hope is that one day such techniques will significantly expand our understanding of fundamental physical and chemical processes. Furthermore, the ability to orient individual molecules is important for simplifying proposed single-shot imaging and structural determination of individual biomolecules. However, these laser alignment methods require relatively strong-fields (~10¹² W/cm² for small molecules) and the effect of these fields on the structure of the molecular framework is essentially unknown, either experimentally or theoretically. In this talk, I will summarize current experiments at the Argonne Advanced Photon Source that use strong-optical fields to create, and resonant polarized x-rays to probe, a macroscopic ensemble of orbitally-aligned krypton ions. By applying an external magnetic field, one can control the dynamics of the ion ensemble, i.e. suppress dealignment and induce coherent spin-precession. Analogous methods can be used to align molecules and detect the alignment. I will discuss the possibility of combining laser alignment techniques with x-ray probes from the proposed ERL to measure quantitatively the structure and dynamics of laser-aligned molecules.