ACCELERATION OF ELECTRONS USING INVERSE ANOMALOUS SCATTERING

Acceleration of electrons in the field of two electromagnetic waves propagating in a slowing down medium in the direction of motion of electrons is considered under conditions when one of the waves propagates with a phase velocity greater than the velocity of electrons, and the other with a phase velocity less than the velocity of electrons. Taking into account deceleration by radiation, we determine dependence of the electron energy on the coordinate along the direction of acceleration. Thus, we obtain the expression for the maximum electron energy and effective acceleration length.

Adaptive Critical Balance and Firehose Instability in an Expanding, Turbulent, Collisionless Plasma

Using a hybrid-kinetic particle-in-cell simulation, we study the evolution of an expanding, collisionless, magnetized plasma in which strong Alfvénic turbulence is persistently driven. Temperature anisotropy generated adiabatically by the plasma expansion (and consequent decrease in the mean magnetic-field strength) gradually reduces the effective elasticity of the field lines, causing reductions in the linear frequency and residual energy of the Alfvénic fluctuations. In response, these fluctuations modify their interactions and spatial anisotropy to maintain a scale-by-scale “critical balance” between their characteristic linear and nonlinear frequencies. Eventually the plasma becomes unstable to kinetic firehose instabilities, which excite rapidly growing magnetic fluctuations at ion-Larmor scales. The consequent pitch-angle scattering of particles maintains the temperature anisotropy near marginal stability, even as the turbulent plasma continues to expand. The resulting evolution of parallel and perpendicular temperatures does not satisfy double-adiabatic conservation laws, but is described accurately by a simple model that includes anomalous scattering. Our results have implications for understanding the complex interplay between macro- and microscale physics in various hot, dilute, astrophysical plasmas, and offer predictions concerning power spectra, residual energy, ion-Larmor-scale spectral breaks, and non-Maxwellian features in ion distribution functions that may be tested by measurements taken in high-beta regions of the solar wind.

Dynamics of KPI lumps

A family of nonsingular rational solutions of the Kadomtsev-Petviashvili (KP) I equation are investigated. These solutions have multiple peaks whose heights are time-dependent and the peak trajectories in the xy-plane are altered after collision. Thus they differ from the standard multi-peaked KPI simple n-lump solutions whose peak heights as well as peak trajectories remain unchanged after interaction.The anomalous scattering occurs due to a non-trivial internal dynamics among the peaks in a slow time scale. This phenomena is explained by relating the peak locations to the roots of complex heat polynomials. It follows from the long time asymptotics of the solutions that the peak trajectories separate as O(√|t|) as |t| → ∞, and all the peak heights approach the same constant value corresponding to that of the simple 1-lump solution. Consequently, a multi-peaked n-lump solution evolves to a superposition of n 1-lump solutions asymptotically as |t| →∞.

Time-varying metamaterials and metasurfaces for antennas and propagation applications

In this contribution we present the most recent results from our group about the opportunities offered by time-varying metamaterials and metasurfaces for conceiving antenna systems and devices exhibiting artificial non-reciprocity, frequency conversion, energy accumulation and temporal electromagnetic scattering. Such artificial metastructures are characterized by constitutive parameters (permittivity, permeability and/or surface impedance) that are modulated in time through an external control or requires modulated excitation signal for enabling anomalous scattering behaviour. Here, we briefly describe the physical insights of the unusual interaction arising between the electromagnetic field and such metamaterials and metasurfaces, and then we present some antennas and propagation applications, showing the performances of non-reciprocal antenna systems, magnet-less isolators, Doppler cloaks, temporal devices and metasurface-based virtual absorbers.

