Prediction of significant tokamak turbulence at electron gyroradius scales

The experimental conditions under which tokamak turbulence at hyperfine (electron gyroradius) scales is predicted to be significant and observable are described. The first quantitative predictions of fluctuation amplitudes, spectral features, and the associated electron energy transport are presented. A novel theoretical model which quantitatively describes the boundaries of the high-amplitude streamer transport regime is presented and shown to explain the gyrokinetic simulation results. This model uniquely includes consideration of two distinct secondary instabilities.     

The global version of the gyrokinetic turbulence code GENE

The understanding and prediction of transport due to plasma microturbulence is a key open problem in modern plasma physics, and a grand challenge for fusion energy research. Ab initio simulations of such small-scale, low-frequency turbulence are to be based on the gyrokinetic equations, a set of nonlinear integro-differential equations in reduced (five-dimensional) phase space. In the present paper, the extension of the well-established and widely used gyrokinetic code GENE [F. Jenko, W. Dorland, M. Kotschenreuther, B.N. Rogers, Electron temperature gradient driven turbulence, Phys. Plasmas 7 (2000) 1904–1910] from a radially local to a radially global (nonlocal) version is described. The necessary modifications of both the basic equations and the employed numerical methods are detailed, including, e.g., the change from spectral methods to finite difference and interpolation techniques in the radial direction and the implementation of sources and sinks. In addition, code verification studies and benchmarks are presented.     

Construction of a New Class of Tetracycline Lead Structures with Potent Antibacterial Activity through Biosynthetic Engineering

Antimicrobial resistance and the shortage of novel antibiotics have led to an urgent need for new antibacterial drug leads. Several existing natural product scaffolds (including chelocardins) have not been developed because their suboptimal pharmacological properties could not be addressed at the time. It is demonstrated here that reviving such compounds through the application of biosynthetic engineering can deliver novel drug candidates. Through a rational approach, the carboxamido moiety of tetracyclines (an important structural feature for their bioactivity) was introduced into the chelocardins, which are atypical tetracyclines with an unknown mode of action. A broad‐spectrum antibiotic lead was generated with significantly improved activity, including against all Gram‐negative pathogens of the ESKAPE panel. Since the lead structure is also amenable to further chemical modification, it is a platform for further development through medicinal chemistry and genetic engineering.     

Electron microscopy studies of potassium sodium niobate ceramics.

Using electron microscopy, K0.5Na0.5NbO3 (KNN) ceramics sintered at 1030 degrees C for 8 h and 1100 degrees C for 2 and 24 h was studied. The scanning electron microscopy and X-ray spectrometry revealed that the materials consisted of a matrix phase in which the (Na+K)/Nb ratio corresponded closely to the nominal composition and a small amount of Nb-rich secondary phase. A bimodal microstructure of cube-shaped grains was revealed in the fracture and thermally-etched surfaces of the KNN. In the ceramics sintered at 1100 degrees C, the larger grains (up to 30 mum across), contained angular trapped pores. The transmission electron microscopy analysis revealed that the crystal planes of the grains bordering the intragranular pore faces were of the {100} family with respect to the simple perovskite cell. Ferroelectric domains were observed in the grains of this material.     

Anomalous diffusion of particles with inertia in external potentials

Recently a new type of Kramers-Fokker-Planck equation has been proposed [Friedrich, R.; Jenko, F.; Baule, A.; Eule, S. Phys. Rev. Lett. 2006, 96, 230601] describing anomalous diffusion in external potentials. In the present paper, the explicit cases of a harmonic potential and a velocity-dependent damping are incorporated. Exact relations for moments for these cases are presented, and the asymptotic behavior for long times is discussed. Interestingly, the bounding potential and the additional damping by itself lead to a subdiffusive behavior. While acting together, the particle becomes localized for long periods of time.     

Collisional gyrokinetic full-f particle-in-cell simulations on open field lines with PICLS

Applying gyrokinetic simulations in theoretical turbulence and transport studies for the plasma edge and scrape-off layer (SOL) presents significant challenges. To particularly account for steep density and temperature gradients in the SOL, the “full-f” code PICLS was developed. PICLS is a gyrokinetic particle-in-cell (PIC) code, is based on an electrostatic model with a linearized field equation, and uses kinetic electrons. In previously published results, we applied PICLS to the well-studied 1D parallel transport problem during an edge-localized mode (ELM) in the SOL without collisions. As an extension to this collision-less case and in preparation for 3D simulations, in this work, a collisional model will be introduced. The implemented Lenard–Bernstein collision operator and its Langevin discretization will be shown. Conservation properties of the collision operator, as well as a comparison of the collisional and non-collisional case, will be discussed.     

Saturation of gyrokinetic turbulence through damped eigenmodes

In the context of toroidal gyrokinetic simulations, it is shown that a hierarchy of damped modes is excited in the nonlinear turbulent state. These modes exist at the same spatial scales as the unstable eigenmodes that drive the turbulence. The larger amplitude subdominant modes are weakly damped and exhibit smooth, large-scale structure in velocity space and in the direction parallel to the magnetic field. Modes with increasingly fine-scale structure are excited to decreasing amplitudes. In aggregate, damped modes define a potent energy sink. This leads to an overlap of the spatial scales of energy injection and peak dissipation, a feature that is in contrast with more traditional turbulent systems.     

Nonlinear gyrokinetic simulations of ion-temperature-gradient turbulence for the optimized Wendelstein 7-X stellarator

Ion-temperature-gradient turbulence constitutes a possibly dominant transport mechanism for optimized stellarators, in view of the effective suppression of neoclassical losses characterizing these devices. Nonlinear gyrokinetic simulation results for the Wendelstein 7-X stellarator [G. Grieger, in (IAEA, Vienna, 1991) Vol. 3, p. 525]-assuming an adiabatic electron response-are presented. Several fundamental features are discussed, including the role of zonal flows for turbulence saturation, the resulting flux-gradient relationship, and the coexistence of ion-temperature-gradient modes with trapped ion modes in the saturated state.     

Crystal structure and phase transitions of sodium potassium niobate perovskites

This paper presents the crystal structure and the phase transitions of K_xNa_1?xNbO₃ (0.4 ≤ x ≤ 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100–800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° ≤ β ≤ 90.34°). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic–tetragonal and tetragonal–cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.     

Density functional theory study of selenium adsorption on Fe(1 1 0)

The adsorption of atomic Se on a Fe(1 1 0) surface is examined using the density functional theory (DFT). Selenium is adsorbed in high-symmetry adsorption sites: the -short and long-bridge, and atop sites at 1/2, 1/4, and 1 monolayer (ML) coverages. The long bridge (LB) site is found to be the most stable, followed by the short bridge (SB) and top sites (T). The following overlayer structures were examined, p(2 × 2), c(2 × 2), and p(1 × 1), which correspond to 1/4 ML, 1/2 ML, and 1 ML respectively. Adsorption energy is −5.23 eV at 1/4 ML. Se adsorption results in surface reconstruction, being more extensive for adsorption in the long bridge site at 1/2 ML, with vertical displacements between +8.63 and −6.69% -with regard to the original Fe position-, affecting the 1st and 2nd neighbours. The largest displacement in x or y-directions was determined to be 0.011, 0.030, and 0.021 Å for atop and bridge sites. Comparisons between Se-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the Se. At the long bridge site, the presence of Se causes a decrease in the surface Fe d-orbital density of states between 4 and 5 eV below Fermi level. The density of states present a contribution of Se states at −3.1 eV and −12.9 eV. stabilized after adsorption. The Fe-Fe overlap population decrease and a Fe-Se bond are formed at the expense of the metallic bond.