Heat convection and transport barriers in low-magnetic-shear rijnhuizen tokamak project plasmas

Layers of reduced electron heat transport ("transport barriers") have been observed in the Rijnhuizen Tokamak Project when the plasma is dominantly heated by electron cyclotron heating (ECH). Experiments into the properties of the transport barriers are reported. Modulation of the ECH power was used to probe electron heat transport in the barriers by means of propagating electron temperature perturbations. The observed propagation shows that transport inside the barriers is dominated by heat convection. This convection is inward, i.e., up the temperature gradient.     

Computer simulation of nonlinear coupling of high-power microwaveswith slots

Numerical calculations are used to determine the effects of air breakdown on the coupling of intense microwave pulses passing through enclosed slot apertures. The calculations are based on Maxwell's equations and a set of electron-fluid equations. Variations in the energies of microwaves transmitted through slot apertures as a function of incident-field intensity and aperture size are addressed. Results show that air-breakdown threshold energies in apertures have a functional dependence on pressure similar to that of air-breakdown threshold energies in the atmosphere     

Modeling the propagation of long, intense microwave pulses inmixtures of air and SF6

Self-consistent equations-Maxwell's equations and the electron fluid transport equations-are used to study the interaction of long, intense microwave pulses with gases. The threshold for breakdown is a factor of seven higher in SF₆ than in air. This difference is attributed to the fact that SF₆ is an insulator. When SF₆ was mixed with air, the breakdown threshold increased with the increase in the percentage of SF₆. Introducing 5-10% of SF₆ in air increased the breakdown threshold by a factor of 3-4. The electron density and conductivity and the plasma frequency increased as the pulse intensity increased. The growth rates of the electron density and conductivity were much higher in air than in SF₆     

Two methanes are better than one: a density functional theory study of the reactions of Mo2Oy- (y = 2-5) with methane

The mechanisms of chemical reactions of molybdenum suboxide clusters Mo(2)O(n)- (n = 2-5) with methane are investigated using B3LYP hybrid density functional theory and polarized basis sets. In particular, we focus on the reactions of the most stable structural isomers of Mo(2)O(2,3,4,5)- that lead to single molybdenum species such as HMoO(2)CH(3)-, as seen in the recent experimental study of Jarrold and co-workers. We find that, while all experimentally observed products are unfavorable due to the high amount of energy required to cleave the metal oxide, the formation of HMoO(2)CH(3)- is least endothermic. Even in this case, the thermodynamics of these reactions is very unfavorable when a single methane is reacted with the metal oxide. However, we find that the sequential addition of two methanes produces HMoO(2)CH(3)- (and another neutral molecule whose identity depends on the number of oxygens in the metal oxide) at a much lower thermodynamic cost. Further, the overall reaction barriers are much lower when the second methane adds prior to the Mo(2)O(2,3,4,5)- cleavage. The methane addition at each metal center oxidizes the metals to produce a species that is then stable enough to afford the Mo-Mo cleavage.     

Ion/molecule reactions provide new evidence for the structure and origin of

Ion/molecule reactions leading to formation of the diagnostically useful [C(3)H(4)N](+) ion (m/z 54), from acetonitrile CI plasma, have been studied using a tandem quadrupole mass spectrometer. Evidence is presented to demonstrate that [C(3)H(4)N](+) is produced from an ion/molecule reaction between [C(2)H(2)N](+) (m/z 40) and neutral acetonitrile, via a [C(4)H(5)N(2)](+) (m/z 81) intermediate. Loss of HCN, where the H atom arises from neutral acetonitrile and the CN group from [C(2)H(2)N](+) (m/z 40), leads to the production of [C(3)H(4)N](+) (m/z 54). These results are consistent with a proposed concerted elimination of HCN, generating m/z 54 as a methylene vinylidene ammonium ion. Copyright 1999 John Wiley & Sons, Ltd.     

100-Gb/s undersea transmission with high spectral efficiency using pre-filtered QPSK modulation format

We provide a review of our recent 100-Gb/s, high spectral efficiency (SE) experiment targeting transoceanic and regional undersea transmission distances. We demonstrated that simple pre-filtering at the transmitter together with a maximum a posteriori probability (MAP) detection algorithm can significantly improve SE. We transmitted 96×100-Gb/s pre-filtered polarization division multiplexed return-to-zero quadrature phase shift keyed (PDM-RZ-QPSK) channels with 300% SE over 10608 km using 52-km spans of 150-μm 2 fiber and simple single-stage erbium-doped fiber amplifiers (EDFAs). We also achieved 400% SE over 4368 km using similar techniques.     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.