Guidelines for de novo phasing using multiple small-wedge data collection

Intense micro-focus X-ray beamlines available at synchrotron facilities have achieved high-quality data collection even from the microcrystals of membrane proteins. The automatic data collection system developed at SPring-8, named ZOO, has contributed to many structure determinations of membrane proteins using small-wedge synchrotron crystallography (SWSX) datasets. The `small-wedge' (5–20°) datasets are collected from multiple crystals and then merged to obtain the final structure factors. To our knowledge, no systematic investigation on the dose dependence of data accuracy has so far been reported for SWSX, which is between `serial crystallography' and `rotation crystallography'. Thus, herein, we investigated the optimal dose conditions for experimental phasing with SWSX. Phase determination using anomalous scattering signals was found to be more difficult at higher doses. Furthermore, merging more homogeneous datasets grouped by hierarchical clustering with controlled doses mildly reduced the negative factors in data collection, such as `lack of signal' and `radiation damage'. In turn, as more datasets were merged, more probable phases could be obtained across a wider range of doses. Therefore, our findings show that it is essential to choose a lower dose than 10 MGy for de novo structure determination by SWSX. In particular, data collection using a dose of 5 MGy proved to be optimal in balancing the amount of signal available while reducing the amount of damage as much as possible.

Cesium based phasing of macromolecules: a general easy to use approach for solving the phase problem

Over the last decades the phase problem in macromolecular x-ray crystallography has become more controllable as methods and approaches have diversified and improved. However, solving the phase problem is still one of the biggest obstacles on the way of successfully determining a crystal structure. To overcome this caveat, we have utilized the anomalous scattering properties of the heavy alkali metal cesium. We investigated the introduction of cesium in form of cesium chloride during the three major steps of protein treatment in crystallography: purification, crystallization, and cryo-protection. We derived a step-wise procedure encompassing a “quick-soak”-only approach and a combined approach of CsCl supplement during purification and cryo-protection. This procedure was successfully applied on two different proteins: (i) Lysozyme and (ii) as a proof of principle, a construct consisting of the PH domain of the TFIIH subunit p62 from Chaetomium thermophilum for de novo structure determination. Usage of CsCl thus provides a versatile, general, easy to use, and low cost phasing strategy.

Phosphorus and sulfur SAD phasing of the nucleic acid-bound DNA-binding domain of interferon regulatory factor 4

Pivotal to the regulation of key cellular processes such as the transcription, replication and repair of DNA, DNA-binding proteins play vital roles in all aspects of genetic activity. The determination of high-quality structures of DNA-binding proteins, particularly those in complexes with DNA, provides crucial insights into the understanding of these processes. The presence in such complexes of phosphate-rich oligonucleotides offers the choice of a rapid method for the routine solution of DNA-binding proteins through the use of long-wavelength beamlines such as I23 at Diamond Light Source. This article reports the use of native intrinsic phosphorus and sulfur single-wavelength anomalous dispersion methods to solve the complex of the DNA-binding domain (DBD) of interferon regulatory factor 4 (IRF4) bound to its interferon-stimulated response element (ISRE). The structure unexpectedly shows three molecules of the IRF4 DBD bound to one ISRE. The sole reliance on native intrinsic anomalous scattering elements that belong to DNA–protein complexes renders the method of general applicability to a large number of such protein complexes that cannot be solved by molecular replacement or by other phasing methods.

Likelihood-based estimation of substructure content from single-wavelength anomalous diffraction (SAD) intensity data

SAD phasing can be challenging when the signal-to-noise ratio is low. In such cases, having an accurate estimate of the substructure content can determine whether or not the substructure of anomalous scatterer positions can successfully be determined. Here, a likelihood-based target function is proposed to accurately estimate the strength of the anomalous scattering contribution directly from the measured intensities, determining a complex correlation parameter relating the Bijvoet mates as a function of resolution. This gives a novel measure of the intrinsic anomalous signal. The SAD likelihood target function also accounts for correlated errors in the measurement of intensities from Bijvoet mates, which can arise from the effects of radiation damage. When the anomalous signal is assumed to come primarily from a substructure comprising one anomalous scatterer with a known value of f′′ and when the protein composition of the crystal is estimated correctly, the refined complex correlation parameters can be interpreted in terms of the atomic content of the primary anomalous scatterer before the substructure is known. The maximum-likelihood estimation of substructure content was tested on a curated database of 357 SAD cases with useful anomalous signal. The prior estimates of substructure content are highly correlated to the content determined by phasing calculations, with a correlation coefficient (on a log–log basis) of 0.72.