A comparative study of the excitation conditions in the plasma and the electrical parameters of a d.c. arc when using power-stabilized,current-stabilized and manually controlled d.c. sources

The abilities of three d.c. arc sources—a manually controlled source, a constant current source and a constant power source—to control variations in arc temperature, electron pressure and anode temperature were compared, and the dependency of these factors on the electrical parameters was investigated. It was shown that with well buffered samples there is little advantage to begained in using any one of the three sources. With poorly buffered samples, however, the constant power source showed greater control over the variations in arc temperature, electron pressure and anode temperature. It was also shown that the only electrical. parameter which could be used to obtain an estimate of the arc temperature is the arc power.     

The genus of the 2-amalgamations of graphs

A graph G is called the 2-amalgamation of subgraphs G₁ and G₂ if G = G₁ ∪ G₂ and G₁ ∩ G₂ = {x, y}, 2 distinct points. in this case we write G = G₁∪_{{x, y}} G₂. in this paper we show that the orientable genus, γ(G), satisfies the inequalities γ(G₁) + γ(G₂) ? 1 ≤ γ(G₁ ∪_{{x, y}} G₂) ≤ γ(G₁) + γ(G₂) + 1 and that this is the best possible result, i. e., the resulting three values for γ(G₁ ∪_{{x, y}} G₂) which are possible can actually be realized by appropriate choices for G₁ and G₂.     

Studies of the cathode layer of a d.c. arc

Some factors which affect the cathode layer of a d.c. arc were investigated. It is shown that: (i) varying concentrations of elements with low ionisation potentials in the sample can markedly affect the distribution of atoms in the cathode layer and hence the detection limits of the technique; (ii) variations in arc current and arc gap have only a small effect on the cathode layer provided that these factors are maintained at values above certain minimum levels; (iii) the use of artifical atmospheres and inhomogeneous magnetic fields generally increase detection limits; (iv) detection limits obtained with samples containing relatively high concentrations of elements with low ionisation potentials can be two or three times better than those obtained with conventional anode evaporation techniques provided wandering of the cathode spot is kept to a minimum, a condition which often cannot be realised in practice.     

Kontrastreiche und von Aufladungsstörungen freie Abbildung hochisolierender Oberflächen mittels Sekundärelektronen

The apparent secondary electron yield from metal surfaces (Au, W, Al, Zn, Ag, Ni and Cu) with a polymeride layer of varying thickness was measured. It was found that even with a layer thickness of as much as 40 Å and a primary energy of 25 kev there were clearly noticeable differences in emission from the various substances. This effect can be used for the electron emission microscopy of specimens consisting of insulating materials. In some cases, satisfactory images of such specimens could not be obtained in a reproducible way if other methods, e.g. Cevales' partial evaporation method were applied. If a carbon layer 30 Å in thickness was evaporated onto the insulating specimen surface byBradley's procedure, well contrasted images free from disturbances by surface charges could be obtained from specimens such as glass, thermally etched A1₂O₃ and other minerals. In these images, a resolution limit of 800 Å was attained.     

Determination of alkali metals leached from glass ampoules into injection fluids

Direct determination of the alkali metal output of glass ampoules by atomic emission and absorption spectrometry is described. The methods provide suitable alternatives to acidimetric titration for characterizing the hydrolytic resistance of container glasses for pharmaceutical use.     

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.     

Near-field optical experiments on low-symmetry split-ring-resonator arrays

Effective symmetric and antisymmetric eigenmodes of coupled plasmonic resonances play a crucial role in many photonic metamaterials. Recently, we discussed a particular arrangement of metallic split-ring resonators that is planar, hence enabling direct experimental access to the different eigenmodes via near-field optical microscopy. In this Letter, corresponding optical experiments are presented and compared with simple theoretical modeling, providing a direct confirmation of our previous, more indirect conclusions.     

Critical threshold behavior for steady-state internal transport barriers in burning plasmas

Burning tokamak plasmas with internal transport barriers are investigated by means of integrated modeling simulations. The barrier sustainment in steady state, differently from the barrier formation process, is found to be characterized by a critical behavior, and the critical number of the phase transition is determined. Beyond a power threshold, alignment of self-generated and noninductively driven currents occurs and steady state becomes possible. This concept is applied to simulate a steady-state scenario within the specifications of the International Thermonuclear Experimental Reactor.