The fluctuation-dissipation measurement instrument at the Linac Coherent Light Source.

The development of new modes at x-ray free electron lasers has inspired novel methods for studying fluctuations at different energies and timescales. For closely spaced x-ray pulses that can be varied on ultrafast time scales, we have constructed a pair of advanced instruments to conduct studies targeting quantum materials. We first describe a prototype instrument built to test the proof-of-principle of resonant magnetic scattering using ultrafast pulse pairs. This is followed by a description of a new endstation, the so-called fluctuation-dissipation measurement instrument, which was used to carry out studies with a fast area detector. In addition, we describe various types of diagnostics for single-shot contrast measurements, which can be used to normalize data on a pulse-by-pulse basis and calibrate pulse amplitude ratios, both of which are important for the study of fluctuations in materials. Furthermore, we present some new results using the instrument that demonstrates access to higher momentum resolution.

Direct experimental observation of the gas density depression effect using a two-bunch X-ray FEL beam.

The experimental observation of the depression effect in gas devices designed for X-ray free-electron lasers (FELs) is reported. The measurements were carried out at the Linac Coherent Light Source using a two-bunch FEL beam at 6.5 keV with 122.5 ns separation passing through an argon gas cell. The relative intensities of the two pulses of the two-bunch beam were measured, after and before the gas cell, from X-ray scattering off thin targets by using fast diodes with sufficient temporal resolution. At a cell pressure of 140 hPa, it was found that the after-to-before ratio of the intensities of the second pulse was about 17% ±â€…6% higher than that of the first pulse, revealing lower effective attenuation of the gas cell due to heating by the first pulse and subsequent gas density reduction in the beam path. This measurement is important in guiding the design and/or mitigating the adverse effects in gas devices for high-repetition-rate FELs such as the LCLS-II and the European XFEL or other future high-repetition-rate upgrades to existing FEL facilities.

Nanosecond X-Ray Photon Correlation Spectroscopy on Magnetic Skyrmions.

We report an x-ray photon correlation spectroscopy method that exploits the recent development of the two-pulse mode at the Linac Coherent Light Source. By using coherent resonant x-ray magnetic scattering, we studied spontaneous fluctuations on nanosecond time scales in thin films of multilayered Fe/Gd that exhibit ordered stripe and Skyrmion lattice phases. The correlation time of the fluctuations was found to differ between the Skyrmion phase and near the stripe-Skyrmion boundary. This technique will enable a significant new area of research on the study of equilibrium fluctuations in condensed matter.

High-intensity double-pulse X-ray free-electron laser.

The X-ray free-electron laser has opened a new era for photon science, improving the X-ray brightness by ten orders of magnitude over previously available sources. Similar to an optical laser, the spectral and temporal structure of the radiation pulses can be tailored to the specific needs of many experiments by accurately manipulating the lasing medium, that is, the electron beam. Here we report the generation of mJ-level two-colour hard X-ray pulses of few femtoseconds duration with an XFEL driven by twin electron bunches at the Linac Coherent Light Source. This performance represents an improvement of over an order of magnitude in peak power over state-of-the-art two-colour XFELs. The unprecedented intensity and temporal coherence of this new two-colour X-ray free-electron laser enable an entirely new set of scientific applications, ranging from X-ray pump/X-ray probe experiments to the imaging of complex biological samples with multiple wavelength anomalous dispersion.

Experimental demonstration of a soft x-ray self-seeded free-electron laser.

The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000-5000, wavelength stability of 10(-4), and an increase in peak brightness by a factor of 2-5 across the photon energy range of 500-1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.

Few-femtosecond time-resolved measurements of X-ray free-electron lasers.

X-ray free-electron lasers, with pulse durations ranging from a few to several hundred femtoseconds, are uniquely suited for studying atomic, molecular, chemical and biological systems. Characterizing the temporal profiles of these femtosecond X-ray pulses that vary from shot to shot is not only challenging but also important for data interpretation. Here we report the time-resolved measurements of X-ray free-electron lasers by using an X-band radiofrequency transverse deflector at the Linac Coherent Light Source. We demonstrate this method to be a simple, non-invasive technique with a large dynamic range for single-shot electron and X-ray temporal characterization. A resolution of less than 1 fs root mean square has been achieved for soft X-ray pulses. The lasing evolution along the undulator has been studied with the electron trapping being observed as the X-ray peak power approaches 100 GW.

Demonstration of single-crystal self-seeded two-color x-ray free-electron lasers.

A scheme for generating two simultaneous hard-x-ray free-electron laser pulses with a controllable difference in photon energy is described and then demonstrated using the self-seeding setup at the Linac Coherent Light Source (LCLS). The scheme takes advantage of the existing LCLS equipment, which allows two independent rotations of the self-seeding diamond crystal. The two degrees of freedom are used to select two nearby crystal reflections, causing two wavelengths to be present in the forward transmitted seeding x-ray pulse. The free-electron laser system must support amplification at both desired wavelengths.

Femtosecond x-ray pulse characterization in free-electron lasers using a cross-correlation technique.

We report the first measurements of x-ray single-pulse duration and two-pulse separation at the Linac Coherent Light Source using a cross-correlation technique involving x rays and electrons. An emittance-spoiling foil is adopted as a very simple and effective method to control the output x-ray pulse. A minimum pulse duration of about 3 fs full width at half maximum has been measured together with a controllable pulse separation (delay) between two pulses. This technique provides critical temporal diagnostics for x-ray experiments such as x-ray pump-probe studies.

Location and chemical composition of microbially induced phosphorus precipitates in anaerobic and aerobic granular sludge.

This work focuses on combined scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDX) applied to granular sludge used for biological treatment of high-strength wastewater effluents. Mineral precipitation is shown to occur in the core of microbial granules under different operating conditions. Three dairy wastewater effluents, from three different upflow anaerobic sludge blanket (UASB) reactors and two aerobic granular sequenced batch reactors (GSBR) were evaluated. The relationship between the solid phase precipitation and the chemical composition of the wastewater was investigated with PHREEQC software (calculation of saturation indexes). Results showed that pH, Ca:P ratios and biological reactions played a major role in controlling the biomineralization phenomena. Thermodynamics calculations can be used to foresee the nature of bio-precipitates, but the location of the mineral concretions will need further investigation as it is certainly due to local microbial activity.

Thermal activation of mass transport and charge transfer at Pt in the I(3)(-)/I(-) electrolyte of a dye-sensitized solar cell.

The electrochemical properties of the I(3)(-)/I(-) reaction mediator as a function of temperature in the range from 30 degrees C to 80 degrees C were investigated by means of symmetric Pt electrodes thin-layer cells (TLC), using three electro-analytical techniques: Electrochemical Impedance Spectroscopy (EIS), Slow Scan Cyclic Voltammetry (SSCV) and Chronoamperometry (CA). Our study pointed out that raising the cell temperature has a beneficial effect both on charge transfer and on mass transport, with an activation energy for the electron transfer process at equilibrium of 24 kJ mol(-1), and of 12 kJ mol(-1) for the mass transfer process at equilibrium. Viscosity and conductivity measurements have demonstrated that most of the ionic mass transport in the solvent (methoxypropionitrile) follows the Stokes' law and that the Walden product is constant, in the temperature range investigated. The diffusion of I(3)(-), however, was found to be partly "non-Stokesian" at lower temperature where the viscosity of the electrolyte is higher. We have shown that EIS and chronoamperometry are both valid methods to derive diffusion coefficients of redox ions in TLC, even if their exact concentration in the electrolyte is not known.

Measurements and simulations of ultralow emittance and ultrashort electron beams in the linac coherent light source.

The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser project presently in a commissioning phase at the SLAC National Accelerator Laboratory. We report here on very low-emittance measurements made at low bunch charge, and a few femtosecond bunch length produced by the LCLS bunch compressors. Start-to-end simulations associated with these beam parameters show the possibilities of generating hundreds of GW at 1.5 A x-ray wavelength and nearly a single longitudinally coherent spike at 1.5 nm with 2-fs duration.

Halo formation and emittance growth of positron beams in plasmas.

An ultrarelativistic 28.5 GeV, 700-microm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n(e) between approximately equal to 10(13) and approximately equal to 5 x 10(14) cm(-3). Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of approximately equal to 3 in the high emittance plane of the beam approximately equal to 1 m downstream from the plasma exit. As n(e) increases, the formation of a beam halo containing approximately 40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of approximately 3 and emittance ratio of approximately 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.

Observation of polarized positrons from an undulator-based source.

An experiment (E166) at the Stanford Linear Accelerator Center has demonstrated a scheme in which a multi-GeV electron beam passed through a helical undulator to generate multi-MeV, circularly polarized photons which were then converted in a thin target to produce positrons (and electrons) with longitudinal polarization above 80% at 6 MeV. The results are in agreement with GEANT4 simulations that include the dominant polarization-dependent interactions of electrons, positrons, and photons in matter.

Ionization-induced electron trapping in ultrarelativistic plasma wakes.

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.

Hosing instability in the blow-out regime for plasma-wakefield acceleration.

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)10.1103/PhysRevLett.96.165002]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)10.1103/PhysRevLett.67.991] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al.

Positron production by x rays emitted by betatron motion in a plasma wiggler.

Positrons in the energy range of 3-30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1-50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.

Measurement and DFT calculation of Fe(cp)(2) redox potential in molecular monolayers covalently bound to H-Si(100).

The electron transfer to self-assembled molecular monolayers carrying a ferrocene (Fc) center, grafted on a flat Si(100) surface, is a recent subject of experimental investigation. We report here the density functional theory (DFT) ab initio calculation of Fc-silicon hybrid redox potentials. The systems were modeled with a slab of H-terminated Si(100) 1 x 1 and 2 x 1 surfaces: geometries were optimized using the ONIOM method, and solute-solvent interactions were included through the polarizable continuum model (PCM) method. Two new routes for Si functionalization with ethyl- (EtFC) and ethynyl-Fc (EFC) differing only in the unsaturation degree of the anchoring arm have been successfully explored, and the redox potential of the resulting hybrids has been measured by cyclic voltammetry: 0.675 and 0.851 V versus NHE for the EtFC and EFC derivatives, respectively. These values, along with the previously measured potential (0.700 V) for the mono-unsaturated derivative, vinyl-Fc, allow the relation between the unsaturation degree and the adduct redox potential to be studied. The comparison among the measured and computed potentials allows one to discriminate between different adduct isomers for the saturated species and more importantly provides strong indications that the carbon-carbon unsaturation initially present in the molecular arm used for anchoring to the surface is preserved upon addition, in contrast with the commonly accepted reaction mechanism.

Hose instability and wake generation by an intense electron beam in a self-ionized gas.

The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam's space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.

Multi-GeV energy gain in a plasma-wakefield accelerator.

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8 x 10(10) electrons is compressed to 20 microm longitudinally and focused to a transverse spot size of 10 microm at the entrance of a 10 cm long column of lithium vapor with density 2.8 x 10(17) atoms/cm3. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

Observation of parity nonconservation in møller scattering.

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A(PV)=[-175+/-30(stat)+/-20(syst)] x 10(-9). This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron's weak charge at low energy Q(e)(W)=-0.053+/-0.011. This is consistent with the standard model expectation at the current level of precision: sin((2)theta(W)(M(Z))((-)MS)=0.2293+/-0.0024(stat)+/-0.0016(syst)+/-0.0006(theory).